Presynaptic depolarization in myelinated vagal afferent fibres terminating in the nucleus of the tractus solitarius in the cat

Abstract

The presynaptic influences that act on terminals of slowly adapting lung stretch receptor afferents and aortic baroreceptor afferents within the nucleus of the solitary tract were assessed using intracellular recording and antidromic stimulation techniques. Central respiratory influences on the excitability of lung stretch receptor terminals were observed in 29% (4 of 14) of measurements. These were confirmed in intracellular recordings where membrane depolarizations in synchrony with phrenic nerve discharge were seen in 17% (4 of 24) of fibres. In three cases membrane depolarization also occurred synchronously with artificial lung inflation. Neither tests of excitability nor intracellular recording revealed any evidence for equivalent presynaptic influences on 16 myelinated aortic baroreceptor terminals. Stimulation of the superior laryngeal nerve evoked depolarizations in 50% (7 of 14) of lung stretch receptor terminals. These took the form of complex waves of depolarization with both short (3-8 ms) and long latency (27-35 ms) components. The amplitude of the long latency response increased during the period of phrenic nerve discharge, i.e. during "central inspiration". These effects are discussed in relation to the central respiratory influences on both respiratory and cardiovascular reflexes.