The human brain is the central
organ of the human
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
, and with the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
makes up the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. The
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...
consists of the
cerebrum
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb ...
, the
brainstem and the
cerebellum. It controls most of the activities of the
body
Body may refer to:
In science
* Physical body, an object in physics that represents a large amount, has mass or takes up space
* Body (biology), the physical material of an organism
* Body plan, the physical features shared by a group of anima ...
, processing, integrating, and coordinating the information it receives from the
sense organs
A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...
, and making decisions as to the instructions sent to the rest of the body. The brain is contained in, and protected by, the
skull bones of the
head.
The cerebrum, the largest part of the human brain, consists of two
cerebral hemisphere
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s. Each hemisphere has an inner core composed of
white matter
White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distributi ...
, and an outer surface – the
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
– composed of
grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingu ...
. The cortex has an outer layer, the
neocortex, and an inner
allocortex
The allocortex or heterogenetic cortex, and neocortex are the two types of cerebral cortex in the brain. The allocortex is the much smaller area of cortex taking up just 10 %, the neocortex takes up the remaining 90 %. It is characterized by havi ...
. The neocortex is made up of six
neuronal layers, while the allocortex has three or four. Each hemisphere is conventionally divided into four
lobes – the
frontal
Front may refer to:
Arts, entertainment, and media Films
* ''The Front'' (1943 film), a 1943 Soviet drama film
* ''The Front'', 1976 film
Music
* The Front (band), an American rock band signed to Columbia Records and active in the 1980s and e ...
,
temporal,
parietal, and
occipital lobe
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
s. The frontal lobe is associated with
executive functions
In cognitive science and neuropsychology, executive functions (collectively referred to as executive function and cognitive control) are a set of cognitive processes that are necessary for the cognitive control of behavior: selecting and suc ...
including
self-control
Self-control, an aspect of inhibitory control, is the ability to regulate one's emotions, thoughts, and behavior in the face of temptations and impulses. As an executive function, it is a cognitive process that is necessary for regulating one' ...
,
planning,
reason
Reason is the capacity of consciously applying logic by drawing conclusions from new or existing information, with the aim of seeking the truth. It is closely associated with such characteristically human activities as philosophy, science, ...
ing, and
abstract thought
Abstraction in its main sense is a conceptual process wherein general rules and concepts are derived from the usage and classification of specific examples, literal ("real" or "concrete") signifiers, first principles, or other methods.
"An abstr ...
, while the
occipital lobe
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
is dedicated to vision. Within each lobe, cortical areas are associated with specific functions, such as the
sensory,
motor
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power g ...
and
association
Association may refer to:
*Club (organization), an association of two or more people united by a common interest or goal
*Trade association, an organization founded and funded by businesses that operate in a specific industry
*Voluntary associatio ...
regions. Although the left and right hemispheres are broadly similar in shape and function, some functions are
associated with one side, such as
language
Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of met ...
in the left and
visual-spatial ability in the right. The hemispheres are connected by
commissural nerve tracts, the largest being the
corpus callosum
The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental m ...
.
The cerebrum is connected by the brainstem to the spinal cord. The brainstem consists of the
midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
, the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
, and the
medulla oblongata. The
cerebellum is connected to the brainstem by three pairs of
nerve tracts called
cerebellar peduncle
Cerebellar peduncles connect the cerebellum to the brain stem. There are six cerebellar peduncles in total, three on each side:
* Superior cerebellar peduncle is a paired structure of white matter that connects the cerebellum to the mid-brain ...
s. Within the cerebrum is the
ventricular system
The ventricular system is a set of four interconnected cavities known as cerebral ventricles in the brain. Within each ventricle is a region of choroid plexus which produces the circulating cerebrospinal fluid (CSF). The ventricular system is co ...
, consisting of four interconnected
ventricles in which
cerebrospinal fluid
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.
CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the ...
is produced and circulated. Underneath the cerebral cortex are several important structures, including the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
, the
epithalamus
The epithalamus is a posterior (dorsal) segment of the diencephalon. The epithalamus includes the habenular nuclei and their interconnecting fibers, the habenular commissure, the stria medullaris and the pineal gland.
Functions
The function ...
, the
pineal gland, the
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
, the
pituitary gland
In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...
, and the
subthalamus; the
limbic structures, including the
amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
and the
hippocampus
The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
; the
claustrum
The claustrum (Latin, meaning "to close" or "to shut") is a thin, bilateral collection of neurons and supporting glial cells, that connects to cortical (e.g., the pre-frontal cortex) and subcortical regions (e.g., the thalamus) of the brain. It ...
, the various
nuclei of the
basal ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an exter ...
; the
basal forebrain
Part of the human brain, the basal forebrain structures are located in the forebrain to the front of and below the striatum. They include the ventral basal ganglia (including nucleus accumbens and ventral pallidum), nucleus basalis, diagonal b ...
structures, and the three
circumventricular organ
Circumventricular organs (CVOs) ( circum-: around ; ventricular: of ventricle) are structures in the brain characterized by their extensive and highly permeable capillaries, unlike those in the rest of the brain where there exists a blood†...
s. The
cells of the brain include
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s and supportive
glial cells
Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form myel ...
. There are more than 86 billion neurons in the brain, and a more or less equal number of other cells. Brain activity is made possible by the interconnections of neurons and their release of
neurotransmitters in response to
nerve impulses
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
. Neurons connect to form
neural pathway
In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to an ...
s,
neural circuits, and elaborate
network systems. The whole circuitry is driven by the process of
neurotransmission
Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron), ...
.
The brain is protected by the
skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
, suspended in
cerebrospinal fluid
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.
CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the ...
, and isolated from the
bloodstream
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
by the
blood–brain barrier. However, the brain is still susceptible to
damage
Damage is any change in a thing, often a physical object, that degrades it away from its initial state. It can broadly be defined as "changes introduced into a system that adversely affect its current or future performance".Farrar, C.R., Sohn, H., ...
,
disease
A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
, and
infection
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...
. Damage can be caused by
trauma
Trauma most often refers to:
* Major trauma, in physical medicine, severe physical injury caused by an external source
* Psychological trauma, a type of damage to the psyche that occurs as a result of a severely distressing event
*Traumatic i ...
, or a loss of blood supply known as a
stroke. The brain is susceptible to
degenerative disorders, such as
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
,
dementia
Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
s including
Alzheimer's disease, and
multiple sclerosis.
Psychiatric condition
A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitti ...
s, including
schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
and
clinical depression
Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Intro ...
, are thought to be associated with brain dysfunctions. The brain can also be the site of
tumours
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
, both
benign
Malignancy () is the tendency of a medical condition to become progressively worse.
Malignancy is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous ''benign'' tumor in that a malignancy is not s ...
and
malignant
Malignancy () is the tendency of a medical condition to become progressively worse.
Malignancy is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous ''benign'' tumor in that a malignancy is not s ...
; these mostly
originate from other sites in the body.
The study of the anatomy of the brain is
neuroanatomy
Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...
, while the study of its function is
neuroscience
Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developme ...
. Numerous techniques are used to study the brain.
Specimens from other animals, which may be
examined microscopically, have traditionally provided much information.
Medical imaging technologies such as
functional neuroimaging
Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions. It is primarily used a ...
, and
electroencephalography (EEG) recordings are important in studying the brain. The
medical history
The medical history, case history, or anamnesis (from Greek: ἀνά, ''aná'', "open", and μνήσις, ''mnesis'', "memory") of a patient is information gained by a physician by asking specific questions, either to the patient or to other peo ...
of people with
brain injury
An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, or o ...
has provided insight into the function of each part of the brain. Brain research has evolved over time, with philosophical, experimental, and theoretical phases. An emerging phase may be to
simulate
A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of Conceptual model, models; the model represents the key characteristics or behaviors of the selected system or proc ...
brain activity.
In culture, the
philosophy of mind
Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are add ...
has for centuries attempted to address the question of the nature of
consciousness
Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
and the
mind–body problem
The mind–body problem is a philosophical debate concerning the relationship between thought and consciousness in the human mind, and the brain as part of the physical body. The debate goes beyond addressing the mere question of how mind and bo ...
. The
pseudoscience
Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable clai ...
of
phrenology attempted to localise personality attributes to regions of the cortex in the 19th century.
In science fiction, brain transplants are imagined in tales such as the 1942 ''
Donovan's Brain
''Donovan's Brain'' is a 1942 science fiction novel by American writer Curt Siodmak.
The novel was an instant success and has been adapted to film three times. Since then the book has become something of a cult classic, with fans including Ste ...
