Modulation of premotoneuron interneurons in normal and hemiplegic subjects

Abstract

Ischemia of the cerebral cortex causes damage to human brain resulting in spastic hemiplegia. The aim of this study was to investigate transmission in premotoneuron interneurons projecting to forearm motoneurons in normal and hemiplegic subjects performing different types of contractions. Three hundred and twenty-three experiments were performed on five hemiplegic subjects and eleven normal subjects. Measurement of change in the amount of facilitation evoked in flexor carpi radialis H-reflexes by ulnar conditioning provided a measure of transmission through premotoneuron interneurons. Premotoneuron facilitation of motoneurons was greater in hemiplegic compared to normal subjects at rest, 150 ms prior to onset and at the onset of weak, ballistic concentric and isometric contractions of flexor carpi ulnaris. Premotoneuron facilitation of motoneurons increased with muscle activity compared to rest and this was more pronounced in hemiplegic subjects. These results suggested a decrease in threshold and an increase in gain in premotoneuron pathways in hemiplegic subjects. The absence of separate peaks of premotoneuron facilitation and the longer central latency in hemiplegic subjects indicated abnormal spatial and temporal summation of cortical and peripheral inputs at premotoneurons. Premotoneurons demonstrated modulation in normal but not in hemiplegic subjects 150 ms prior to and at the onset of maximal fatiguing concentric and isometric contractions of flexor carpi ulnaris. Experiments on spatial facilitation using cortical and ulnar conditioning suggested an increase in excitability of corticospinal pathways projecting to premotoneurons during fatigue. This was associated with a decrease in spatial facilitation of cortical and peripheral inputs at premotoneurons. After fatigue, there was a decrease in corticospinal activation and spatial facilitation of cortical and peripheral inputs at premotoneurons, indicating inhibition of premotoneurons during and after fatigue in normal but not in hemiplegic subjects. From the experiments conducted in this thesis it is therefore concluded that premotoneurons provide considerable facilitation of flexor motoneurons during weak, ballistic contractions of forearm flexor muscles. Premotoneurons were modulated during and after fatigue and this mechanism was deficient in hemiplegic subjects which may contribute to the movement disorder or compensate for a reduced cortical drive in hemiplegic subjects.