Effects of stretch on single myelinated nerve fibres

Impulses from those sensory receptors which respond to mechanical stimulation are probably initiated by some distortion ofthe terminations of the sensory axons (Katz, 1950; Gray & Sato, 1953). Furthermore, it appears possible that in some receptors, e.g. muscle spindles and Pacinian corpuscles (Adrian & Umrath, 1929), this distortion takes the form of stretching the axon. One explanation of the mechanism of such sensory receptors could be that the axon membrane of the sensory terminals is the same as that at other points on the axon, but that at the nerve ending the membrane is held under a suitable initial tension so that any movement stretches the nerve membrane. The nerve trunk, on the other hand, is well protected against stretch because the fibres zig-zag inside a strong connective tissue sheath; Schneider (1952) estimates that the length of a nerve trunk must be increased by more than 50% before the nerve fibres begin to stretch. If the axon membrane at sensory terminals and at other points on the axon are the same, stretching a node of Ranvier held under a suitable initial tension might have a result similar to stretching a sensory receptor. In the experiments described in the first section of the Results, single nodes of Ranvier held under varying degrees of tension were subjected to rapid stretching. These experiments were, for technical reasons, done on motor fibres; however, as the results were entirely negative, a few experiments on sensory fibres were attempted, but it was unfortunately not possible to get any results before the work had to be stopped. During the course of these experiments changes in the conducted impulse were noticed. These changes were associated with the length to which an axon had been stretched and some experiments designed to investigate these changes are reported.