''.
Structure
Gross anatomy
The adult human brain weighs on average about which is about 2% of the total body weight,
with a volume of around 1260
cm3 in men and 1130 cm
3 in women.
There is substantial individual variation,
[ with the standard ]reference range
In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood, o ...
for men being and for women .
The cerebrum
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb ...
, consisting of the cerebral hemisphere
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s, forms the largest part of the brain and overlies the other brain structures. The outer region of the hemispheres, the cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
, is grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingu ...
, consisting of cortical layers
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of a ...
of neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s. Each hemisphere is divided into four main lobes – the frontal lobe
The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove be ...
, parietal lobe
The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.
The parietal lobe integrates sensory informa ...
, temporal lobe
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in pro ...
, and occipital lobe
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
. Three other lobes are included by some sources which are a ''central lobe'', a limbic lobe
The limbic lobe is an arc-shaped region of cortex on the medial surface of each cerebral hemisphere of the mammalian brain, consisting of parts of the frontal, parietal and temporal lobes. The term is ambiguous, with some authors including the p ...
, and an insular lobe. The central lobe comprises the precentral gyrus
The precentral gyrus is a prominent gyrus on the surface of the posterior frontal lobe of the brain. It is the site of the primary motor cortex that in humans is cytoarchitecturally defined as Brodmann area 4.
Structure
The precentral gyrus l ...
and the postcentral gyrus and is included since it forms a distinct functional role.
The brainstem, resembling a stalk, attaches to and leaves the cerebrum at the start of the midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
area. The brainstem includes the midbrain, the pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
, and the medulla oblongata. Behind the brainstem is the cerebellum ( la , little brain).
The cerebrum, brainstem, cerebellum, and spinal cord are covered by three membranes called meninges. The membranes are the tough dura mater; the middle arachnoid mater and the more delicate inner pia mater. Between the arachnoid mater and the pia mater is the Meninges#Subarachnoid space, subarachnoid space and subarachnoid cisterns, which contain the cerebrospinal fluid
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.
CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the ...
. The outermost membrane of the cerebral cortex is the basement membrane of the pia mater called the glia limitans and is an important part of the blood–brain barrier.
The living brain is very soft, having a gel-like consistency similar to soft tofu. The cortical layers of neurons constitute much of the cerebral grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingu ...
, while the deeper subcortical regions of myelinated axons, make up the white matter
White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distributi ...
. The white matter of the brain makes up about half of the total brain volume.
Cerebrum
The cerebrum is the largest part of the brain, and is divided into nearly Symmetry in biology#Bilateral symmetry, symmetrical left and right cerebral hemisphere, hemispheres by a deep groove, the longitudinal fissure. Asymmetry between the lobes is noted as a Paleoneurobiology#Asymmetry, petalia. The hemispheres are connected by five Commissural fiber#Structure, commissures that span the longitudinal fissure, the largest of these is the corpus callosum
The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental m ...
.
Each hemisphere is conventionally divided into four main lobes; the frontal lobe
The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove be ...
, parietal lobe
The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.
The parietal lobe integrates sensory informa ...
, temporal lobe
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in pro ...
, and occipital lobe
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
, named according to the skull , skull bones that overlie them. Each lobe is associated with one or two specialised functions though there is some functional overlap between them.[ The surface of the brain is gyrification, folded into ridges (gyrus, gyri) and grooves (sulcus (neuroanatomy), sulci), many of which are named, usually according to their position, such as the frontal gyrus of the frontal lobe or the central sulcus separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds.
The outer part of the cerebrum is the ]cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
, made up of grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingu ...
arranged in layers. It is thick, and deeply folded to give a convoluted appearance. Beneath the cortex is the cerebral white matter
White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distributi ...
. The largest part of the cerebral cortex is the neocortex, which has six neuronal layers. The rest of the cortex is of allocortex
The allocortex or heterogenetic cortex, and neocortex are the two types of cerebral cortex in the brain. The allocortex is the much smaller area of cortex taking up just 10 %, the neocortex takes up the remaining 90 %. It is characterized by havi ...
, which has three or four layers.
The cortex is brain mapping, mapped by divisions into about fifty different functional areas known as Brodmann's areas. These areas are distinctly different when Histology, seen under a microscope. The cortex is divided into two main functional areas – a motor cortex and a sensory cortex. The primary motor cortex, which sends axons down to motor neurons in the brainstem and spinal cord, occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The primary sensory areas receive signals from the sensory nerves and nerve tract, tracts by way of Thalamus#Thalamic nuclei, relay nuclei in the thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
. Primary sensory areas include the visual cortex of the occipital lobe
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
, the auditory cortex in parts of the temporal lobe
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in pro ...
and insular cortex, and the somatosensory cortex in the parietal lobe
The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.
The parietal lobe integrates sensory informa ...
. The remaining parts of the cortex are called the association areas. These areas receive input from the sensory areas and lower parts of the brain and are involved in the complex cognition, cognitive processes of perception, thought, and decision-making. The main functions of the frontal lobe are to Attentional control, control attention, abstract thinking, behaviour, problem-solving tasks, and physical reactions and personality. The occipital lobe is the smallest lobe; its main functions are visual reception, visual-spatial processing, movement, and Color vision#Color in the human brain, colour recognition. There is a smaller occipital lobule in the lobe known as the cuneus. The temporal lobe controls Echoic memory, auditory and visual memory, visual memories, Language processing in the brain, language, and some hearing and speech.
The cerebrum contains the ventricular system, ventricles where the cerebrospinal fluid is produced and circulated. Below the corpus callosum is the septum pellucidum, a membrane that separates the lateral ventricles. Beneath the lateral ventricles is the thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
and to the front and below this is the hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
. The hypothalamus leads on to the pituitary gland
In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...
. At the back of the thalamus is the brainstem.
The basal ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an exter ...
, also called basal nuclei, are a set of structures deep within the hemispheres involved in behaviour and movement regulation. The largest component is the striatum, others are the globus pallidus, the substantia nigra and the subthalamic nucleus. The striatum is divided into a ventral striatum, and a dorsal striatum, subdivisions that are based upon function and connections. The ventral striatum consists of the nucleus accumbens and the olfactory tubercle whereas the dorsal striatum consists of the caudate nucleus and the putamen. The putamen and the globus pallidus lie separated from the lateral ventricles and thalamus by the internal capsule, whereas the caudate nucleus stretches around and abuts the lateral ventricles on their outer sides. At the deepest part of the lateral sulcus between the insular cortex and the striatum is a thin neuronal sheet called the claustrum
The claustrum (Latin, meaning "to close" or "to shut") is a thin, bilateral collection of neurons and supporting glial cells, that connects to cortical (e.g., the pre-frontal cortex) and subcortical regions (e.g., the thalamus) of the brain. It ...
.
Below and in front of the striatum are a number of basal forebrain
Part of the human brain, the basal forebrain structures are located in the forebrain to the front of and below the striatum. They include the ventral basal ganglia (including nucleus accumbens and ventral pallidum), nucleus basalis, diagonal b ...
structures. These include the nucleus basalis, diagonal band of Broca, substantia innominata, and the medial septal nucleus. These structures are important in producing the neurotransmitter, acetylcholine, which is then distributed widely throughout the brain. The basal forebrain, in particular the nucleus basalis, is considered to be the major cholinergic output of the central nervous system to the striatum and neocortex.
Cerebellum
The cerebellum is divided into an anterior lobe of cerebellum, anterior lobe, a posterior lobe of cerebellum, posterior lobe, and the flocculonodular lobe. The anterior and posterior lobes are connected in the middle by the cerebellar vermis, vermis. Compared to the cerebral cortex, the cerebellum has a much thinner outer cortex that is narrowly furrowed into numerous curved transverse fissures.
Viewed from underneath between the two lobes is the third lobe the flocculonodular lobe. The cerebellum rests at the back of the posterior cranial fossa, cranial cavity, lying beneath the occipital lobes, and is separated from these by the cerebellar tentorium, a sheet of fibre.
It is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncle
Cerebellar peduncles connect the cerebellum to the brain stem. There are six cerebellar peduncles in total, three on each side:
* Superior cerebellar peduncle is a paired structure of white matter that connects the cerebellum to the mid-brain ...
s. The superior cerebellar peduncle, superior pair connects to the midbrain; the middle cerebellar peduncle, middle pair connects to the medulla, and the inferior cerebellar peduncle, inferior pair connects to the pons. The cerebellum consists of an inner medulla of white matter and an outer cortex of richly folded grey matter. The cerebellum's anterior and posterior lobes appear to play a role in the coordination and smoothing of complex motor movements, and the flocculonodular lobe in the maintenance of Equilibrioception, balance although debate exists as to its cognitive, behavioural and motor functions.
