Mayston, Margaret Joy; (1996) Mechanisms underlying co-contraction during development and in pathology in man. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Postural and motor skills require the synergistic or co-ordinated action of a number of different muscles. These muscle synergies change during childhood and are disordered in developmental abnormalities. The underlying mechanisms have been investigated using a number of neurophysiological techniques. Focal magnetic brain stimulation was used to investigate the integrity and distribution of the corticospinal pathways; cutaneous reflexes and phasic stretch reflexes to investigate spinal and transcortical reflex pathways; somatosensory evoked potentials to examine afferent pathways and cross-correlation analysis of multi-unit EMG signals to look for the presence of common synaptic drive to motoneurone pools. Investigation of children performing a unimanual task showed the occurrence of contralateral involuntary activity (mirror movements). This decreased with age. A study of a group of 4-11 year old children provided evidence to suggest that the mirror movements in children are due to a lack of interhemispheric inhibition by the corpus callosum. No bilateral activity was observed in adults unless there was contralateral background activity. In subjects with X-linked Kallmann's Syndrome (XKS) who show obligatory mirror movements, it was concluded that there is an abnormal ipsilateral projection which accounts for the occurrence of common synaptic drive to left and right homologous motoneurone pools and underlies the mirror movements in XKS. In the group of children with cerebral palsy (CP) the associated activity was not due to a common synaptic drive although in some patients there was evidence of a novel ipsilateral projection. In the adult, antagonistic muscles can either co-contract to stabilise a joint or act reciprocally to produce force or movement about a joint. In young children, and those with cerebral palsy, agonist and antagonist muscles have been found to co-contract. The synaptic input to the antagonistic motoneurone pools was investigated using cross-correlation analysis. In adults the presence of a short duration central trough in the cross-correlogram indicated the presence of a reciprocal inhibitory mechanism. This was not observed in the young children or those with CP. Nor was there evidence of any shared excitatory input in these children.

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