The spinal connections of the inspiratory neurones of the ventrolateral nucleus of the cat's tractus solitarius

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(1) In a preceding study the inspiratory neurones of the solitary complex (TS neurones) were found to fall into two separate groups: (1) with afferent vagal connections but without spinal projections, and (2) with direct rapidly conducting spinal connections but without vagal afferent input. In the present paper the properties of the spinal connections of this latter group of TS neurones has been examined, whether formed by the axons of the cells or orthodromic in character. (2) These inspiratory ‘spinal, non-vagal’ TS cells could be driven by single shocks to the ventrolateral funiculi of the contralateral side of the spinal cord with fairly short latencies which did not vary by more than 0.5 msec. (3) Critically time-controlled collision tests showed that there was a period after a spontaneous spike during which a stimulus applied to the spinal cord was ineffective in eliciting a response in the TS neurone. (4) The duration of this period, the ‘critical delay’, in 16 cases out of 17, was shorter than the sum of the driving latency and the minimal value for axon refractoriness (0.5 msec). This indicated that, after a spontaneous spike, spinal driving of the cell at a time slightly longer than the critical delay could not be due to antidromic activation but was transmitted by an orthodromic pathway. After a spontaneous spike the TS soma was thus not responsive to spinal stimulation for a period of 3–4 msec which corresponds to the sum of the critical delay and the conduction time from the spinal stimulus site to the soma. (5) The TS neurone was not refractory for the whole of the period when it was unresponsive to spinal stimulation. During this period it could fire a second spike and it could be driven antidromically at high rates with interstimulus intervals as short as 1.3 msec.