Brainstem
The brainstem lies beneath the cerebrum and consists of the midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
, pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
and medulla oblongata, medulla. It lies in the posterior cranial fossa, back part of the skull, resting on the part of the base of the skull, base known as the clivus (anatomy), clivus, and ends at the foramen magnum, a large :wikt:foramen, opening in the occipital bone. The brainstem continues below this as the spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, protected by the vertebral column.
Ten of the twelve pairs of cranial nerves emerge directly from the brainstem. The brainstem also contains many cranial nerve nucleus, cranial nerve nuclei and nucleus (neuroanatomy), nuclei of nerve, peripheral nerves, as well as nuclei involved in the regulation of many essential processes including breathing, control of eye movements and balance. The reticular formation, a network of nuclei of ill-defined formation, is present within and along the length of the brainstem. Many nerve tracts, which transmit information to and from the cerebral cortex to the rest of the body, pass through the brainstem.
Microanatomy
The human brain is primarily composed of neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s, glial cells, neural stem cells, and blood vessels. Types of neuron include interneurons, pyramidal cells including Betz cells, motor neurons (upper motor neuron, upper and lower motor neurons), and cerebellar Purkinje cells. Betz cells are the largest cells (by size of cell body) in the nervous system. The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells. Out of these neurons, 16 billion (19%) are located in the cerebral cortex, and 69 billion (80%) are in the cerebellum.
Types of glial cell are astrocytes (including Bergmann glia), oligodendrocytes, ependymal cells (including tanycytes), radial glial cells, microglia, and a subtype of oligodendrocyte progenitor cells. Astrocytes are the largest of the glial cells. They are stellate cells with many processes radiating from their soma (biology), cell bodies. Some of these processes end as perivascular end-feet on capillary walls. The glia limitans of the cortex is made up of astrocyte foot processes that serve in part to contain the cells of the brain.
Mast cells are white blood cells that interact in the neuroimmune system in the brain. Mast cells in the central nervous system are present in Mast cell#In the nervous system, a number of structures including the meninges; they mediate neuroimmune responses in inflammatory conditions and help to maintain the blood–brain barrier, particularly in brain regions where the barrier is absent. Mast cells serve the same general functions in the body and central nervous system, such as effecting or regulating allergic responses, Innate immune system, innate and Adaptive immune system, adaptive immunity, autoimmunity, and inflammation. Mast cells serve as the main effector cell through which pathogens can affect the gut-brain axis, biochemical signaling that takes place between the gastrointestinal tract and the central nervous system.
Some 400 genes are shown to be brain-specific. In all neurons, ELAVL3 is expressed, and in pyramidal neurons, NRGN and REEP2 are also expressed. GAD1 – essential for the biosynthesis of the neurotransmitter GABA – is expressed in interneurons. Proteins expressed in glial cells include astrocyte markers Glial fibrillary acidic protein, GFAP and S100B whereas myelin basic protein and the transcription factor OLIG2 are expressed in oligodendrocytes.
Cerebrospinal fluid
Cerebrospinal fluid is a clear, colourless transcellular fluid that circulates around the brain in the subarachnoid space, in the ventricular system
The ventricular system is a set of four interconnected cavities known as cerebral ventricles in the brain. Within each ventricle is a region of choroid plexus which produces the circulating cerebrospinal fluid (CSF). The ventricular system is co ...
, and in the central canal of the spinal cord. It also fills some gaps in the subarachnoid space, known as subarachnoid cisterns. The four ventricles, two lateral ventricle, lateral, a third ventricle, third, and a fourth ventricle, all contain a choroid plexus that produces cerebrospinal fluid. The third ventricle lies in the midline and Interventricular foramina (neuroanatomy), is connected to the lateral ventricles. A single duct (anatomy), duct, the cerebral aqueduct between the pons and the cerebellum, connects the third ventricle to the fourth ventricle. Three separate openings, the Medial aperture, middle and two lateral apertures, drain the cerebrospinal fluid from the fourth ventricle to the cisterna magna, one of the major cisterns. From here, cerebrospinal fluid circulates around the brain and spinal cord in the subarachnoid space, between the arachnoid mater and pia mater.
At any one time, there is about 150mL of cerebrospinal fluid – most within the subarachnoid space. It is constantly being regenerated and absorbed, and is replaced about once every 5–6 hours.
A glymphatic system has been described as the lymphatic drainage system of the brain. The brain-wide glymphatic pathway includes drainage routes from the cerebrospinal fluid, and from the meningeal lymphatic vessels that are associated with the dural sinuses, and run alongside the cerebral blood vessels. The pathway drains interstitial fluid from the tissue of the brain.
Blood supply
The internal carotid arteries supply Blood#Oxygen transport, oxygenated blood to the front of the brain and the vertebral arteries supply blood to the back of the brain. These two circulations anastomosis, join in the circle of Willis, a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons.
The internal carotid arteries are branches of the common carotid arteries. They enter the cranium through the carotid canal, travel through the cavernous sinus and enter the subarachnoid space. They then enter the circle of Willis, with two branches, the anterior cerebral arteries emerging. These branches travel forward and then upward along the longitudinal fissure, and supply the front and midline parts of the brain. One or more small anterior communicating artery, anterior communicating arteries join the two anterior cerebral arteries shortly after they emerge as branches. The internal carotid arteries continue forward as the middle cerebral arteries. They travel sideways along the sphenoid bone of the orbit (anatomy), eye socket, then upwards through the insula cortex, where final branches arise. The middle cerebral arteries send branches along their length.
The vertebral arteries emerge as branches of the left and right subclavian arteries. They travel upward through Vertebra#Cervical vertebrae, transverse foramina which are spaces in the cervical vertebrae. Each side enters the cranial cavity through the foramen magnum along the corresponding side of the medulla. They give off Posterior inferior cerebellar artery, one of the three cerebellar branches. The vertebral arteries join in front of the middle part of the medulla to form the larger basilar artery, which sends multiple branches to supply the medulla and pons, and the two other Anterior inferior cerebellar artery, anterior and Superior cerebellar artery, superior cerebellar branches. Finally, the basilar artery divides into two posterior cerebral arteries. These travel outwards, around the superior cerebellar peduncles, and along the top of the cerebellar tentorium, where it sends branches to supply the temporal and occipital lobes. Each posterior cerebral artery sends a small posterior communicating artery to join with the internal carotid arteries.
Blood drainage
Cerebral veins drain Blood#Oxygen transport, deoxygenated blood from the brain. The brain has two main networks of veins: an exterior or Superior cerebral veins, superficial network, on the surface of the cerebrum that has three branches, and an Internal cerebral veins, interior network. These two networks communicate via anastomosis, anastomosing (joining) veins. The veins of the brain drain into larger cavities of the dural venous sinuses usually situated between the dura mater and the covering of the skull. Blood from the cerebellum and midbrain drains into the great cerebral vein. Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into the spinal veins or into adjacent cerebral veins.
The blood in the Anatomical terms of location#deep, deep part of the brain drains, through a venous plexus into the cavernous sinus at the front, and the superior petrosal sinus, superior and inferior petrosal sinuses at the sides, and the inferior sagittal sinus at the back. Blood drains from the outer brain into the large superior sagittal sinus, which rests in the midline on top of the brain. Blood from here joins with blood from the straight sinus at the confluence of sinuses.
Blood from here drains into the left and right transverse sinuses. These then drain into the sigmoid sinuses, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large internal jugular veins.
The blood–brain barrier
The larger arteries throughout the brain supply blood to smaller capillaries. These smallest of blood vessels in the brain, are lined with cells joined by tight junctions and so fluids do not seep in or leak out to the same degree as they do in other capillaries; this creates the blood–brain barrier. Pericytes play a major role in the formation of the tight junctions. The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including anaesthetics and alcohol). The blood-brain barrier is not present in the circumventricular organswhich are structures in the brain that may need to respond to changes in body fluidssuch as the pineal gland, area postrema, and some areas of the hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
. There is a similar Choroid plexus#Function, blood–cerebrospinal fluid barrier, which serves the same purpose as the blood–brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems.
Development
At the beginning of the third week of human embryonic development, development, the embryonic ectoderm forms a thickened strip called the neural plate. By the fourth week of development the neural plate has widened to give a broad cephalization, cephalic end, a less broad middle part and a narrow caudal end. These swellings are known as the Brain vesicle, primary brain vesicles and represent the beginnings of the forebrain (prosencephalon), midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
(mesencephalon), and hindbrain (rhombencephalon).
