Afferent nerve fiber

Afferent nerve fiber
Afferent (PSF).png
Details
SystemNervous system
Identifiers
Latinneurofibrae afferentes
TAA14.2.00.017
THH2.00.06.1.00015
FMA76570
Anatomical terms of neuroanatomy

Afferent nerve fibers refer to axonal projections that arrive at a particular 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).

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.[1][2][3]

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.

Structure

Nervous System Organization - The Motor and Sensory Systems

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.[4] 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.[4] All of these sensations travel along the same general pathway towards the brain. From the dorsal root ganglion they travel to the spinal cord.[4] 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.

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).

Type Primary/secondary Response
Type Ia primary Respond to the rate of change in muscle length, as well to change in velocity, rapidly adapting
Type Ib N/A In Golgi tendon organ, responds to muscle tension changes
Type II secondary Provide position sense of a still muscle, fire when muscle is static [5]
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