Afferent nerve fiber
|Afferent nerve fibre|
|Anatomical terms of neuroanatomy|
In the peripheral nervous system, an afferent nerve fiber is the nerve fiber (axon) of an afferent neuron (sensory neuron). It is a long process (projection) extending far from the nerve cell body that carries nerve impulses from sensory receptors or sense organs toward the central nervous system. The opposite direction of neural activity is efferent conduction.
In the nervous system there is a "closed loop" system of sensation, decision, and reactions. This process is carried out through the activity of afferent neurons (sensory neurons), interneurons, and efferent neurons (motor neurons).
A touch or painful stimulus, for example, creates a sensation in the brain only after information about the stimulus travels there via afferent nerve pathways. Afferent neurons are pseudounipolar neurons that have a single long axon with a short central and a long peripheral branch. These cells do not have dendrites. They have a smooth and rounded cell body. Just outside the spinal cord, thousands of afferent neuronal cell bodies are aggregated in a swelling in the dorsal root known as the dorsal root ganglion.
Afferent neurons' somas are located in the ganglia of the peripheral nervous system, and the axons of these cells travel from ganglion to ganglion and lead back to the spinal cord. The majority of these are unipolar neurons in that they have a single axon leaving the cell body and is sent towards the sensory organ. 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 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 pathway towards the brain, from the dorsal root ganglion they travel to the spinal cord. From the spinal cord to the medulla, which then leads to the medial lemniscus of the midbrain. From here it travels to the primary somatosensory cortex of the parietal lobe.
Etymology and mnemonics
Afferent is derived from Latin 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.
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