Stabilization of the thalamocortical motor system by cerebellar stimulation.

Epilepsy typifies instability in a complex control system. We have previously identified parameters of the thalamocortical motor system of the cat which correlate with epileptiform activity and are controlled by common anticonvulsants. This study is concerned with the control of such parameters by cerebellar stimulation, potencially promoting stability within the motor system. Under computer control, stimuli were delivered to both ventrolateral thalamus and cerebellar cortex, with multichannel recording of evoked responses obtained from sensorimotor cortex. Cortical evoked responses were plotted as an excitability curve (mean response amplitude as a function of pulse interval) or a family of threshold curves (mean response amplitude as a function of stimulus amplitude at various fixed intervals). The present study reveals that cerebellar epidural stimulation can reduce both the height and duration of the excitability curve, as well as increase the response threshold and reduce the saturation level of the threshold curve. The degree and direction of these parameter changes are dependent on the frequency, polarity, and amplitude of cerebellar stimulation, with the amount of parameter change exceeding that achieved by monotoxic doses of anticonvulsants. These data suggest that control of parameters related to excitability and threshold in the thalamocortical motor system may be the means by which cerebellar stimulation accomplishes control of clinical seizures.