Neural crest, Neural crest cells (derived from the ectoderm) populate the lateral edges of the plate at the neural folds. In the fourth weekduring the neurulation , neurulation stagethe Neural fold#Folding mechanism, neural folds close to form the neural tube, bringing together the neural crest cells at the neural crest. The neural crest runs the length of the tube with cranial neural crest cells at the cephalic end and caudal neural crest cells at the tail. Cells detach from the crest and cell migration, migrate in a craniocaudal (head to tail) wave inside the tube. Cells at the cephalic end give rise to the brain, and cells at the caudal end give rise to the spinal cord.
The tube Flexure (embryology), flexes as it grows, forming the crescent-shaped cerebral hemispheres at the head. The cerebral hemispheres first appear on day 32.
Early in the fourth week, the cephalic part bends sharply forward in a cephalic flexure. This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure becomes the hindbrain (rhombencephalon). These areas are formed as swellings known as the three brain vesicle, primary brain vesicles. In the fifth week of development five brain vesicle, secondary brain vesicles have formed. The forebrain separates into two vesicles – an anterior telencephalon and a posterior diencephalon. The telencephalon gives rise to the cerebral cortex, basal ganglia, and related structures. The diencephalon gives rise to the thalamus and hypothalamus. The hindbrain also splits into two areas – the metencephalon and the myelencephalon. The metencephalon gives rise to the cerebellum and pons. The myelencephalon gives rise to the medulla oblongata. Also during the fifth week, the brain divides into segmentation (biology), repeating segments called neuromeres. In the hindbrain these are known as rhombomeres.
A characteristic of the brain is the cortical folding known as gyrification. For just over five months of prenatal development the cortex is smooth. By the gestational age of 24 weeks, the wrinkled morphology showing the fissures that begin to mark out the lobes of the brain is evident. Why the cortex wrinkles and folds is not well-understood, but gyrification has been linked to intelligence and neurological disorders, and a Gyrification#Theories on causality in gyrification, number of gyrification theories have been proposed. These theories include those based on Gyrification#Mechanical buckling, mechanical buckling, Gyrification#Axonal tension, axonal tension, and Gyrification#Differential tangential expansion, differential tangential expansion. What is clear is that gyrification is not a random process, but rather a complex developmentally predetermined process which generates patterns of folds that are consistent between individuals and most species.
The first groove to appear in the fourth month is the lateral cerebral fossa. The expanding caudal end of the hemisphere has to curve over in a forward direction to fit into the restricted space. This covers the fossa and turns it into a much deeper ridge known as the lateral sulcus and this marks out the temporal lobe. By the sixth month other sulci have formed that demarcate the frontal, parietal, and occipital lobes. A gene present in the human genome (ARHGAP11B) may play a major role in gyrification and encephalisation.
Function
Motor control
The frontal lobe is involved in reasoning, motor control, emotion, and language. It contains the motor cortex, which is involved in planning and coordinating movement; the prefrontal cortex, which is responsible for higher-level cognitive functioning; and Broca’s area, which is essential for language production. The motor system of the brain is responsible for the motor control, generation and control of movement. Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles. The corticospinal tract carries movements from the brain, through the spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, to the torso and limbs. The cranial nerves carry movements related to the eyes, mouth and face.
Gross movement – such as Animal locomotion, locomotion and the movement of arms and legs – is generated in the motor cortex, divided into three parts: the primary motor cortex, found in the precentral gyrus
The precentral gyrus is a prominent gyrus on the surface of the posterior frontal lobe of the brain. It is the site of the primary motor cortex that in humans is cytoarchitecturally defined as Brodmann area 4.
Structure
The precentral gyrus l ...
and has sections dedicated to the movement of different body parts. These movements are supported and regulated by two other areas, lying anterior to the primary motor cortex: the premotor area and the supplementary motor area. The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in a Cortical homunculus#Types, motor homunculus. Impulses generated from the motor cortex travel along the corticospinal tract along the front of the medulla and cross over (decussate) at the medullary pyramids (brainstem), medullary pyramids. These then travel down the spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, with most connecting to spinal interneuron, interneurons, in turn connecting to lower motor neurons within the grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingu ...
that then transmit the impulse to move to muscles themselves. The cerebellum and basal ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an exter ...
, play a role in fine, complex and coordinated muscle movements. Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as the extrapyramidal system.
Sensory
The sensory nervous system is involved with the reception and processing of sense, sensory information. This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centres of the brain exposed to the blood. The brain also receives and interprets information from the special senses of visual perception, vision, Olfaction, smell, hearing, and taste. Sensory-motor coupling, Mixed motor and sensory signals are also integrated.
From the skin, the brain receives information about touch, fine touch, pressure, pain, vibration and temperature. From the joints, the brain receives information about proprioception, joint position. The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord. The dorsal column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. The pathway fibres travel up the back part of the spinal cord to the back part of the medulla, where they connect with dorsal column–medial lemniscus pathway#Second-order neurons, second-order neurons that immediately Decussation, send fibres across the midline. These fibres then travel upwards into the ventrobasal complex in the thalamus where they connect with dorsal column–medial lemniscus pathway#Third-order neurons, third-order neurons which send fibres up to the sensory cortex. The spinothalamic tract carries information about pain, temperature, and gross touch. The pathway fibres travel up the spinal cord and connect with second-order neurons in the reticular formation of the brainstem for pain and temperature, and also terminate at the ventrobasal complex of the thalamus for gross touch.
Visual perception, Vision is generated by light that hits the retina of the eye. Photoreceptor cell, Photoreceptors in the retina visual phototransduction, transduce the sensory stimulus of electromagnetic radiation, light into an electrical action potential, nerve signal that is sent to the visual cortex in the occipital lobe. Visual signals leave the retinas through the optic nerves.
Optic nerve fibres from the retinas' nasal halves Optic chiasm, cross to the opposite sides joining the fibres from the temporal halves of the opposite retinas to form the optic tracts.
The arrangements of the eyes' optics and the visual pathways mean vision from the left visual field is received by the right half of each retina, is processed by the right visual cortex, and vice versa.
The optic tract fibres reach the brain at the lateral geniculate nucleus, and travel through the optic radiation to reach the visual cortex.
Hearing and Equilibrioception, balance are both generated in the inner ear. Sound results in vibrations of the ossicles which continue finally to Hair cell, the hearing organ, and change in balance results in movement of Vestibular system, liquids within the inner ear. This creates a nerve signal that passes through the vestibulocochlear nerve. From here, it passes through to the cochlear nuclei, the superior olivary nucleus, the medial geniculate nucleus, and finally the auditory radiation to the auditory cortex.
The sense of Olfaction, smell is generated by Olfactory receptor neuron, receptor cells in the olfactory epithelium, epithelium of the olfactory mucosa in the nasal cavity. This information passes via the olfactory nerve which goes into the skull through cribiform plate, a relatively permeable part. This nerve transmits to the neural circuitry of the olfactory bulb from where information is passed to the olfactory system, olfactory cortex.
Taste is generated from Taste receptor, receptors on the tongue and passed along the facial and glossopharyngeal nerves into the solitary nucleus in the brainstem. Some taste information is also passed from the pharynx into this area via the vagus nerve. Information is then passed from here through the thalamus into the gustatory cortex.
Regulation
Autonomic nervous system, Autonomic functions of the brain include the regulation, or Neuroscience of rhythm, rhythmic control of the heart rate and respiratory rate, rate of breathing, and maintaining homeostasis.
Blood pressure and heart rate are influenced by the vasomotor center, vasomotor centre of the medulla, which causes arteries and veins to be somewhat constricted at rest. It does this by influencing the sympathetic nervous system, sympathetic and parasympathetic nervous systems via the vagus nerve. Information about blood pressure is generated by baroreceptors in aortic body, aortic bodies in the aortic arch, and passed to the brain along the general visceral afferent fibers, afferent fibres of the vagus nerve. Information about the pressure changes in the carotid sinus comes from carotid body, carotid bodies located near the common carotid artery, carotid artery and this is passed via a Hering's nerve, nerve joining with the glossopharyngeal nerve. This information travels up to the solitary nucleus in the medulla. Signals from here influence the vasomotor centre to adjust vein and artery constriction accordingly.
