Correlation between tremor, voluntary contraction, and firing pattern of motor units in Parkinson's disease

Patients with tremor of Parkinsonism show three characteristics of motor unit activity: rhythmic spontaneous resting discharge, abnormally low firing rates during voluntary contraction, and consistent differences in firing pattern between small and large motor units. Smaller units discharge once per tremor beat at weak contractions but change into bursts of two or three spikes per beat at stronger forces. Large units are later recruited and fire preferentially once per beat. The large tremor amplitudes can be partly explained by synchronization of unfused twitches of low frequency units which summate more powerfully than the partially fused contractions during physiological tremor, which is about twice as rapid. Tremor is strongly influenced by the force of voluntary contraction. It is strongest at rest or during weak muscular effort and with increasing force becomes continuously of higher frequency and smaller amplitude. Both changes are the consequence of increasing discharge rates of motoneurones at stronger contractions.