Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from

sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The ...

s to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers are carried by efferent nerves and ''exit'' a region to act on muscles and glands. In the

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brai ...

afferent and efferent nerve fibers are part of the somatic nervous system and arise from outside of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...

. Sensory nerves carry the afferent fibers to enter into the spinal cord, and motor nerves carry the efferent fibers out of the spinal cord to act on

skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...

s. In the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

non-motor efferents are carried in efferent nerves to act on

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

Structure

Afferent neurons are pseudounipolar neurons that have a single process leaving the cell body dividing into two branches: the long one towards the sensory organ, and the short one toward the central nervous system (e.g. spinal cord). These cells do have sensory afferent

dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the ...

s, similar to those typically inherent in

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...

s. They have a smooth and rounded cell body located in the ganglia of the peripheral nervous system. Just outside the

, thousands of afferent neuronal cell bodies are aggregated in a swelling in the dorsal root known as the dorsal root ganglion. All of the axons in the dorsal root, 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.Casagrand, Janet (2008) ''Action and Movement: Spinal Control o ...

is a type of afferent neuron that lies parallel to the extrafusal muscle fibers 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 ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsa ...

begins with sensation from the periphery being sent via afferent nerve fiber of the dorsal root ganglion (first order neuron) through the spinal cord to the dorsal column nuclei (second order neuron) in the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

. The second order neuron's projection decussates at the medulla through medial lemniscus to the third order neurons in the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...

. The third order neuron's axon terminates at the

primary somatosensory cortex In neuroanatomy, the primary somatosensory cortex is located in the postcentral gyrus of the brain's parietal lobe, and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and pa ...

of the 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 A Special visceral afferent fibers (SVA) is a afferent fiber that develop in association with the gastrointestinal tract. They carry the special senses of smell (olfaction) and taste (gustation). The cranial nerves containing SVA fibers are the olfa ...

(SVA). Alternatively, in the sensory system, 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, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

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 that connect two brain regions, i.e. not direct motor neurons or sensory neurons. ...

s, and

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirect ...

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 defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...

ful stimulus, 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 a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

participle ''afferentem'' (''af- = ad-'' : to + ''ferre'' : bear, carry), meaning carrying into. ''Ad'' and ''ex'' give an easy mnemonic device 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.

References

Other References