Conduction Velocity

In a myelinated nerve fiber, successive 1- to 2-mm segments of axon, called internodes, are enveloped by multiple layers of Schwann cell membrane. Between these segments are short lengths of axon with little or no covering, called nodes of Ranvier. According to the saltatory conduction theory, myelin increases the transverse resistance of the internodes, while the resistance at the nodes remains normal. As a result, when the axonal membrane at a node becomes active (part A), the passive outward currents produced by this activity are prevented from flowing through the membrane of the adjacent internode; instead, they flow through the membrane of the next node.

The resulting depolarization triggers an action potential at this node. Thus, unlike impulse propagation in an unmyelinated axon (part B), which proceeds continuously in very small steps, the impulse in a myelinated axon jumps from node to node and results in a much greater conduction velocity.

As shown in C, mammalian peripheral nerves contain myelinated fibers with diameters of 0.5 to 20 µm and conduction velocities of 3 to 120 m/sec, and un-myelinated fibers with diameters of less than 2µm and conduction velocities of 0.5 to 2.0 m/sec. In 1930, Erlanger and Gasser published a classification of peripheral nerve fibers, based on conduction velocity. Three groups of fibers were defined according to descending conduction velocity, designated A (with subgroups α, β and γ), B, and C. A further subgroup, Aδ, was added later. This classification refers to both afferent (sensory) and efferent (motor) fibers, whereas a more recent classification of nerve fibers into groups I, II, III, and IV refers only to afferent fibers.

The properties and functions of the different classes of nerve fibers are summarized in part C. In the somatic efferent system, fibers supplying skeletal muscle fibers (alpha motor axons) have conduction velocities ranging from 50 to 100 m/sec (Aα and Aβ ranges), and fibers supplying the intrafusal muscle fibers of muscle spindles (gamma motor axons) have conduction velocities ranging from 10 to 40 m/sec (Aγ and Aδ ranges). Autonomic efferent fibers fall either into group B (preganglionic fibers) or group C (postganglionic fibers). In the afferent system, the larger myelinated fibers carry information from specialized receptors that respond to only one type of stimulus, whereas many smaller myelinated fibers carry information about noxious stimuli that give rise to the sensation of prickling pain. The function of unmyelinated sensory fibers (group IV, or C, fibers) is not entirely clear. Stimulation of these fibers as a group evokes only the sensation of burning pain, but experiments have shown that many of these fibers carry information about a specific type of stimulus (touch, pressure, temperature), and only a restricted group is specifically sensitive to noxious stimuli.

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