Clonus: the role of central mechanisms.

The peripheral and central components in sustained clonus were investigated. The excitability of the motoneurons responding to maintained stretch by clonus was examined by tendon taps, trains of vibratory stimuli and by H-reflex afferent volleys. Every burst of clonic discharge of the motoneurons was shown to be followed by a refractory period, which was followed by a shorter excitatory period. It was concluded that the motoneurons responding clonically to a continuous stretch cannot respond until their excitability has been regained after the refractory period. Attempts to change the rate of clonus in various ways failed to do so. Whether motoneurons of clonic muscles tend to respond maximally to other Ia volleys at the rate of clonus was examined by applying repeated taps to the tendon at rates from 1 to 15 Hz. There was a maximal response at the rate of clonus. Inputs other than those induced by stretch cause clonus; examples of cutaneous inputs causing it are given.