Intrathalamic regulation of activity in a cerebellocortical projection pathway

Brachium conjunctivum (BC) stimulation elicits short-latency (1 msec) activity in n. ventralis lateralis (VL) and specific responses in motor cortex that are associated with 3- to 4-msec latency relayed discharges in the medullary pyramidal tract. Secure synaptic drives in this cerebellocortical pathway are achieved with BC-stimulus frequencies up to 140 per sec. Low-frequency (7/sec) medialthalamic stimulation produces early facilitation and long-latency prolonged inhibition of BC-evoked activity in VL. Inhibitory and facilitatory interactions are related to components of focal responses in VL initiated during thalamocortical recruiting responses. Inhibitory effects in VL of medial-thalamic stimulation are frequency specific and are eliminated during 25 to 50 per sec stimulation of the nonspecific system. Low-frequency (7/sec) stimulation in rostral portions of VL produces effects on BC-evoked activity that are similar to those of medial-thalamic stimulation. Inhibitory interactions are more pronounced and persist beyond the period of stimulation especially at high frequencies. Short-latency facilitatory interactions between augmenting responses evoked by rostral VL stimulation and BC-evoked activity are prominent at depths of barbiturate narcosis at which BC-evoked relayed activity is eliminated. The data indicate that intrathalamic synaptic pathways arising in medial-thalamic nuclei and rostral portions of VL converge on relay elements in the cerebellocortical projection system. These excitatory and inhibitory internuclear pathways play a major role in the regulation of afferent drives on corticospinal neurons.