Abstract
Effects of stimulation of midline thalamic nuclei, caudate nucleus and reticular formation on responses evoked in n. ventralis lateralis by brachium conjunctivum stimulation have been studied to further elucidate the properties of synaptic pathways influencing input-output relations in n. ventralis lateralis. Low-frequency midline thalamic or caudate stimulation produces early facilitation and later developing inhibition of brachium conjunctivum-evoked responses in n. ventralis lateralis. These effects in n. ventralis lateralis are associated with diphasic negative-positive, focal potentials which are fundamentally similar in overt characteristics. Caudate evoked responses in n. ventralis lateralis have a longer latency and are more temporally dispersed than those elicited by midline thalamic stimulation. Inhibitory interactions between midline thalamic and brachium conjunctivum-evoked activities in n. ventralis lateralis are abolished by high-frequency caudate stimulation. The caudate-induced blockade of inhibitory interactions in n. ventralis lateralis is similar in many respects to that observed following high-frequency brain stem reticular formation stimulation. However, additional effects of reticular stimulation are seen in facilitation of brachium conjunctivum-evoked responses in n. ventralis lateralis, motor cortex and corticospinal tract. Motor cortex stimulation elicits a focal positivity in n. ventralis lateralis which is associated with inhibition of brachium conjunctivum-evoked activity. This inhibition mediated by corticofugal projections is not preceded by a phase of facilitation as in the case of midline thalamic or caudate stimulation. The results indicate that transmission of activity in cerebellothalamocortical projections may be dramatically influenced in n. ventralis lateralis by inhibitory and excitatory drives in intrathalamic, striothalamic and reticulothalamic synaptic pathways. Involvement of these synaptic pathways is considered in the genesis of movement disorders and the clinical effects of n. ventralis lateralis lesions in the treatment of dyskinesias.
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