Motoneurone and monosynoptic reflex excitability studied in man

Abstract

Natural stimulation of the skin and muscle was found to modulate the monosynaptic reflex and motoneurone excitability as judged by the H - reflex. Skin cold and mechanoreceptors inhibit the reflex as does muscle vibration while cooling of the muscle resulted in facilitation. The effective skin areas were of the same spinal segments as the muscle investigated..The motoneurone pool was inhibited for 50 cosec. after a conditioning electrical stimulus. The postulated mechanism for this is transmitter depletion and the evidence obtained supports this. Moreover the evidence excluded the involvement of Golgi tendon organs.The motoneurone excitability measured by the recovery curves was tested in young and old subjects and significant differences were found. Degeneration in the large diameter afferent fibres and motoneurones could account for the findings.Electro stimulation of the spinal cord in MS improves the abnormal recovery curves towards normal.. Other electrophysiological studies using the implanted electrodes were made and demonstrated changes in monosynaptic reflex excitability. Monosynaptic and motoneurone excitability were 'studied at the unitary level using single fibre EMG techniques and early physiological conclusions were confirmed. Motoneurones recovered from inhibition in a definite pattern. Different properties were noticed for units of the soleus which suggested a division into two groups according to their order of recruitment during incremental stimuli; blockage and inhibition. One group was recruited and blocked early followed by the other group with increasing stimuli. An opposite order was demonstrated during mechanoreceptor stimulation. A new hypothesis was explored, controversial to Henneman's size principle and implies that the external requirements of the movements determine the order of recruitment of the different motoneurones. Clinically in myotonic dystrophy significant changes were_ found in monosynaptic and motoneurone excitability. Neurological defects were shown which confirm the involvement of the nervous system in this disease.

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