Activity of muscle spindles during muscle twitch and clonus in normal and spastic human subjects

Abstract

Muscle spindle single unit activity was recorded with tungsten electrodes from the median and tibial nerve in unanesthetized normal and spastic human subjects during an electrically or reflexly induced muscle twitch and during clonus. Muscle spindle activity was silenced during the contraction phase and reactivated during twitch relaxation. In the normal subject and the patient, the relaxation activity was facilitated during a voluntary remote muscle contraction. In spastic patients, in whom at rest no clonus could be elicited by a tendon tap, clonic reflex oscillations were provoked in conjunction with the increased spindle relaxation activity due to the remote muscle contraction. The latency of spindle relaxation activity was decreased during the reinforcement maneuver and the clonus frequency was accordingly increased. Using the relaxation activity as an indicator of the dynamic spindle sensitivity, it is concluded, that a remote muscle contraction in normal and in spastic human subjects affects a specific dynamic gamma drive to muscles which are not involved in posture in movement. In clonus, a pronounced bursting firing of muscle spindle primaries was seen during twitch relaxation which never was observed as markedly in normal subjects. The results suggest that twitch relaxation phase activity of spindle primaries plays a crucial role in the activation of clonus during periods of increased central excitability.