HOME

TheInfoList




In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, the classical doctrine of the nervous system determines that it is a highly complex part of an
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
that coordinates its actions and
sensory
sensory
information by transmitting
signals
signals
to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the
endocrine system The endocrine system is a messenger system comprising feedback loops of the hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported ...

endocrine system
to respond to such events.
Nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, se ...
first arose in wormlike organisms about 550 to 600 million years ago. However, this classical doctrine has been challenged in recent decades by discoveries about the existence and use of electrical signals in plants. On the basis of these findings, some scientists have proposed that a plant nervous system exists and that a scientific field called plant neurobiology should be created. This proposal has led to a dispute in the scientific community between those who think we should talk about the nervous system of plants and those who are against it. The inflexibility of the positions in the scientific debate on both sides has led to proposing a solution to the debate, consisting of redefining the concept of the nervous system by using only physiological criteria and avoiding phylogenetic criteria. In vertebrates it consists of two main parts, the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
(CNS) and the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
(PNS). The CNS consists of the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
and
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
. The PNS consists mainly of
nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, ...

nerve
s, which are enclosed bundles of the long fibers or
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called
motor nerve A motor nerve is a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, s ...
s or '' efferent'' nerves, while those nerves that transmit information from the body to the CNS are called
sensory nerve A sensory nerve, or afferent nerve, is a general anatomic term for a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and ...
s or '' afferent''.
Spinal nerve A spinal nerve is a , which carries motor, sensory, and autonomic signals between the and the body. In the there are 31 pairs of spinal nerves, one on each side of the . These are grouped into the corresponding , , , and regions of the spine. ...

Spinal nerve
s are
mixed nerve A nerve is an enclosed, cable-like bundle of fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the electro ...
s that serve both functions. The PNS is divided into three separate subsystems, the
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syste ...
,
autonomic
autonomic
, and
enteric The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syst ...
nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the
parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulating t ...
nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the
gastrointestinal The gastrointestinal tract, (GI tract, GIT, digestive tract, digestion tract, alimentary canal) is the tract from the mouth to the anus which includes all the organs of the digestive system The human digestive system consists of the human ...
system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
while those exiting from the spinal cord are called
spinal nerves A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...
. At the cellular level, the nervous system is defined by the presence of a special type of cell, called the
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
, also known as a "nerve cell." Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical impulses traveling along thin fibers called
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s, which can be directly transmitted to neighboring cells through
electrical synapse An electrical synapse is a mechanical and electrically conductive In physics and electrical engineering, a conductor is an object or type of material A material is a substance Substance may refer to: * Substance (Jainism), a term in Jain on ...
s or cause chemicals called
neurotransmitter A neurotransmitter is a signaling molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, governs the basic activities of cells and coordinates multiple-cell actions. A signal is an entity that ...
s to be released at
chemical synapse Chemical synapses are biological junctions through which neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims t ...
s. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise
modulated In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplifi ...
. The connections between neurons can form
neural pathway A neural pathway is the connection formed by axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Lat ...
s,
neural circuit A neural circuit is a population of neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a ...
s, and larger
networks Network and networking may refer to: Arts, entertainment, and media * Network (1976 film), ''Network'' (1976 film), a 1976 American film * Network (2019 film), ''Network'' (2019 film), an Indian film * Network (album), ''Network'' (album), a 2004 ...
that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called
glial cells Glia, also called glial cells (singular ''gliocyte'') or neuroglia, are non-neuronal cell (biology), cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They main ...

glial cells
(or simply glia), which provide structural and metabolic support. Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s,
placozoa The Placozoa are a basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a ...

placozoa
ns, and
mesozoa The Mesozoa are minuscule, worm Worms are many different distantly related animals that typically have a long cylindrical tube-like body, no limbs, and no eyes. Worms vary in size from microscopic to over in length for marine polychaete ...
ns, which have very simple body plans. The nervous systems of the radially symmetric organisms
ctenophores Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly r ...
(comb jellies) and
cnidarians Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...
(which include ,
hydras ''Hydra'' ( ) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including t ...
,
corals Corals are within the of the . They typically form compact of many identical individual s. Coral species include the important builders that inhabit tropical oceans and secrete to form a hard skeleton. A coral "group" is a colony of myri ...

