Chapter 6 - Pathophysiology of impaired neural transmission

Summary

The symptoms of MS are determined not only by the localization of plaques, but also by changes in signal transmission. Electrophysiological methods are therefore established in the diagnosis of the disease.

ELECTROPHYSIOLOGY OF NORMAL NERVE FIBERS

A neuron is composed typically of a cell body with thin, branched processes, the dendrites, and an axon of variable length. Dendrites receive impulses from the axons of numerous other neurons by synaptic contact. A complex network is formed of neurons communicating with each other. Sensible unipolar neurons are an exception to this rule insofar as they do not have any dendrites. Their participation in the disease process of MS is by means of a long afferent axon in the peripheral nerve, and a long efferent axon in the dorsal column of the spinal cord. Central nerve fibers also consist of axons, as they do in the peripheral nerve, and may reach a length of up to 1 m in the CNS. The velocity of impulse conduction in nerve fibers or axons depends primarily on whether or not the fiber is myelinated. Unmyelinated fibers conduct very slowly, at about 1 m/s, whereas impulse conduction velocity in myelinated fibers depends on their total thickness. For a fiber thickness of 10–20 μm, the conduction velocity is about 50–120 m/s; for a fiber thickness of 5 μm, it is about 25 m/s. A myelinated nerve fiber consists of an axon cylinder and the myelin sheath.