Fractionation of motor units by curare

Abstract

The effect of curare upon the population of muscle fibers innervated by a single motor neuron has been investigated. Observations were made on motor units driven by stimulation of ventral root filaments and on units discharging reflexly in response to pinch of the ipsilateral hind paw. Recordings were made from the surface of the gastrocnemius and gracilis muscles of the rat with pairs of closely spaced electrodes. Curare was found to reduce reversibly the amplitude of responses from motor units in either of two patterns: (1) the amplitude remained at control levels until it abruptly dropped to zero or a very low value; (2) the amplitude declined gradually, with one or more abrupt downward “steps” and a large terminal “step.” Analysis of these effects with a fast sweep showed that decline in the amplitude of response was accompanied by an alteration in its form. Units which declined gradually in amplitude with “steps” were found to be made up of two or more components, each of which responded in all-or-none fashion. As the effect of curare developed, these components became distinguishable as separate entities because of unequal delays at their neuromuscular junctions. The originally synchronized responses, that is, began to fall more and more out of phase, which produced a gradual decline in amplitude. “Steps” occurred whenever a component dropped out entirely. An electronic analog of this desynchronization and fractionation of motor units was devised to help explain the results.