corals
and
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

jellyfish
) consist of a diffuse
nerve net A nerve net consists of interconnected neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims to develop electron ...
. All other animal species, with the exception of a few types of worms, have a nervous system containing a brain, a central cord (or two cords running in
parallel Parallel may refer to: Computing * Parallel algorithm In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their a ...
), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants. The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply . The medical specialty of
neurology Neurology (from el, , "string, nerve" and the suffix , "study of") is a branch of dealing with . Neurology deals with the diagnosis and treatment of all categories of conditions and disease involving the and s (and their subdivisions, the ...
studies disorders of the nervous system and looks for interventions that can prevent or treat them. In the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including
diabetic neuropathy Diabetic neuropathy refers to various types of nerve damage associated with diabetes mellitus. Symptoms depend on the site of nerve damage and can include motor changes such as weakness; sensory symptoms such as numbness, tingling, or pain; or auton ...
and demyelinating disorders such as
multiple sclerosis Multiple sclerosis (MS), also known as encephalomyelitis disseminata, is the most common demyelinating disease, in which the Myelin, insulating covers of nerve cells in the Human brain, brain and spinal cord are damaged. This damage disrupts the ...
and
amyotrophic lateral sclerosis Amyotrophic lateral sclerosis (ALS; also known as Lou Gehrig's disease in Canada Canada is a country in the northern part of North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the A ...
.
Neuroscience Neuroscience is the of the . It is a science that combines , , , , , and to understand the fundamental and emergent properties of s, and s. The understanding of the biological basis of , , , , and has been described by as the "epic chal ...

Neuroscience
is the field of science that focuses on the study of the nervous system.


Structure

The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s of
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
s), that emanate from the brain and
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
, and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope. The author Michael Nikoletseas wrote:
"It is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain (
Santiago Ramón y Cajal Santiago Ramón y Cajal (; 1 May 1852 – 17 October 1934) was a Spanish neuroscientist, pathology, pathologist, and Histology, histologist specializing in neuroanatomy and the central nervous system. He and Camillo Golgi received the Nobel Prize ...
). Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 (
Henry Hallett Dale Sir Henry Hallett Dale (9 June 1875 – 23 July 1968) was an English pharmacologist and physiologist. For his study of acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals ( ...

Henry Hallett Dale
and
Otto Loewi Otto Loewi (; 3 June 1873 – 25 December 1961) was a German-born pharmacologist Pharmacology is a branch of medicine and pharmaceutical sciences concerned with drug or medication action, where a drug may be defined as any artificial, natu ...

Otto Loewi
). We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves, the action potential, in the 1950s (
Alan Lloyd Hodgkin Sir Alan Lloyd Hodgkin (5 February 1914 – 20 December 1998) was an English physiology, physiologist and biophysics, biophysicist who shared the 1963 Nobel Prize in Physiology or Medicine with Andrew Huxley and John Eccles (neurophysiologist), ...

Alan Lloyd Hodgkin
,
Andrew Huxley Sir Andrew Fielding Huxley (22 November 191730 May 2012) was an English physiologist and biophysicist. He was born into the prominent Huxley family. After graduating from Westminster School in Central London, from where he won a scholarship ...
and John Eccles). It was in the 1960s that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible ( David H. Hubel and
Torsten Wiesel Torsten Nils Wiesel (born 3 June 1924) is a Swedish neurophysiologist. Together with David H. Hubel, he received the 1981 Nobel Prize in Physiology or Medicine ) , name = The Nobel Prize in Physiology or Medicine , image = Nobel Pr ...
). The molecular revolution swept across US universities in the 1980s. It was in the 1990s that molecular mechanisms of behavioral phenomena became widely known ()."
A microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into fascicles. The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain,
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
, or peripheral
ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, high ...

ganglia
. All animals more advanced than sponges have nervous systems. However, even
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a
nerve net A nerve net consists of interconnected neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims to develop electron ...
, a diffuse network of isolated cells. In
bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material ...
n animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the
Ediacaran The Ediacaran Period ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of ...
period, over 550 million years ago.


