Physiological study of presumed ventralis intermedius neurons in the human thalamus

Unitary or a group of unitary responses recorded extracellularly in the rostral part of the thalamic sensory nucleus during the course of thalamotomy for parkinsonian and other kinds of tremor were studied. They were the first sensory neurons encountered in the frontocaudal approach of the stereotaxic needle to the therapeutic target at the lower border of the ventralis intermedius nucleus. The spike discharges, isolated from high neural noise background, responded to passive and/or active movement of joint or muscle on the contralateral extremity but never to light tactile stimuli. In five cases, electrical stimulation of the corresponding peripheral nerves elicited the same spike discharge at short and fixed latency of about 12 msec. Weak electrical stimulation given to each thalamic point produced paresthetic or electric sensation around the receptive field, and stronger stimulation induced tremor-like movement in the area involving the receptive field. Finally, a small coagulation lesion in this thalamic point resulted in immediate relief of tremor without any sensory deficit. From these findings, the possibility is considered that in the human, this particular thalamic zone could be the ventralis intermedius nucleus, which is separable from the surrounding nuclei and is related to tremor mechanism.