Spasticity is one type of hypertonus, which increases with the velocity of joint movement. It is attributed to hyperactive stretch reflexes mediated by muscle spindle stretch receptor. Mechanisms underlying the development of spasticity after spinal cord injury are not understood. One spinal interneuron likely to be affected is Renshaw cell, which acts to produce recurrent inhibition in motor neurons as well as inhibiting Ia interneuron.
The Renshaw cell activity was evaluated in 20 normal subjects as control and in 5 patients with spasticity after spinal cord injury using the conditioned H-reflex technique of Pierrot-Deseilligny and Bussel to investigate the role of Renshaw cell activity in the development of spasticity after spinal cord injury. The H/M ratio was increased (p<0.1) and H'/H ratio was decreased (p<0.05) in spinal cord injured patients and correlates well with clinical measurements of spasticity although the absolute values of amplitude of M response, H reflex and the conditioned H-reflex were not significantly different between two groups. These results suggest that the recurrent inhibition via Renshaw cell activity is increased in spinal cord injured patients and recurrent inhibition can be measured using the conditioned H reflex technique.
