Reciprocal Ia inhibition in spastic paralysis in man

Reciprocal reflex connections were studied in capsular hemiplegia and spastic paresis with spinal cord lesions, using Lloyd's technique. Effects of conditioning stimulation of the tibial or peroneal nerve on the H reflex in the antagonists were examined. Stimulus intensity was controlled with reference to the threshold of the M wave. Weaker stimulation than this threshold was regarded as stimulation of group I afferents. It aroused no subjection sensation in intact subjects. Early and strong inhibition, comparable to Ia inhibition in the cat (Lloyd 1946), was observed from weak stimulation of the tibial nerve on the pre-tibial (flexor) H reflex, but not from the peroneal nerve on the triceps surae (extensor) H reflex in capsular hemiplegia. Alcohol block of extensor motor points resulted in reduction of spasticity without further paralysis in the blocked muscle and a remarkable increase in strength of the antagonist pre-tibial muscles. These results suggest that an extensor spasticity withe flexor weakness, which is common in capsular hemiplegia, may be due to an imbalance of reflex activities via Ia muscle afferents, and that a part of flexor weakness can be restored by "disinhibition' by reduction of Ia inflow from extensor muscles. Ia inhibition was also observed in one third of cases with spinal cord lesions at rest. It returned to normal after recovery from spastic paresis by radical therapy in some cases.