Review of Medical Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Excitable Tissue: Nerve
Types of Nerve Fibers and Their Function

Erlanger and Gasser classified mammalian nerve fibers into groups A, B, and C, further subdividing group A into a-, ß-, y-, and δ-fibers. By comparing neurological deficits following careful experimental transection of the dorsal ROOT and other neural structures with subsequent histological Changes in the nerve fibers, researchers successfully determined the Functions and histological characteristics of each fiber group, stimulation of which generates distinct peaks in the compound Action Potential. It was found that larger axons are primarily associated with Proprioception, somatic motor function, and conscious Touch and pressure, whereas smaller ones transmit pain and Temperature sensations, as well as impulses mediating autonomic functions. Table 2-1 lists the various fiber types along with their diameters, electrical characteristics, and functions. Dorsal root C-fibers transmit impulses generated by nociceptors and thermoreceptors. Further research revealed that not all components encompassed by this Classification are homogeneous; consequently, some physiologists use an alternative numerical system (Ia, Ib, II, III, IV) to classify sensory fibers, although it also has limitations. Table 2-2 provides a comparison of the letter and numerical classification systems.

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Fig. 2-13. Compound action potential. Shown are recording traces obtained via recording electrodes placed on a mixed nerve at various distances from the stimulating electrodes.

In addition to differences in conduction velocity and diameter, various peripheral nerve fiber types exhibit distinct sensitivities to Hypoxia and local anesthetics (Table 2-3). This is of both physiological and clinical significance. Local anesthetics block conduction in group C fibers more rapidly than in group A touch fibers. Conversely, nerve compression can cause conduction loss in large-diameter motor, touch, and pressure fibers, while pain perception remains largely unaffected. The characteristics of this fiber type vulnerability can be observed in nerve compression occurring in the shoulder area during deep, prolonged Sleep when individuals sleep with their arm under their HEAD. Because such prolonged sleep in this position frequently occurs following alcohol intoxication during weekends, the resulting injury has been termed "Saturday night palsy" or "Sunday morning paralysis".