Synaptic mechanisms involved in the inspiratory modulation of vagal cardio‐inhibitory neurones in the cat.

Abstract

The respiratory modulation of the activity of vagal cardio-inhibitory neurons of the nucleus ambiguus of the cat was investigated by electrophysiological and neuropharmacological techniques. All 24 vagal efferent neurons studied had axons with conduction velocities indicative of B fibers and projected to the right cardiac branches of the vagus. Their spontaneous of DL-homocysteic acid (DLH)-evoked activity showed a marked reduction during the phase of inspiration and all showed signs of receiving a baroreceptor input. Ionophoretic application of DLH always excited cardiac vagal montoneurons (c.v.m.s.). Application of acetylcholine to these same cells provoked a decrease in firing rate in 12 of the 15 neurons tested. In 10 of these 12 cells simultaneous application of atropine antagonized the effect of acetylcholine. Atropine applied alone enhanced neuronal firing, particularly in inspiration. Stable intracellular recordings were made from 2 c.v.m.s. These were inhibited during inspiration. Input resistance fell markedly during inspiration and injection of Cl reversed this wave of hyperpolarization to a wave of depolarization, indicating that this resulted from Cl-mediated inhibitory post-synaptic potentials (i.p.s.p.s.). These c.v.m.s. were activated during stage I expiration, showed a weak and variable wave of inhibition in stage II expiration. Pulse-rhythmic depolarizing potentials were reduced in their amplitudes during the periods of decreased neuron input resistance. Evidently, c.v.m.s receive an excitatory input during post-inspiration and a powerful inhibitory synaptic input during inspiration. The implications of these observations for the physiology of cardiorespiratory reflexes are discussed.