Afferent nerve fiber
Afferent nerve fibers refer to axonal 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 (PNS) and central nervous system (CNS).
|Afferent nerve fiber|
|Anatomical terms of neuroanatomy|
In the PNS, afferent and efferent projections are always from the perspective of the spinal cord (see figures). PNS afferents are the axons of sensory neurons carrying sensory information from all over the body, into the spine. PNS efferents are the axons of spinal cord motor neurons that carry motor-movement signals out of the spine to the muscles.
In the CNS, afferent and efferent projections can be 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.
Afferent neurons are pseudounipolar neurons that have a single axon 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 not have dendrites. They have a smooth and rounded cell body located in the ganglia of the peripheral nervous system. 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.
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 pathways towards the brain. One pathway—dorsal column-medial lemniscus pathway—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 second order neuron's projection decussates at the medulla through medial lemniscus to the third order neurons in the thalamus. The third order neuron's axon terminates at the primary somatosensory cortex of the parietal lobe.
- For further information on the different pathways, see Somatosensory system#General somatosensory pathway, Mechanoreceptor#Mechanism of sensation, Proprioception#Conscious and non-conscious, Nociceptor#Pathway, Thermoreceptor#Location, Dorsal column-medial lemniscus pathway, Anterior trigeminothalamic tract, Spinothalamic tract, Solitary tract, Dorsal spinocerebellar tract, and Cuneocerebellar tract
Alternatively, in the sensory system, afferent fibers can be classified by sizes with category specifications depending on if they innervate the skins or muscles.
|Myelination||Diameter (µm)||Speed (m/s)||From muscles||From skin||Receptors|
|Thick||12-20||72-120||I||Aα||Proprioceptors (Muscle spindle, Golgi tendon organ)|
|Medium||6-12||35-75||II||Aβ||Merkel nerve ending, Tactile corpuscle, Lamellar corpuscle, Bulbous corpuscle|
|Thin||1-6||4-36||III||Aδ||Free nerve ending|
|None||0.2-1.5||0.4-2.0||IV||C||Free nerve ending|
In the nervous system there is a "closed loop" system of sensation, decision, and reactions. This process is carried out through the activity of sensory neurons, interneurons, and 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.
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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