Effects of vagal volleys on units of intralaminar and juxtalaminar thalamic nuclei in monkeys.

As part of an attempt to clarify the nature of inputs to the limbic cortex, the thalamic intralaminar and juxtalaminar nuclei were explored for unit responses to vagal volleys in awake, sitting squirrel monkeys. Vagal shocks elicited responses of a large percentage of units in the anterior medial, paracentral, lateral dorsal, and lateral, and medial dorsal nuclei, as well as in part of the ventral lateral nucleus adjacent to the paracentral. Responsive units showed either initial excitation or initial inhibition. As in the preceding study on the cingulate and supracingulate cortex, there were two main types of initially excited units: type 1 responded with a discharge of 1-3 spikes at relatively short and constant latencies, while type 2 units were characterized by a burst of 3-14 spikes at longer and more variable latencies. Although the findings were compatible with the hypothesis that the anterior and paracentral nuclei transmit vagal impulses to the cingulate and supracingulate cortex, an analysis of latencies suggested that a more rapidly conducting pathway(s) accounts for latencies as short as 12 msec of some cingulate units. Twenty-eight percent of 367 units in the medial dorsal nucleus responded to vagal volleys. This finding gives substantial support to the traditional view that the medial dorsal nucleus transmits interoceptive information to limbic and neocortical areas of the orbitofrontal region.