Ipsilateral pallidal control on the sternocleidomastoid muscle in cats: relationship to the side of thalamotomy for torticollis.

To elucidate the role of the basal ganglia on neck movement control and thus clarify the side of targets for stereotactic surgery of spasmodic torticollis, effects of electrical stimulation to the globus pallidus-entopeduncular nucleus complex (GP-EP) upon the activities of the sternocleidomastoid (SCM) muscle motoneurons were studied in cats. Repetitive stimulation of the ipsilateral GP-EP increased discharge rate in the majority of the SCM motoneurons, whereas contralateral GP-EP stimulation caused a decrease in discharge rate. These effects were gradually enhanced in the course of repetitive stimulation and lasted even after its cessation. Bilateral ablation of the motor and premotor cortices did not influence the effects of GP-EP stimulation. Latency studies suggested that the pallidal control of the SCM motoneuron is mediated by polysynaptic pathways on both sides; however, these same studies produced evidence of an ipsilateral oligosynaptic connection, as well. Repetitive antidromic stimulation of the accessory nerve caused a gradual increase in discharge rate of the SCM motoneurons, similar to the augmentation induced by ipsilateral GP-EP stimulation. Features of the responses indicate that the pallidal control on the neck muscles involves reverberating circuits in the brain stem and spinal cord. These experimental findings suggest that stereotactic thalamotomy of the ventrolateral nucleus for spasmodic torticollis should be performed on the side ipsilateral to the contracting SCM muscle.