The brain controls the respiratory rate, rate of breathing, mainly by respiratory center, respiratory centres in the medulla and pons. The respiratory centres control respiration (physiology), respiration, by generating motor signals that are passed down the spinal cord, along the phrenic nerve to the Thoracic diaphragm, diaphragm and other muscles of respiration. This is a spinal nerve, mixed nerve that carries sensory information back to the centres. There are four respiratory centres, three with a more clearly defined function, and an apneustic centre with a less clear function. In the medulla a dorsal respiratory group causes the desire to inhalation, breathe in and receives sensory information directly from the body. Also in the medulla, the ventral respiratory group influences exhalation, breathing out during exertion. In the pons the pneumotaxic center, pneumotaxic centre influences the duration of each breath, and the apneustic center, apneustic centre seems to have an influence on inhalation. The respiratory centres directly senses blood carbon dioxide and pH. Information about blood oxygen, carbon dioxide and pH levels are also sensed on the walls of arteries in the peripheral chemoreceptors of the aortic and carotid bodies. This information is passed via the vagus and glossopharyngeal nerves to the respiratory centres. High carbon dioxide, an acidic pH, or low oxygen stimulate the respiratory centres. The desire to breathe in is also affected by pulmonary stretch receptors in the lungs which, when activated, prevent the lungs from overinflating by transmitting information to the respiratory centres via the vagus nerve.
The hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
in the diencephalon, is involved in regulating many functions of the body. Functions include neuroendocrine regulation, regulation of the circadian rhythm, control of the autonomic nervous system, and the regulation of fluid, and food intake. The circadian rhythm is controlled by two main cell groups in the hypothalamus. The anterior hypothalamus includes the suprachiasmatic nucleus and the ventrolateral preoptic nucleus which through gene expression cycles, generates a roughly 24 hour circadian clock. In the circadian clock, circadian day an ultradian rhythm takes control of the sleeping pattern. Sleep is an essential requirement for the body and brain and allows the closing down and resting of the body's systems. There are also findings that suggest that the daily build-up of toxins in the brain are removed during sleep. Whilst awake the brain consumes a fifth of the body's total energy needs. Neuroscience of sleep, Sleep necessarily reduces this use and gives time for the restoration of energy-giving Adenosine triphosphate, ATP. The effects of sleep deprivation show the absolute need for sleep.
The lateral hypothalamus contains orexinergic neurons that control appetite and arousal through their projections to the ascending reticular activating system. The hypothalamus controls the pituitary gland
In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...
through the release of peptides such as oxytocin, and vasopressin, as well as dopamine into the median eminence. Through the autonomic projections, the hypothalamus is involved in regulating functions such as blood pressure, heart rate, breathing, sweating, and other homeostatic mechanisms. The hypothalamus also plays a role in thermal regulation, and when stimulated by the immune system, is capable of generating a fever. The hypothalamus is influenced by the kidneys: when blood pressure falls, the renin released by the kidneys stimulates a need to drink. The hypothalamus also regulates food intake through autonomic signals, and hormone release by the digestive system.
Language
While language functions were traditionally thought to be localized to Wernicke's area and Broca's area, it is now mostly accepted that a wider network of Cortex (anatomy), cortical regions contributes to language functions.
The study on how language is represented, processed, and language acquisition, acquired by the brain is called neurolinguistics, which is a large multidisciplinary field drawing from cognitive neuroscience, cognitive linguistics, and psycholinguistics.
Lateralisation
The cerebrum has a contralateral brain, contralateral organisation with each hemisphere of the brain interacting primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa. The developmental cause for this is uncertain. Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both decussation, cross sides in the brainstem. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibres from each nerve split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain. Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field.
The left and right sides of the brain appear symmetrical, but they function asymmetrically. For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several important exceptions, involving language and spatial cognition. The left frontal lobe is dominant for language. If a key language area in the left hemisphere is damaged, it can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.
A substantial part of current understanding of the interactions between the two hemispheres has come from the study of "split-brain patients"—people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures. These patients do not show unusual behaviour that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it. These patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown.
Emotion
Emotions are generally defined as two-step multicomponent processes involving Human intelligence (intelligence gathering), elicitation, followed by psychological feelings, appraisal, expression, autonomic responses, and action tendencies. Attempts to localize basic emotions to certain brain regions have been controversial; some research found no evidence for specific locations corresponding to emotions, but instead found circuitry involved in general emotional processes. The amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
, orbitofrontal cortex, mid and anterior insula cortex and lateral prefrontal cortex, appeared to be involved in generating the emotions, while weaker evidence was found for the ventral tegmental area, ventral pallidum and nucleus accumbens in incentive salience. Others, however, have found evidence of activation of specific regions, such as the basal ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an exter ...
in happiness, the corpus callosum, subcallosal cingulate cortex in sadness, and amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
in fear.
Cognition
The brain is responsible for cognition, which functions through numerous cognitive process, processes and executive functions.
Figure 4: Executive functions and related terms
Executive functions include the ability to filter information and tune out irrelevant stimuli with attentional control and cognitive inhibition, the ability to process and manipulate information held in working memory, the ability to think about multiple concepts simultaneously and task switching (psychology), switch tasks with cognitive flexibility, the ability to inhibit impulse (psychology), impulses and prepotent responses with inhibitory control, and the ability to determine the relevance of information or appropriateness of an action. Higher order executive functions require the simultaneous use of multiple basic executive functions, and include planning, prospection and fluid intelligence (i.e., reasoning and problem solving).
The prefrontal cortex plays a significant role in mediating executive functions. Planning involves activation of the dorsolateral prefrontal cortex (DLPFC), anterior cingulate cortex, angular prefrontal cortex, right prefrontal cortex, and supramarginal gyrus. Working memory manipulation involves the DLPFC, inferior frontal gyrus, and areas of the parietal cortex. Inhibitory control involves multiple areas of the prefrontal cortex, as well as the caudate nucleus and subthalamic nucleus.
Physiology
Neurotransmission
Brain activity is made possible by the interconnections of neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s that are linked together to reach their targets. A neuron consists of a soma (biology), cell body, axon, and dendrites. Dendrites are often extensive branches that receive information in the form of signals from the axon terminals of other neurons. The signals received may cause the neuron to initiate an action potential (an electrochemical signal or nerve impulse) which is sent along its axon to the axon terminal, to connect with the dendrites or with the cell body of another neuron. An action potential is initiated at the Axon#Initial segment, initial segment of an axon, which contains a specialized complex of proteins. When an action potential reaches the axon terminal it triggers the release of a neurotransmitter at a synapse that propagates a signal that acts on the target cell. These chemical neurotransmitters include dopamine, serotonin, gamma-Aminobutyric acid, GABA, glutamate (neurotransmitter), glutamate, and acetylcholine. GABA is the major inhibitory neurotransmitter in the brain, and glutamate is the major excitatory neurotransmitter. Neurons link at synapses to form neural pathway
In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to an ...
s, neural circuits, and large elaborate large scale brain networks, network systems such as the salience network and the default mode network, and the activity between them is driven by the process of neurotransmission
Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron), ...
.
Metabolism
The brain consumes up to 20% of the energy used by the human body, more than any other organ. In humans, blood glucose is the primary food energy, source of energy for most cells and is critical for normal function in a number of tissues, including the brain. The human brain consumes approximately 60% of blood glucose in fasted, sedentary individuals. Brain metabolism normally relies upon blood glucose as an energy source, but during times of low glucose (such as fasting, endurance exercise, or limited carbohydrate intake), the brain uses ketone bodies for fuel with a smaller need for glucose. The brain can also utilize Lactic acid#Exercise and lactate, lactate during exercise. The brain stores glucose in the form of glycogen, albeit in significantly smaller amounts than that found in the liver or skeletal muscle. Fatty acid#Length of free fatty acid chains, Long-chain fatty acids cannot cross the blood–brain barrier, but the liver can break these down to produce ketone bodies. However, short-chain fatty acids (e.g., butyric acid, propionic acid, and acetic acid) and the Fatty acid#Length of free fatty acid chains, medium-chain fatty acids, octanoic acid and heptanoic acid, can cross the blood–brain barrier and be metabolised by brain cells.
Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. The brain mostly uses glucose for energy, and deprivation of glucose, as can happen in hypoglycemia, can result in loss of consciousness. The energy consumption of the brain does not vary greatly over time, but active regions of the cortex consume somewhat more energy than inactive regions, which forms the basis for the functional neuroimaging
Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions. It is primarily used a ...
methods of Positron emission tomography, PET and fMRI. These techniques provide a three-dimensional image of metabolic activity. A preliminary study showed that brain metabolic requirements in humans peak at about five years old.
