Production and suppression of tremor by mesencephalic tegmental lesions in monkeys

In a group of rhesus monkeys two consecutive lesions were placed in the mesencephalic tegmental region. The first lesion, for tremor production, was placed ventromedially in a region where multiunit activity was organized in rhythmic bursts of 4 to 8 cps and threshold electrical stimulation induced III nerve type of responses. The second lesion, for tremor suppression, was placed dorsolateral to the previous one, in a region where nonrhythmic multiunit activity was enhanced by peripheral nociceptive stimulus and threshold electrical stimulation produced awakening. Studying the lesions by planimetry in a quantitative manner, it was found that tremor production depended on the involvement of a critical amount of the pars compacta of the substantia nigra and the region immediately behind it, whereas tremor suppression depended on the involvement of a critical amount of the nucleus reticularis mesencephali. Involvement of other adjacent structures was not significant for either tremor production or suppression.