Properties of Unmyelinated Fibers of Nerve. ∗

Abstract

Previous reports on the physiology of nerve by Erlanger, Gasser and Bishop 1 have dealt with the properties of the myelinated fibers. In this paper are presented data on the properties of fibers characteristic of the involuntary nervous system especially the unmyelinated ones. The analyses have been made by means of the cathode ray oscillograph and the use of amplification greater than was usually considered necessary for the study of the myelinated fibers.

The unmyelinated fibers can be identified by a conduction rate slower than that of the ordinary myelinated fibers of peripheral nerves (1.5 to 0.2 M. per sec. in the turtle). These rates are comparable to the rates reported by Chauchard, A. et B., 2 as determined from the effect of stimulation at different levels along the course of nerves containing unmyelinated fibers. A second characteristic of these fibers is their high threshold. An induction shock at least 10 to 100 times that of the most irritable fibers in the same nerve is required for a threshold response in the turtle, in the cat 50 to 100 times.

A more suitable method of stimulating these fibers is by the galvanic current. The nerve is arranged in a bridge balanced for resistance and capacity as described by Bishop. 3 Relative thresholds can then be measured either by change in duration or by change in intensity of stimulating current. On the basis of either threshold or conduction rate the fibers, if myelinated, would have to be not more than 0.1μ in diameter in order to fit the size conduction rate ratio of Gasser and Erlanger 4 for the myelinated fibers. Myelinated fibers of this diameter are not found in the nerves under consideration.

Using the galvanic current in the manner indicated above, the chronaxie of these fibers can be measured.