Motoneuron membrane changes associated with spinal shock and the Schiff-Sherrington phenomenon

Abstract

Following transection of the thoracic spinal cord, in a decerebrate cat, the forelimbs exhibit an enhanced extensor stretch reflex while the ipsilateral flexion reflex is more difficult to elicit (Schiff-Sherrington Phenomenon). The hindlimbs on the other hand have an increased threshold for the extensor stretch reflex while flexion reflexes are hyperactive (spinal shock). This investigation was designed to examine the synaptic events mediating the Schiff-Sherrington Phenomenon (SSP) and spinal shock; and to characterize any differences in the response of flexor and extensors alpha-motoneurons. This was accomplished by measuring membrane potential and input resistance of identified forelimb and hindlimb alpha-motoneurons before, during, and after coldblock of the low thoracic cord. During post-brachial spinal cord coldblock, forelimb extensor motoneurons depolarized while flexor motoneurons hyperpolarized. Both flexor and extensor motoneurons in the lumbar cord hyperpolarized. These observations account at least in part for the behavioral manifestation of increased extensor tone seen during the SSP and for the reflex depression seen during spinal shock. The membrane potential changes observed in this study were mediated through a direct effect on alpha-motoneurons since these animals were flaxedilized and gamma loop activity was probably negligible. The hyperpolarization of hindlimb motoneurons was apparently due to the removal of descending facilitation, while the depolarization seen in forelimb extensor motoneurons appeared to be due to a release of facilitation. The mechanism of the hyperpolarization observed in some forelimb motoneurons was unclear.