Disynaptic reciprocal inhibition of ankle extensors in spastic patients

The soleus Hoffmann-reflex (H-reflex) was conditioned by a preceding stimulation of the common peroneal nerve in 74 healthy control subjects and 39 patients with spasticity in the lower extremities due to multiple sclerosis. At a conditioning-test interval of 1-3 ms a decrease of the size of the soleus H-reflex was seen in the healthy subjects. The decrease was most likely caused by disynaptic reciprocal Ia inhibition (Crone et al., 1987). In the spastic patients a similar short-latency inhibition was rarely seen. On the contrary, in several patients a facilitation was seen at a conditioning-test interval of 3-4 ms. A short-latency inhibition as pronounced as in healthy subjects was seen in four patients. These four patients did not differ from the other patients regarding the degree of spasticity or any other clinical parameter. However, they all used an external peroneal nerve stimulator daily as a walking aid. It is suggested that the lack of short-latency reciprocal inhibition reflects a deficient control of the interneurons which mediate this inhibitory spinal mechanism between antagonistic muscles in man. This might contribute to the pathophysiology of spasticity and it might be related to the frequent occurrence of co-contraction of functionally antagonistic muscles during gait in spastic patients. The existence of a pronounced reciprocal inhibition in patients receiving frequent stimulation of the peroneal nerve may suggest that regular activation of peripheral nerves is of importance for the maintenance of the activity in spinal pathways.