| Afferent nerve fiber | |
|---|---|
 Afferent nerve fibers transmit information from the peripheral to the central nervous system. | |
| Details | |
| System | Nervous system |
| Identifiers | |
| Latin | neurofibrae afferentes |
| TA98 | A14.2.00.017 |
| TH | H2.00.06.1.00015 |
| FMA | 76570 |
| Anatomical terms of neuroanatomy | |
Afferent nerve fibers areaxons (nerve fibers) ofsensory neurons that carrysensory information fromsensory receptors to thecentral nervous system. Many afferent projectionsarrive at a particular brain region.
In theperipheral nervous system, afferent nerve fibers are part of the sensory nervous system and arise from outside of the central nervous system. Sensory and mixed nerves contain afferent fibers.
Afferent neurons arepseudounipolar neurons that have a single process leaving the cell body dividing into two branches: the long one towards thesensory organ, and the short one toward the central nervous system (e.g. spinal cord).These cells do have sensory afferentdendrites, similar to those typically inherent inneurons.[1]They have a smooth and rounded cellbody located in theganglia of the peripheral nervous system.Just outside thespinal cord, thousands of afferent neuronal cell bodies are aggregated in a swelling in thedorsal root known as thedorsal root ganglion.[1][2]
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 anintrafusal muscle fiber is a type of afferent neuron that lies parallel to theextrafusal muscle fibers thus functions as a stretch receptor by detecting muscle length.[2]
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 thedorsal column nuclei (second order neuron) in thebrainstem. The second order neuron's projectiondecussates at themedulla throughmedial lemniscus to the third order neurons in thethalamus. The third order neuron's axon terminates at theprimary somatosensory cortex of theparietal lobe.[3]
Types of afferent fibers include thegeneral somatic, thegeneral visceral, thespecial somatic and thespecial visceral afferent fibers.
Alternatively, in thesensory system, afferent fibers can be classified by sizes with category specifications depending on if they innervate the skins or muscles.[4][5]
| Myelination | Diameter (μm) | Speed (m/s) | Frommuscles | Fromskin | 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 thenervous system, there is a "closed loop" system of sensation, decision, and reactions. This process is carried out through the activity of sensory neurons,interneurons, andmotor neurons. A touch orpainful stimulus, for example, creates a sensation in the brain only after information about thestimulus travels there via afferent nerve pathways.
Afferent is derived fromLatin participleafferentem (af- =ad- : to +ferre : bear, carry), meaningcarrying into, whereas efferent is derived fromex ferens, meaningcarrying away (e- =ex- means 'from').Ad andex give anmnemonic device for remembering the relationship betweenafferent andefferent :afferent connectionarrives and anefferent connectionexits.[6]
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.