The function of sleep is not fully understood; however, there is evidence that sleep enhances the clearance of metabolic waste products, some of which are potentially neurotoxic, from the brain and may also permit repair. Evidence suggests that the increased clearance of metabolic waste during sleep occurs via increased functioning of the glymphatic system. Sleep may also have an effect on cognitive function by weakening unnecessary connections.
Research
The brain is not fully understood, and research is ongoing. Neuroscientists, along with researchers from allied disciplines, study how the human brain works. The boundaries between the specialties of neuroscience
Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developme ...
, neurology and other disciplines such as psychiatry have faded as they are all influenced by basic research in neuroscience.
Neuroscience research has expanded considerably in recent decades. The "Decade of the Brain", an initiative of the United States Government in the 1990s, is considered to have marked much of this increase in research, and was followed in 2013 by the BRAIN Initiative. The Human Connectome Project was a five-year study launched in 2009 to analyse the anatomical and functional connections of parts of the brain, and has provided much data.
Methods
Information about the structure and function of the human brain comes from a variety of experimental methods, including animals and humans. Information about brain trauma and stroke has provided information about the function of parts of the brain and the effects of brain damage. Neuroimaging is used to visualise the brain and record brain activity. Electrophysiology is used to measure, record and monitor the electrical activity of the cortex. Measurements may be of local field potentials of cortical areas, or of the activity of a single neuron. An electroencephalography, electroencephalogram can record the electrical activity of the cortex using electrodes placed non-invasively on the scalp.
Invasive measures include electrocorticography, which uses electrodes placed directly on the exposed surface of the brain. This method is used in cortical stimulation mapping, used in the study of the relationship between cortical areas and their systemic function. By using much smaller microelectrodes, single-unit recordings can be made from a single neuron that give a high Angular resolution, spatial resolution and high temporal resolution. This has enabled the linking of brain activity to behaviour, and the creation of neuronal maps.
The development of cerebral organoids has opened ways for studying the growth of the brain, and of the cortex, and for understanding disease development, offering further implications for therapeutic applications.
Imaging
Functional neuroimaging techniques show changes in brain activity that relate to the function of specific brain areas. One technique is functional magnetic resonance imaging (fMRI) which has the advantages over earlier methods of SPECT and positron emission tomography, PET of not needing the use of Nuclear medicine, radioactive materials and of offering a higher resolution. Another technique is functional near-infrared spectroscopy. These methods rely on the haemodynamic response that shows changes in brain activity in relation to changes in cerebral circulation, blood flow, useful in brain mapping, mapping functions to brain areas. Resting state fMRI
looks at the interaction of brain regions whilst the brain is not performing a specific task. This is also used to show the default mode network.
Any electrical current generates a magnetic field; neural oscillations induce weak magnetic fields, and in functional magnetoencephalography the current produced can show localised brain function in high resolution. Tractography uses MRI and image analysis to create 3D modeling, 3D images of the nerve tracts of the brain. Connectograms give a graphical representation of the connectome, neural connections of the brain.
Differences in brain morphometry, brain structure can be measured in some disorders, notably schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
and dementia
Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
. Different biological approaches using imaging have given more insight for example into the disorders of biology of depression, depression and biology of obsessive-compulsive disorder, obsessive-compulsive disorder. A key source of information about the function of brain regions is the effects of damage to them.
Advances in neuroimaging have enabled objective insights into mental disorders, leading to faster diagnosis, more accurate prognosis, and better monitoring.
Gene and protein expression
Bioinformatics is a field of study that includes the creation and advancement of databases, and computational and statistical techniques, that can be used in studies of the human brain, particularly in the areas of Bioinformatics#Gene and protein expression, gene and protein expression. Bioinformatics and studies in genomics, and functional genomics, generated the need for DNA annotation, a Transcriptomics technologies, transcriptome technology, identifying genes, their locations and functions. GeneCards is a major database.
, just under 20,000 Human genome#Coding sequences (protein-coding genes), protein-coding genes are seen to be expressed in the human, and some 400 of these genes are brain-specific. The data that has been provided on gene expression in the brain has fuelled further research into a number of disorders. The long term use of alcohol for example, has shown altered gene expression in the brain, and cell-type specific changes that may relate to alcoholism, alcohol use disorder. These changes have been noted in the Synapse, synaptic transcriptome in the prefrontal cortex, and are seen as a factor causing the drive to alcohol dependence, and also to other substance abuses.
Other related studies have also shown evidence of synaptic alterations and their loss, in the ageing brain. Changes in gene expression alter the levels of proteins in various neural pathways and this has been shown to be evident in synaptic contact dysfunction or loss. This dysfunction has been seen to affect many structures of the brain and has a marked effect on inhibitory neurons resulting in a decreased level of neurotransmission, and subsequent cognitive decline and disease.
Clinical significance
Injury
Brain damage, Injury to the brain can manifest in many ways. Traumatic brain injury, for example received in contact sport, after a Falling (accident), fall, or a traffic collision, traffic or work accident, can be associated with both immediate and longer-term problems. Immediate problems may include intracerebral haemorrhage, bleeding within the brain, this may compress the brain tissue or damage its blood supply. Cerebral contusion, Bruising to the brain may occur. Bruising may cause widespread damage to the nerve tracts that can lead to a condition of diffuse axonal injury. A skull fracture, fractured skull, injury to a particular area, deafness, and concussion are also possible immediate developments. In addition to the site of injury, the opposite side of the brain may be affected, termed a Coup contrecoup injury, contrecoup injury. Longer-term issues that may develop include posttraumatic stress disorder, and hydrocephalus. Chronic traumatic encephalopathy can develop following multiple head injury, head injuries.
Disease
Neurodegenerative diseases result in progressive damage to different parts of the brain's function, and Aging brain, worsen with age. Common examples include dementia
Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
such as Alzheimer's disease, alcoholic dementia or vascular dementia; Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
; and other rarer infectious, genetic, or metabolic causes such as Huntington's disease, motor neuron diseases, HIV dementia, Neurosyphilis, syphilis-related dementia and Wilson's disease. Neurodegenerative diseases can affect different parts of the brain, and can affect movement, memory, and cognition.
Cerebral atherosclerosis is atherosclerosis that affects the brain. It results from the build-up of atheroma, plaques formed of cholesterol, in the large arteries of the brain, and can be mild to significant. When significant, arteries can become narrowed enough to reduce blood flow. It contributes to the development of dementia, and has protein similarities to those found in Alzheimer’s disease.
The brain, although protected by the blood–brain barrier, can be affected by infections including viruses, bacteria and fungi. Infection may be of the meninges (meningitis), the brain matter (encephalitis), or within the brain matter (such as a cerebral abscess). Rare prion diseases including Creutzfeldt–Jakob disease and its Variant Creutzfeldt–Jakob disease, variant, and Kuru (disease), kuru may also affect the brain.
Tumours
Brain tumor, Brain tumours can be either benign or malignant, cancerous. Most malignant tumours metastasis, arise from another part of the body, most commonly from the lung cancer, lung, breast cancer, breast and melanoma, skin. Cancers of brain tissue can also occur, and originate from any tissue in and around the brain. Meningioma, cancer of the meninges around the brain, is more common than cancers of brain tissue. Cancers within the brain may cause symptoms related to their size or position, with symptoms including headache and nausea, or the gradual development of focal symptoms such as gradual difficulty seeing, swallowing, talking, or as a change of mood. Cancers are in general investigated through the use of CT scans and MRI scans. A variety of other tests including blood tests and lumbar puncture may be used to investigate for the cause of the cancer and evaluate the type and cancer staging, stage of the cancer. The corticosteroid dexamethasone is often given to decrease the oedema, swelling of brain tissue around a tumour. Surgery may be considered, however given the complex nature of many tumours or based on tumour stage or type, radiotherapy or chemotherapy may be considered more suitable.
Mental disorders
Mental disorders, such as major depressive disorder, depression, schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
, bipolar disorder, posttraumatic stress disorder, attention deficit hyperactivity disorder, obsessive-compulsive disorder, Tourette syndrome, and addiction, are known to relate to the functioning of the brain. Treatment for mental disorders may include psychotherapy, psychiatry, social interventionism, social intervention and personal Recovery model, recovery work or cognitive behavioural therapy; the underlying issues and associated prognoses vary significantly between individuals.
