Peripheral Nerves, Sensory Receptors, and Spinal Roots
Peripheral nerves are composed of the axons of sensory and motor neurons, along with the investing connective tissue sheaths (endoneurium, perineurium, and epineurium) (see Landon, 1976). The axons may be myelinated or unmyelinated. In cutaneous nerves, the largest axons belong to the Aαβ class (Erlanger and Gasser, 1937), while the small myelinated fibers belong to the Aδ group. Aαβ fibers conduct at 30–100 m/s, and Aδ fibers at 4–30 misec (Boivie and Perl, 1975). Unmyelinated afferents are often designated C fibers; they conduct at less than 2.5 m/s (Gasser, 1950). Joint and visceral nerves share the terminology of cutaneous nerve axons. However, a different terminology is used for muscle nerves. The myelinated fibers are subdivided into groups I, II, and III (Lloyd and Chang, 1948), conducting at 72–120, 24–71, and 6– 23 m/s, respectively (Hunt, 1954). Muscle nerves also contain numerous unmyelinated, or group IV, afferents (Stacey, 1969), conducting at less than 2.5 m/s.
KeywordsDorsal Root Ganglion Dorsal Root Myelinated Fiber Sensory Receptor Ventral Root
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