Cells

The nervous system contains two main categories or types of cells:
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
s and
glial cell Glia, also called glial cells or neuroglia, are non-neuronal cell (biology), 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 m ...
s.


Neurons

The nervous system is defined by the presence of a special type of cell—the
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
(sometimes called "neurone" or "nerve cell"). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via
synapse In the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiol ...

synapse
s, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
, a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts; axons typically extend throughout the body in bundles called nerves. Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include
sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called sy ...
s that transmute physical stimuli such as light and sound into neural signals, and
motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * ...
s that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons.


Glial cells

Glial cell Glia, also called glial cells or neuroglia, are non-neuronal cell (biology), 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 m ...
s (named from the Greek for "glue") are non-neuronal cells that provide support and
nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: st ...
, maintain
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
, form
myelin Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons (the nervous system's "wires") to Insulator (electricity), insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the a ...
, and participate in signal transmission in the nervous system. In the
human brain The human brain is the central organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

human brain
, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
s and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets. A very important type of glial cell (
oligodendrocyte Oligodendrocytes (), or oligodendroglia, are a type of neuroglia Glia, also called glial cells (singular ''gliocyte'') or neuroglia, are non-neuronal cell (biology), cells in the central nervous system (brain and spinal cord) and the periphera ...
s in the central nervous system, and
Schwann cell Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann Theodor Schwann (; 7 December 181011 January 1882) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic gr ...
s in the peripheral nervous system) generates layers of a fatty substance called
myelin Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons (the nervous system's "wires") to Insulator (electricity), insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the a ...
that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system.


Anatomy in vertebrates

The nervous system of
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s (including humans) is divided into the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
(CNS) and the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
(PNS). The (CNS) is the major division, and consists of the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
and the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
. The
spinal canal The spinal canal (or vertebral canal or spinal cavity) is the canal that contains the spinal cord within the vertebral column. The spinal canal is formed by the vertebrae through which the spinal cord passes. It is a process of the dorsal body cavi ...
contains the spinal cord, while the
cranial cavity The cranial cavity, also known as intracranial space, is the space within the skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a s ...
contains the brain. The CNS is enclosed and protected by the
meninges In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any i ...

meninges
, a three-layered system of membranes, including a tough, leathery outer layer called the
dura mater Dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. The other two mening ...
. The brain is also protected by the skull, and the spinal cord by the
vertebra In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform c ...
e. The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syste ...
and
visceral An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...

visceral
parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in
dorsal root ganglia A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synap ...
of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the
sympathetic nervous system The sympathetic nervous system (SNS) is one of two divisions of the autonomic nervous system, along with the parasympathetic nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes co ...
and the
parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system th ...
. Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them. The vertebrate nervous system can also be divided into areas called
gray matter Grey matter (or gray matter) is a major component of the central nervous system, consisting of neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse ...
and
white matter White matter refers to areas of the central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anato ...
. Gray matter (which is only gray in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons. White matter is composed mainly of
myelin Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons (the nervous system's "wires") to Insulator (electricity), insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the a ...
ated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Gray matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
, whereas a cluster of neurons in the periphery is called a
ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies li ...

ganglion
. There are, however, a few exceptions to this rule, notably including the part of the forebrain called the
basal ganglia The basal ganglia (or basal nuclei) are a group of subcortical The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue Nervous tissue, also called neural tissue, is the main tissue (biology), tissue compon ...

basal ganglia
.


Comparative anatomy and evolution


Neural precursors in sponges

Sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

Sponge
s have no cells connected to each other by , that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a
postsynaptic density Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in neuromuscular junction, muscles or glands. Chemical synapses allow neurons to form biological neural n ...

postsynaptic density
(the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.


