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Afferent nerve fibers refer to
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 ...
al projections that ''arrive'' at a particular brain region, as opposed to efferent projections that ''exit'' the region. These terms have a slightly different meaning in the context of 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) and
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). In the PNS, afferent and efferent projections are always from the perspective of the spinal cord (see figures). PNS afferents are 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
sensory neurons Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of Stimulus (physiology), stimulus, via their Receptor (biochemistry), receptors, into action potentials or graded potentials. This ...
carrying sensory information from all over the body, ''into'' the spine. PNS efferents are 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 spinal cord motor neurons that carry motor-movement signals ''out of'' the spine to the
muscle Skeletal muscles (commonly referred to as muscles) are organs 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 cat ...

muscle
s. In the CNS, afferent and efferent projections are determined from the perspective of any given brain region. That is, each brain region has its own unique set of afferent and efferent projections. In the context of a given brain region, afferents are arriving fibers while efferents are exiting fibers.


Structure

Afferent neurons are
pseudounipolar neuron A pseudounipolar neuron is a type of neuron which has one extension from its cell body. This type of neuron contains an axon that has split into two branches; one branch travels to the peripheral nervous system and the other to the central nervous ...
s that have a single axon leaving the cell body dividing into two branches: the long one towards the
sensory organ A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to Stimulus (physiology), stimuli. (For example, in the human body, the brain receives signals from the senses ...
, and the short one toward the central nervous system (e.g. spinal cord). These cells do not have
dendrite Dendrites (from 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 approx ...

dendrite
s that are typically inherent in
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. They have a smooth and rounded cell
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 animals ...
located in the
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
of the peripheral nervous system. Just outside 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
, thousands of afferent neuronal cell bodies are aggregated in a swelling in the
dorsal root The dorsal root of spinal nerve (or posterior root of spinal nerve or sensory root) is one of two "roots" which emerge from the spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also call ...
known as the
dorsal root ganglion 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 membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with o ...
. All of the axons in the
dorsal root The dorsal root of spinal nerve (or posterior root of spinal nerve or sensory root) is one of two "roots" which emerge from the spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also call ...
, which contains afferent nerve fibers, are used in the transduction of somatosensory information. Somatosensory receptors include senses such as pain, touch, temperature, itch, and stretch. For example, a specific muscle fiber called an
intrafusal muscle fiber Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs ( proprioceptors). They detect the amount and rate of change in length of a muscle Muscle is a soft tissue found in most animals. Muscle cel ...
is a type of afferent neuron that lies parallel to the
extrafusal muscle fibers Extrafusal muscle fibers are the skeletal muscle, skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal stri ...
thus functions as a stretch receptor by detecting muscle length. All of these sensations travel along the same general pathways towards the brain. One pathway
dorsal column-medial lemniscus pathway Dorsal (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Repu ...
begins with sensation from the periphery being sent via afferent nerve fiber of the
dorsal root ganglion 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 membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with o ...
(first order neuron) through the spinal cord to the
dorsal column nuclei In neuroanatomy Neuroanatomy is the study of the structure and organization 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 Sens ...
(second order neuron) in the
brainstem The brainstem (or brain stem) is the posterior stalk-like part 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 anim ...

brainstem
. The second order neuron's projection
decussates Decussation is used in biological contexts to describe a crossing (in Latin, the roman numeral for 10, ten, ''deca'', is an uppercase 'X'). In Latin anatomical terms, the form ''decussatio'' is used, e.g. ''Decussation of pyramids, decussatio pyr ...
at the
medulla Medulla or Medullary may refer to: Science * Medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is ...

medulla
through
medial lemniscus The medial lemniscus, also known as Reil's band or Reil's ribbon, is a large ascending bundle of heavily myelinated axons that decussate in the brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects ...
to the third order neurons in the
thalamus The thalamus (from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter located in the wikt:dorsal, dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the tha ...

thalamus
. The third order neuron's axon terminates at the
primary somatosensory cortex The primary somatosensory cortex is located in the postcentral gyrus The postcentral gyrus is a prominent gyrus In neuroanatomy, a gyrus (pl. ''gyri'') is a ridge on the cerebral cortex. It is generally surrounded by one or more sulci (de ...
of the
parietal lobe The parietal lobe is one of the four Lobes of the brain, 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 integrat ...

parietal lobe
.


Types

Types of afferent fibers include the general somatic (GSA), the general visceral (GVA), the special somatic (SSA) and the special visceral afferent fibers (SVA). Alternatively, in the
sensory system The sensory nervous system is a 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, Molecular bio ...
, afferent fibers can be classified by sizes with category specifications depending on if they innervate the skins or muscles.


Function

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, Physiology, physiological mecha ...

nervous system
there is a "closed loop" system of sensation, decision, and reactions. This process is carried out through the activity of sensory neurons,
interneuron Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons A neuron or nerve cell is an electrically excitable cell that communicates wit ...
s, 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. A touch or
pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain The International Association for the Study of Pain (IASP) is an international learned society A learned societ ...
ful
stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity generally. For example, "The press provides stimulation of political discourse." ...
, for example, creates a sensation in the brain only after information about the stimulus travels there via afferent nerve pathways.


Etymology and mnemonics

Afferent is derived from
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
participle ''afferentem'' (''af- = ad-'' : to + ''ferre'' : bear, carry), meaning carrying into. ''Ad'' and ''ex'' give an easy
mnemonic device A mnemonic (, the first "m" is not pronounced) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the brain by which data or informat ...
for remembering the relationship between ''afferent'' and ''efferent'' : ''afferent connection arrives'' and an ''efferent connection exits''. Another mnemonic device used for remembering afferent and efferent (in terms of the spinal cord, with its dorsal/ventral organization) is SAME DAVE. Sensory Afferent Motor Efferent, Dorsal Afferent Ventral Efferent. Afferent and efferent are connected to affect and effect through their common Latin roots: Afferent nerves affect the subject, whereas efferent nerves allow the subject to effect change.


See also

*
Autonomic nervous system The autonomic nervous system (ANS), formerly the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of viscera, internal organs. The autonomic nervous ...

Autonomic nervous system


References


Other References

* {{DEFAULTSORT:Afferent Nerve Fiber Neurophysiology Sensory systems Medical mnemonics