Electrophysiologic studies on the pallido- and cerebellothalamic projections in squirrel monkeys (Saimiri sciureus)

Thalamic projections of the pallidum and the deep cerebellar nuclei were studied by unitary recordings as well as field potential analysis in the thalamus of squirrel monkeys (Saimiri Sciureus) under sodium pentobarbital anesthesia. Stimulation of the pallidum produced a positive field potential preceded by incoming afferent fiber volleys in the thalamus. Spontaneous discharges of thalamic neurons were suppressed during this positive potential corresponded to that of the hyperpolarizing potential. The hyperpolarization was presumed to be a monosynaptic inhibitory post synaptic potential by the short synaptic delay (about 0.5--0.7 ms) and responsiveness to high frequency stimulation (over 150 Hz). The positive field potential on stimulation of the external pallidal segment was distributed in L.po (VA) and the reticular thalamic nucleus around L.po, whereas that on stimulation of the internal segment was in V.o.a (the anterior basal part of VL) and in Z.o (upper part of VL). The projection of the external segment appeared to be less dense than that of the internal segment. The projection of deep cerebellar nuclei was situated in V.o.a, V.o.p (posterior part of basal part of VL), V.o.i (VLm), the intralaminar nucleus (CL), and some part of V. im (the rostral part of VPLo). Projections of the interpositus and dentate nuclei were distributed in a more anterior part than those of the fastigial nucleus. A certain topographical arrangement of the projections of these three nuclei was found in V.o.p, V.o.i and V.im. No significant overlap was detected between pallidum and the deep cerebellar nuclei within the thalamus.