Radiata

Jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

Jellyfish
, , and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete
ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, high ...

ganglia
. The development of the nervous system in
radiata Radiata or Radiates is a historical taxonomic rank that was used to classify animals Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀρ ...
is relatively unstructured. Unlike
bilaterians The Bilateria or bilaterians are animals with symmetry (biology)#Bilateral symmetry, bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior- ...
, radiata only have two primordial cell layers,
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazo ...
and
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.


Bilateria

The vast majority of existing animals are
bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material ...
ns, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Ediacaran period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain". Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain. Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups ( superphyla) called
protostomes Protostomia is the clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineag ...

protostomes
and
deuterostome Deuterostomia (; in Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
s. Deuterostomes include vertebrates as well as
echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of ...
s,
hemichordates Hemichordata is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

hemichordates
(mainly acorn worms), and
Xenoturbellida ''Xenoturbella'' is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthos, benthic worm-like species. The first known species (''Xenoturbella bocki'') was discovered in 1915 by Sixten Bock, b ...
ns. Protostomes, the more diverse group, include
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s,
mollusc Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s, and numerous types of worms. There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline.


Worms

Worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limb Limb can refer to: *Limb (anatomy), an appendage of a human or animal *Limb Music, a record label *Limb (album), an ...

Worm
s are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example,
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to man ...

earthworm
s have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by
transverse Transverse may refer to: *Transverse engine, an engine in which the crankshaft is oriented side-to-side relative to the wheels of the vehicle *Transverse flute, a flute that is held horizontally *Euler force, Transverse force (or ''Euler force''), ...
nerves like the rungs of a ladder. These transverse nerves help
coordinate In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space t ...

coordinate
the two sides of the animal. Two
ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, high ...

ganglia
at the head (the "
nerve ringA circumesophageal or circumpharyngeal nerve ring is an arrangement of nerve ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes Dorsal root ganglion, dorsal root gang ...
") end function similar to a simple
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
. Photoreceptors on the animal's eyespots provide sensory information on light and dark. The nervous system of one very small roundworm, the
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

nematode
''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a Hybrid word, blend of the Greek ''caeno-'' (recent), ''r ...

Caenorhabditis elegans
'', has been completely mapped out in a
connectome A connectome () is a comprehensive map of Biological neural network, neural connections in the brain, and may be thought of as its "wiring diagram". More broadly, a connectome would include the mapping of all neural connections within an organi ...

connectome
including its synapses. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and female hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In ''C. elegans'', males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons.


Arthropods

Arthropods, such as insects and crustaceans, have a nervous system made up of a series of
ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, high ...

ganglia
, connected by a ventral nerve cord made up of two parallel connectives running along the length of the Abdomen, belly. Typically, each body segment has one
ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies li ...

ganglion
on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the supraesophageal ganglion. In the Insect#Nervous system, insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the Arthropod mouthparts, mouthparts, the salivary glands and certain muscles. Many arthropods have well-developed sense, sensory organs, including compound eyes for vision and antenna (biology), antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain. In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil, in the interior.


"Identified" neurons

A neuron is called ''identified'' if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties. In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique. In the roundworm ''Caenorhabditis elegans, C. elegans'', whose nervous system is the most thoroughly described of any animal's, every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. One notable consequence of this fact is that the form of the ''C. elegans'' nervous system is completely specified by the genome, with no experience-dependent plasticity. The brains of many molluscs and insects also contain substantial numbers of identified neurons. In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish. Every fish has two Mauthner cells, in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a Mauthner cell#The C-start behavior, C-shape, then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response individually, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response. Mauthner cells have been described as command neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually. Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances.


Function

At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells. One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second. At a more integrative level, the primary function of the nervous system is to control the body. It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.


