Motor cortex-pyramidal effects on single ankle flexor and extensor motoneurons of the cat

Abstract

Descending volleys were initiated by stimulation of the pericruciate cortex in the “pyramidal” cat preparation. Single motoneuron units were isolated by subdissection of selected ventral roots of the lumbosacral spinal segments. The identity of the isolated motoneuron unit was ascertained by conventional methods, utilizing stimulation of isolated peripheral nerves of the sciatic distribution. Only neurons identified as belonging to the populations innervating the medial and lateral heads of the gastrocnemius muscle, the soleus muscle and the ankle flexor muscles were studied. The results obtained confirmed and extended our previous observations which demonstrated that flexor motoneurons are predominantly facilitated by motor cortex volleys and extensor motoneurons are inhibited by the same volleys. In addition, it was possible to demonstrate cortical facilitation of some ankle extensor motoneurons. Such facilitation could not be demonstrated in earlier studies in which the monosynaptic reflex of the entire motoneuron population was employed for testing cortical effects. The temporal patterns of cortical effects on single motoneurons of a homonymous population differed for different units in the same preparation. Since systematic variation of cortical stimulation parameters and of site of stimulation of the motor cortex did not change these differences in cortically evoked unit patterns within the same population, it is assumed that these differences in cortically evoked patterns may be of functional significance.