The Differential Action of Pressure on Fibers of Different Sizes in a Mixed Nerve.

Abstract

The effect of pressure on a mixed nerve was studied to determine whether it would shed any light upon the nature of the complex constitution of nerve trunks. Nerve action potentials recorded at a distance from a stimulating electrode have been shown to have a form dependent on the structure of the nerve as revealed histologically. The velocity of conduction in the fibers varies as the diameter; therefore in the action potential wave, recorded as above, the first of the action wave is made up by potentials in large fibers, and the end by potentials in small fibers.

A priori mechanical considerations lead one to suppose that an applied external pressure would block the large fibers before the small ones. This was indeed found to be the case. The nerve was blocked by a method similar to that of Meek and Leaper 1 who first applied fluid pressure through a cuff. A brass T-tube was made 12 mm. long with a bore of 2.5 mm. Through this was threaded a rubber tube with a fairly thin wall, whose outside diameter fitted the bore of the tube. The ends of the rubber tube were everted over the ends of the brass tube and tied, then covered with a brass cap to prevent longitudinal extension of the cuff. The nerve was threaded through a perforation in the cap. Pressure was then conveniently applied from an oxygen cylinder, being accurately controlled by a reducing valve. It was applied between the stimulus and the leads to the recording instrument, a Braun tube oscillograph; therefore, what is obtained is a record of normal waves, the form of the wave being altered either through block in some of the fibers, or a delay in traversing the compressing chamber.