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 and central nervous system.
In the PNS, afferent and efferent projections are always from the perspective of the spinal cord. 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 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 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.These cells do not have dendrites that are typically inherent in neurons.
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 pathwaydorsal column-medial lemniscus pathwaybegins with sensation from the periphery being sent via afferent nerve fiber of the dorsal root ganglion through the spinal cord to the dorsal column nuclei 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.
Types
Types of afferent fibers include the general somatic, the general visceral, the special somatic and the special visceral afferent fibers.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 | Speed | From muscles | From skin | Receptors |
| Thick | 12-20 | 72-120 | I | Aα | Proprioceptors |
| 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 |
| - | 0.2-1.5 | 0.4-2.0 | IV | C | Free nerve ending |
Function
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, 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 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.