Regulatory mechanisms in parkinsonian tremor.

The precise mechanism of the tremor in Parkinson's disease remains a subject for debate and investigation. The number of conflicting views in the standard clinical and physiological texts and journals attests to the difficulty of the problem. Some investigators suggest that central structures are 'released' to periodic discharge (Brain, 1960; Holmes, 1952; Bucy, 1944; Merritt, 1959). Others view the alternating discharge in spinal motorneurones as an uninhibited spinal or bulbo-spinal rhythm (Jung and Hassler, 1960) or disordered function in the reticular formation (French, 1960; Ward, McCulloch, and Magoun, 1948). Peripheral servomechanism function has until lately been somewhat neglected in the analysis of Parkinsonian tremor, although earlier experiments had been interpreted to show that the tremor was not entirely independent of proprioceptive input (Walshe, 1924; Pollock and Davis, 1930). It is now clear that peripheral length sensors do influence all motor output and that they are not simply passive receptors. Due consideration must be given to indirect alpha cell driving by means of periodic discharges into, as well as out of, the loop circuit. It is the purpose of this paper to show that the amplitude, at least, of Parkinsonian tremor is not independent of peripheral influences and that dysfunction in servo mechanisms may play a distinct role.