Fibers in Mixed Nerves and Their Dorsal Roots Responsible for Pain

Abstract

It has been shown that fibers which differ in anatomical type, for instance myelinated and unmyelinated fibers, when stimulated, give electrical potentials which differ characteristically (Bishop and Heinbecker 1 ). The potential picture recorded from a mixed nerve by means of the oscillograph thus serves as an index of the nerve's fiber content. When these potentials are recorded from a nerve still in the body, the correlation between potential record and the response given by the animal as a whole serves to identify the fibers responsible for accomplishing a specific type of activity. The potential record from a nerve, like the electrocardiogram, thus becomes an index of physiological function. We here present data to identify a group of nerve fibers which convey impulses resulting in painful sensations.

Autonomic fibers in mixed nerves can be recognized by the fact that they are slow-acting, and myelinated autonomics can be differentiated from unmyelinated by thresholds and conduction rates. Of somatic fibers, 2 groups can be distinguished, both myelinated, one of which conducts more rapidly than the other. The slower includes the so-called “visceral afferents”. There are thus 4 groups recognizable, general somatic, visceral afferent, myelinated autonomic and unmyelinated autonomic. Pain is mediated by fibers of the second group, of which the visceral afferent is the type. Whether pain is also conveyed by unmyelinated fibers in mixed nerves we have been unable to determine so far, since stimulation of the myelinated fibers causes so much pain that an increase of sensation which might result from stimulation of unmyelinated fibers of higher threshold cannot be detected.

A first experiment was conducted in a conscious human. A branch of the saphenous nerve was dissected out after novocaine anesthesia to the skin only.