Neurons and synapses

Most neurons send signals via their
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s, although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in the form of electrochemical waves called action potentials, which produce cell-to-cell signals at points where axon terminals make synapse, synaptic contact with other cells. Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons, but
chemical synapse Chemical synapses are biological junctions through which neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims t ...
s are much more common, and much more diverse in function. At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with
neurotransmitter A neurotransmitter is a signaling molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, governs the basic activities of cells and coordinates multiple-cell actions. A signal is an entity that ...
chemicals. When the presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft. The neurotransmitter then binds to neurotransmitter receptor, receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter acetylcholine at a synaptic contact between a
motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * ...
and a muscle cell induces rapid contraction of the muscle cell. The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace). There are literally hundreds of different types of synapses. In fact, there are over a hundred known neurotransmitters, and many of them have multiple types of receptors. Many synapses use more than one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as glutamic acid, glutamate or gamma-Aminobutyric acid, GABA, along with one or more peptide neurotransmitters that play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two broad groups: ligand-gated ion channel, chemically gated ion channels and second messenger systems. When a chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription. According to a rule called Dale's principle, which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses. This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on the receptors that it activates. Because different targets can (and frequently do) use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still. Nevertheless, it happens that the two most widely used neurotransmitters, glutamic acid, glutamate and gamma-Aminobutyric acid, GABA, each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking, this is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications. One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. The best-known form of neural memory is a process called long-term potentiation (abbreviated LTP), which operates at synapses that use the neurotransmitter glutamic acid, glutamate acting on a special type of receptor known as the NMDA receptor. The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a channel opens that permits calcium to flow into the target cell. The calcium entry initiates a second messenger cascade that ultimately leads to an increase in the number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time. The reward system, that reinforces desired behaviour for example, depends on a variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine as neurotransmitter. All these forms of synaptic modifiability, taken collectively, give rise to neural plasticity, that is, to a capability for the nervous system to adapt itself to variations in the environment.


Neural circuits and systems

The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. Neural network (biological), Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing, and there are seen to be countless types of information processing possible. Warren Sturgis McCulloch, Warren McCulloch and Walter Pitts showed in 1943 that even artificial neural networks formed from a greatly simplified mathematical abstraction of a neuron are capable of universal computation. Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator. In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Descartes believed that all of the behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically. Charles Scott Sherrington, Charles Sherrington, in his influential 1906 book ''The Integrative Action of the Nervous System'', developed the concept of stimulus-response mechanisms in much more detail, and Behaviorism, the school of thought that dominated Psychology through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms. However, experimental studies of electrophysiology, beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining Membrane potential#Cell excitability, cell excitability and generating patterns of activity intrinsically, without requiring an external stimulus. Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation. When intrinsically active neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more extensive. A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior.


Reflexes and other stimulus-response circuits

The simplest type of neural circuit is a reflex arc, which begins with a sensory system, sensory input and ends with a motor output, passing through a sequence of neurons connected in Series and parallel circuits, series. This can be shown in the "withdrawal reflex" causing a hand to jerk back after a hot stove is touched. The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change the electrical field across the membrane. If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into the brain. One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away. In reality, this straightforward schema is subject to numerous complications. Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex. Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain. For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated. The initial sensory response, in the retina of the eye, and the final motor response, in the oculomotor nuclei of the brain stem, are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cerebral cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei. These areas perform signal-processing functions that include Feature detection (nervous system), feature detection, perception, perceptual analysis, memory recall, decision-making, and motor planning. feature detection (nervous system), Feature detection is the ability to extract biologically relevant information from combinations of sensory signals. In the visual system, for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world. Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming a hierarchy of processing stages. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina.


Intrinsic pattern generation

Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos theory, chaos. A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator. Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is circadian rhythmicity—that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus.


Mirror neurons

A mirror neuron is a neuron that action potential, fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in primate species. Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory cortex and the parietal lobe, inferior parietal cortex. The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception/action coupling (see the common coding theory). They argue that mirror neurons may be important for understanding the actions of other people, and for learning new skills by imitation. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. However, to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation. There are neuroscientists who caution that the claims being made for the role of mirror neurons are not supported by adequate research.