Epilepsy
Epileptic seizures are thought to relate to abnormal electrical activity. Seizure activity can manifest as absence seizure, absence of consciousness, focal seizure, focal effects such as limb movement or impediments of speech, or be generalised seizure, generalized in nature. Status epilepticus refers to a seizure or series of seizures that have not terminated within 5 minutes. Seizures have a large number of causes, however many seizures occur without a definitive cause being found. In a person with epilepsy, risk factors for further seizures may include sleeplessness, drug and alcohol intake, and stress. Seizures may be assessed using blood tests, EEG and various medical imaging techniques based on the medical history
The medical history, case history, or anamnesis (from Greek: ἀνά, ''aná'', "open", and μνήσις, ''mnesis'', "memory") of a patient is information gained by a physician by asking specific questions, either to the patient or to other peo ...
and medical examination findings. In addition to treating an underlying cause and reducing exposure to risk factors, anticonvulsant medications can play a role in preventing further seizures.
Congenital
Some brain disorders such as Tay–Sachs disease are congenital disorder, congenital, and linked to Mutation, genetic and chromosome abnormality, chromosomal mutations. A rare group of congenital cephalic disorders known as lissencephaly is characterised by the lack of, or inadequacy of, cortical folding. Normal prenatal development, development of the brain can be affected during pregnancy by nutritional deficiencies, teratology, teratogens, infectious diseases, and by the use of Recreational drug use, recreational drugs, including alcohol (which may result in fetal alcohol spectrum disorders).
Stroke
A stroke is a ischemia, decrease in blood supply to an area of the brain causing cell death and Brain damage#Causes, brain injury. This can lead to a wide range of Stroke#Signs and symptoms, symptoms, including the "FAST (stroke), FAST" symptoms of facial droop, arm weakness, and speech difficulties (including dysarthria, with speaking and dysphasia, finding words or forming sentences). Symptoms relate to the function of the affected area of the brain and can point to the likely site and cause of the stroke. Difficulties with movement, speech, or sight usually relate to the cerebrum, whereas ataxia, imbalance, diplopia, double vision, vertigo and symptoms affecting more than one side of the body usually relate to the brainstem or cerebellum.
Most strokes result from loss of blood supply, typically because of an embolus, rupture of a atheroma, fatty plaque causing thrombus, or arteriosclerotic, narrowing of small arteries. Strokes can also result from Stroke#Hemorrhagic, bleeding within the brain. Transient ischemic attack, Transient ischaemic attacks (TIAs) are strokes in which symptoms resolve within 24 hours. Investigation into the stroke will involve a medical examination (including a neurological examination) and the taking of a medical history
The medical history, case history, or anamnesis (from Greek: ἀνά, ''aná'', "open", and μνήσις, ''mnesis'', "memory") of a patient is information gained by a physician by asking specific questions, either to the patient or to other peo ...
, focusing on the duration of the symptoms and risk factors (including Hypertension, high blood pressure, atrial fibrillation, and tobacco smoking, smoking). Further investigation is needed in younger patients. An ECG and biotelemetry may be conducted to identify atrial fibrillation; an ultrasound can investigate carotid stenosis, narrowing of the Common carotid artery, carotid arteries; an echocardiogram can be used to look for clots within the heart, Valvular heart disease, diseases of the heart valves or the presence of a patent foramen ovale. Blood tests are routinely done as part of the Medical diagnosis#Other diagnostic procedure methods, workup including Diabetes mellitus#Diagnosis, diabetes tests and a lipid profile.
Some treatments for stroke are time-critical. These include thrombolysis, clot dissolution or embolectomy, surgical removal of a clot for Brain ischemia, ischaemic strokes, and decompression (surgery), decompression for Intracranial hemorrhage, haemorrhagic strokes. As stroke is time critical, hospitals and even pre-hospital care of stroke involves expedited investigations – usually a CT scan to investigate for a haemorrhagic stroke and a CT angiogram, CT or MR angiogram to evaluate arteries that supply the brain. MRI scans, not as widely available, may be able to demonstrate the affected area of the brain more accurately, particularly with ischaemic stroke.
Having experienced a stroke, a person may be admitted to a stroke unit, and treatments may be directed as secondary prevention, preventing future strokes, including ongoing anticoagulation (such as aspirin or clopidogrel), Antihypertensive drug, antihypertensives, and lipid-lowering agent, lipid-lowering drugs. A multidisciplinary team including speech pathologists, physiotherapists, occupational therapists, and psychologists plays a large role in supporting a person affected by a stroke and their physical medicine and rehabilitation, rehabilitation. A history of stroke increases the risk of developing dementia by around 70%, and recent stroke increases the risk by around 120%.
Brain death
Brain death refers to an irreversible total loss of brain function. This is characterised by coma, loss of reflexes, and apnoea,[ however, the declaration of brain death varies geographically and is not always accepted.] In some countries there is also a defined syndrome of brainstem death. Declaration of brain death can have profound implications as the declaration, under the principle of Futile medical care, medical futility, will be associated with the withdrawal of life support, and as those with brain death often have organs suitable for organ donation. The process is often made more difficult by poor communication with patients' families.
When brain death is suspected, reversible differential diagnosis, differential diagnoses such as, electrolyte, neurological and drug-related cognitive suppression need to be excluded. Testing for reflexes can be of help in the decision, as can the absence of response and breathing. Clinical observations, including a total lack of responsiveness, a known diagnosis, and neural imaging evidence, may all play a role in the decision to pronounce brain death.
Society and culture
Neuroanthropology is the study of the relationship between culture and the brain. It explores how the brain gives rise to culture, and how culture influences brain development. Cultural differences and their relation to brain development and structure are researched in different fields.
The mind
The philosophy of mind, philosophy of the mind studies such issues as the problem of understanding consciousness
Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
and the mind–body problem
The mind–body problem is a philosophical debate concerning the relationship between thought and consciousness in the human mind, and the brain as part of the physical body. The debate goes beyond addressing the mere question of how mind and bo ...
. The relationship between the brain and the mind is a significant challenge both philosophically and scientifically. This is because of the difficulty in explaining how mental activities, such as thoughts and emotions, can be implemented by physical structures such as neurons and synapses, or by any other type of physical mechanism. This difficulty was expressed by Gottfried Leibniz in the analogy known as ''Leibniz's Mill'':
Doubt about the possibility of a mechanistic explanation of thought drove René Descartes, and most other philosophers along with him, to Dualism (philosophy of mind), dualism: the belief that the mind is to some degree independent of the brain. There has always, however, been a strong argument in the opposite direction. There is clear empirical evidence that physical manipulations of, or injuries to, the brain (for example by drugs or by lesions, respectively) can affect the mind in potent and intimate ways. In the 19th century, the case of Phineas Gage, a railway worker who was injured by a stout iron rod passing through his brain, convinced both researchers and the public that cognitive functions were localised in the brain. Following this line of thinking, a large body of empirical evidence for a close relationship between brain activity and mental activity has led most neuroscientists and contemporary philosophers to be Materialism, materialists, believing that mental phenomena are ultimately the result of, or reducible to, physical phenomena.
Brain size
The size of the brain and a person's intelligence are not strongly related. Studies tend to indicate small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and Intelligence quotient, IQ. The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampi, and the cerebellum, but these only account for a relatively small amount of variance in IQ, which itself has only a partial relationship to general intelligence and real-world performance.
Other animals, including whales and elephants have larger brains than humans. However, when the brain-to-body mass ratio is taken into account, the human brain is almost twice as large as that of a bottlenose dolphin, and three times as large as that of a Common chimpanzee, chimpanzee. However, a high ratio does not of itself demonstrate intelligence: very small animals have high ratios and the treeshrew has the largest quotient of any mammal.
In popular culture
Earlier ideas about the relative importance of the different History of the location of the soul, organs of the human body sometimes emphasized the heart.
Modern Western popular conceptions, in contrast, have placed increasing focus on the Mind-brain dichotomy, brain.
Research has disproved some common List of common misconceptions#Brain, misconceptions about the brain. These include both ancient and modern myths. It is not true (for example) that neurons are not replaced after the age of two; nor that normal humans use only Ten percent of the brain myth, ten per cent of the brain. Popular culture has also oversimplified the Lateralization of brain function, lateralisation of the brain by suggesting that functions are completely specific to one side of the brain or the other. Akio Mori coined the term "game brain" for the unreliably supported theory that spending long periods playing video games harmed the brain's pre-frontal region, and impaired the expression of emotion and creativity.
Historically, particularly in the early-19th century, the brain featured in popular culture through phrenology, a pseudoscience
Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable clai ...
that assigned personality attributes to different regions of the cortex. The cortex remains important in popular culture as covered in books and satire.
