Orthostatic tremor arises from an oscillator in the posterior fossa.

We tested the hypotheses that orthostatic tremor is generated by a central oscillator and that the tremor is expressed through spinal Ib interneurons. Six patients with orthostatic tremor were examined. The tremor was reset by electrical stimulation over the posterior fossa at intensities that were below the threshold for a motor evoked potential (MEP) but was not reset by transcranial magnetic stimulation over the motor cortex that did produce an MEP. It is argued that the oscillator involves the cerebellum or brainstem. The inhibition of voluntary EMG produced by stimulation over tendons, which has been attributed to effects from Golgi tendon organs (GTO), was not modulated in synchrony with the tremor. We were unable to demonstrate, therefore, that the tremor is expressed through GTO interneurons with this method.