Development

In vertebrates, landmarks of embryonic neural development include the neurogenesis, birth and cellular differentiation, differentiation of
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
s from stem cells, stem cell precursors, the cellular migration, migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s from neurons and axon guidance, guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of
synapse In the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiol ...

synapse
s between these axons and their postsynaptic partners, and finally the lifelong synaptic plasticity, changes in synapses which are thought to underlie learning and memory. All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one end called the animal pole and the other the vegetal pole. The gastrula has the shape of a disk with three layers of cells, an inner layer called the
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazo ...
, which gives rise to the lining of most internal organs, a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer layer called the
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
, which gives rise to the skin and nervous system. In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
(CNS), the outer portion the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
(PNS). As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest, running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation. In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of nervous tissue is "induced" by signals from a group of mesodermal cells called the ''organizer region''. For decades, though, the nature of neural induction defeated every attempt to figure it out, until finally it was resolved by genetic approaches in the 1990s. Induction of neural tissue requires inhibition of the gene for a so-called bone morphogenetic protein, or BMP. Specifically the protein BMP4 appears to be involved. Two proteins called Noggin (protein), Noggin and Chordin, both secreted by the mesoderm, are capable of inhibiting BMP4 and thereby inducing ectoderm to turn into neural tissue. It appears that a similar molecular mechanism is involved for widely disparate types of animals, including arthropods as well as vertebrates. In some animals, however, another type of molecule called Fibroblast Growth Factor or FGF may also play an important role in induction. Induction of neural tissues causes formation of neural precursor cells, called neuroblasts. In drosophila, neuroblasts divide asymmetrically, so that one product is a "ganglion mother cell" (GMC), and the other is a neuroblast. A GMC divides once, to give rise to either a pair of neurons or a pair of glial cells. In all, a neuroblast is capable of generating an indefinite number of neurons or glia. As shown in a 2008 study, one factor common to all symmetry (biology), bilateral organisms (including humans) is a family of secreted Signalling molecule, signaling molecules called neurotrophins which regulate the growth and survival of
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
s. Zhu et al. identified DNT1, the first neurotrophin found in fly, flies. DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in Drosophila. Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation.


Pathology

The central nervous system is protected by major physical and chemical barriers. Physically, the brain and spinal cord are surrounded by tough meninges, meningeal membranes, and enclosed in the bones of the skull and vertebral column, which combine to form a strong physical shield. Chemically, the brain and spinal cord are isolated by the blood–brain barrier, which prevents most types of chemicals from moving from the bloodstream into the interior of the CNS. These protections make the CNS less susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to have more serious consequences. Although nerves tend to lie deep under the skin except in a few places such as the ulnar nerve near the elbow joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Damage to nerves can also be caused by swelling or bruises at places where a nerve passes through a tight bony channel, as happens in carpal tunnel syndrome. If a nerve is completely transected, it will often nerve regeneration, regenerate, but for long nerves this process may take months to complete. In addition to physical damage, peripheral neuropathy may be caused by many other medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory conditions such as Guillain–Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or shingles, or poisoning by toxins such as heavy metals. Many cases have no cause that can be identified, and are referred to as idiopathic. It is also possible for nerves to lose function temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop in temperature, or chemical interactions with local anesthetic drugs such as lidocaine. Physical damage to the spinal cord may result in paralysis, loss of sensation or movement. If an injury to the spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the spine are actually destroyed, the loss of function is usually permanent. Experimental studies have shown that spinal nerve fibers attempt to regrow in the same way as nerve fibers, but in the spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves.


See also

*Circulatory system *Digestive system *Muscular system *Sentience


References


Further reading


Nervous system
William E. Skaggs, Scholarpedia


External links

* (human) * (non-human)
The Human Brain Project Homepage
{{DEFAULTSORT:Nervous System Nervous system,