The human brain can feature in Brain in science fiction, science fiction, with themes such as brain transplants and Cyborgs in fiction, cyborgs (beings with features like partly artificial brains). The 1942 science-fiction book (adapted three times for the cinema) ''Donovan's Brain
''Donovan's Brain'' is a 1942 science fiction novel by American writer Curt Siodmak.
The novel was an instant success and has been adapted to film three times. Since then the book has become something of a cult classic, with fans including Ste ...
'' tells the tale of an isolated brain kept alive ''in vitro'', gradually taking over the personality of the book's protagonist.
History
Early history
The Edwin Smith Papyrus, an ancient Egyptian medical literature, medical treatise written in the 17th century BC, contains the earliest recorded reference to the brain. The hieroglyph for brain, occurring eight times in this papyrus, describes the symptoms, diagnosis, and prognosis of two traumatic injuries to the head. The papyrus mentions the external surface of the brain, the effects of injury (including seizures and aphasia), the meninges, and cerebrospinal fluid.
In the fifth century BC, Alcmaeon of Croton in Magna Grecia, first considered the brain to be the Sensorium, seat of the mind. Also in the Fifth-century Athens, fifth century BC in Athens, the unknown author of ''On the Sacred Disease'', a medical treatise which is part of the Hippocratic Corpus and traditionally attributed to Hippocrates, believed the brain to be the seat of intelligence. Aristotle, in his Aristotle's biology, biology initially believed the heart to be the seat of intelligence, and saw the brain as a cooling mechanism for the blood. He reasoned that humans are more rational than the beasts because, among other reasons, they have a larger brain to cool their hot-bloodedness. Aristotle did describe the meninges and distinguished between the cerebrum and cerebellum.
Herophilus of Chalcedon in the fourth and third centuries BC distinguished the cerebrum and the cerebellum, and provided the first clear description of the Ventricular system, ventricles; and with Erasistratus of Kea (island), Ceos experimented on living brains. Their works are now mostly lost, and we know about their achievements due mostly to secondary sources. Some of their discoveries had to be re-discovered a millennium after their deaths. Anatomist physician Galen in the second century AD, during the time of the Roman Empire, dissected the brains of sheep, monkeys, dogs, and pigs. He concluded that, as the cerebellum was denser than the brain, it must control the muscles, while as the cerebrum was soft, it must be where the senses were processed. Galen further theorized that the brain functioned by movement of animal spirits through the ventricles.[
]
Renaissance
In 1316, Mondino de Luzzi's ''Anathomia'' began the modern study of brain anatomy.
Niccolò Massa discovered in 1536 that the ventricles were filled with fluid.[ Archangelo Piccolomini of Rome was the first to distinguish between the cerebrum and cerebral cortex.] In 1543 Andreas Vesalius published his seven-volume ''De humani corporis fabrica''. The seventh book covered the brain and eye, with detailed images of the ventricles, cranial nerves, pituitary gland
In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...
, meninges, structures of the human eye, eye, the vascular supply to the brain and spinal cord, and an image of the peripheral nerves. Vesalius rejected the common belief that the ventricles were responsible for brain function, arguing that many animals have a similar ventricular system to humans, but no true intelligence.
René Descartes proposed the theory of Mind-body dualism, dualism to tackle the issue of the brain's relation to the mind. He suggested that the pineal gland was where the mind interacted with the body, serving as the seat of the soul and as the connection through which animism, animal spirits passed from the blood into the brain. This dualism likely provided impetus for later anatomists to further explore the relationship between the anatomical and functional aspects of brain anatomy.
Thomas Willis is considered a second pioneer in the study of neurology and brain science. He wrote ''Cerebri Anatome'' ( la , Anatomy of the brain) in 1664, followed by ''Cerebral Pathology'' in 1667. In these he described the structure of the cerebellum, the ventricles, the cerebral hemispheres, the brainstem, and the cranial nerves, studied its blood supply; and proposed functions associated with different areas of the brain. The circle of Willis was named after his investigations into the blood supply of the brain, and he was the first to use the word "neurology." Willis removed the brain from the body when examining it, and rejected the commonly held view that the cortex only consisted of blood vessels, and the view of the last two millennia that the cortex was only incidentally important.
In the middle of 19th century Emil du Bois-Reymond and Hermann von Helmholtz were able to use a galvanometer to show that electrical impulses passed at measurable speeds along nerves, refuting the view of their teacher Johannes Peter Müller that the nerve impulse was a vital function that could not be measured. Richard Caton in 1875 demonstrated electrical impulses in the cerebral hemispheres of rabbits and monkeys. In the 1820s, Jean Pierre Flourens pioneered the experimental method of damaging specific parts of animal brains describing the effects on movement and behavior.
Modern period
Studies of the brain became more sophisticated with the use of the microscope and the development of a silver staining Golgi method, method by Camillo Golgi during the 1880s. This was able to show the intricate structures of single neurons. This was used by Santiago Ramón y Cajal and led to the formation of the neuron doctrine, the then revolutionary hypothesis that the neuron is the functional unit of the brain. He used microscopy to uncover many cell types, and proposed functions for the cells he saw. For this, Golgi and Cajal are considered the founders of History of neuroscience, twentieth century neuroscience, both sharing the Nobel prize in 1906 for their studies and discoveries in this field.
Charles Scott Sherrington, Charles Sherrington published his influential 1906 work ''The Integrative Action of the Nervous System'' examining the function of reflexes, evolutionary development of the nervous system, functional specialisation of the brain, and layout and cellular function of the central nervous system. In 1942 he coined the term ''enchanted loom'' as a metaphor for the brain. John Farquhar Fulton, founded the ''Journal of Neurophysiology'' and published the first comprehensive textbook on the physiology of the nervous system during 1938. History of neuroscience#Twentieth Century, Neuroscience during the twentieth century began to be recognised as a distinct unified academic discipline, with David Rioch, Francis O. Schmitt, and Stephen Kuffler playing critical roles in establishing the field. Rioch originated the integration of basic anatomical and physiological research with clinical psychiatry at the Walter Reed Army Institute of Research, starting in the 1950s. During the same period, Schmitt established the Neuroscience Research Program, an inter-university and international organisation, bringing together biology, medicine, psychological and behavioural sciences. The word neuroscience itself arises from this program.
Paul Broca associated regions of the brain with specific functions, in particular language in Broca's area, following work on brain-damaged patients.[Principles of Neural Science, 4th ed. Eric R. Kandel, James H. Schwartz, Thomas M. Jessel, eds. McGraw-Hill:New York, NY. 2000.] John Hughlings Jackson described the function of the motor cortex by watching the progression of epileptic seizures through the body. Carl Wernicke described Wernicke's area, a region associated with language comprehension and production. Korbinian Brodmann divided regions of the brain based on the appearance of cells. By 1950, Sherrington, James Papez, Papez, and Paul D. MacLean, MacLean had identified many of the brainstem and limbic system functions. The capacity of the brain to re-organise and change with age, and a recognised critical development period, were attributed to neuroplasticity, pioneered by Margaret Kennard, who experimented on monkeys during the 1930-40s.
Harvey Cushing (1869–1939) is recognised as the first proficient neurosurgery, brain surgeon in the world. In 1937, Walter Dandy began the practice of vascular neurosurgery by performing the first surgical clipping of an intracranial aneurysm.
Comparative anatomy
The human brain has many properties that are common to all vertebrate brains. Many of its features are common to all mammalian brains, most notably a six-layered cerebral cortex and a set of associated structures, including the hippocampus and amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
. The cortex is proportionally larger in humans than in many other mammals. Humans have more association cortex, sensory and motor parts than smaller mammals such as the rat and the cat.
As a primate brain, the human brain has a much larger cerebral cortex, in proportion to body size, than most mammals, and a highly developed visual system.
As a hominidae, hominid brain, the human brain is substantially enlarged even in comparison to the brain of a typical monkey. The sequence of human evolution from ''Australopithecus'' (four million years ago) to human, ''Homo sapiens'' (modern humans) was marked by a steady increase in brain size. As brain size increased, this altered the size and shape of the skull, from about 600 Cubic centimetre, cm3 in ''Homo habilis'' to an average of about 1520 cm3 in ''Homo neanderthalensis''. Differences in DNA, gene expression, and gene–environment interactions help explain the differences between the function of the human brain and other primates.
See also
*Cerebral atrophy
*Cortical spreading depression
*Evolution of human intelligence
*Large-scale brain networks
*Outline of neuroscience
*Superficial veins of the brain
References
Bibliography
*
*
*
*
*
*
*
*
Notes
External links
Brain facts and figures
{{DEFAULTSORT:Human Brain
Brain
Human anatomy by organ, Brain
Brain anatomy