Physiological and pathological tremors and rhythmic central motor control

In recent years there has been increasing interest in oscillatory neural ac tivity in the CNS and in the role that such activity may have in motor cont rol, It is thought that physiological tremor may be a manifestation in the periphery of such central oscillatory activity and that some pathological t remors are the result of derangement of these oscillators, This review re-e valuates both early and recent studies on physiological and pathological tr emors and other peripheral oscillations in order to gain a new perspective on the nature and function of their central progenitors, This approach, nam ely using tremor as a 'window' into the! function of central oscillations, is particularly suited to human investigations because of the obvious limit ations of direct central recording. It is argued that physiological tremor is likely to be multifactorial in origin, with contributions not only from CNS 10-Hz range oscillatory activity, but also from motor unit firing prope rties, mechanical resonances and reflex loop resonances. Different origins are likely to dominate under different conditions. While some pathological tremors appear to arise as a distortion of central or peripheral components of physiological tremor, others arise de novo, such as the pathological os cillation of 3- to 6-Hz parkinsonian tremor, CNS oscillations outside the 1 0-Hz range are also found to modulate limb activity in normal individuals, and oscillatory activity exists in other motor systems such as eye movement s, Finally, it is shown how studies of peripheral oscillations may help dev elop hypotheses on the role of CNS oscillations in motor control, including the proposed 'binding' function of synchronized oscillations and the possi bility that motor signals could be coded by frequency of modulating oscilla tion as well as by synaptic connectivity,