Nerve‐mediated excitation and inhibition of the smooth muscle cells of the avian gizzard

Abstract

1. The electrical events evoked in smooth muscle cells of the chick and pigeon gizzards by vagal, perivascular sympathetic and transmural stimulation were recorded with intracellular micro-electrodes.2. Single stimulating pulses applied to the extrinsic nerves, or to intrinsic fibres, produced excitatory junction potentials (E.J.P.s) which were blocked by hyoscine. Repetitive stimulation caused facilitation of E.J.P.s and, at higher frequencies, summation. When the tissue was well oxygenated, maximal stimulation evoked action potentials and the tissue contracted. If the tissue was allowed to become anoxic, action potentials were blocked and thus junctional transmission could be observed uncomplicated by tissue contraction.3. Cholinergic E.J.P.s appeared to be due to stimulation of both post-ganglionic neurones and the pre-ganglionic input to post-ganglionic neurones, since ganglion-blocking drugs abolished the late phase of complex E.J.P.s while leaving unaffected their initial components.4. Repetitive perivascular sympathetic stimulation, after cholinergic E.J.P.s had been blocked by hyoscine, evoked a slow, long-lasting depolarization of the muscle cells. This depolarization was blocked by guanethidine (5 x 10(-6) g/ml.) and was mimicked by noradrenaline (10(-7) g/ml.).5. Under non-anoxic conditions, the tissue underwent rhythmical contractions which were associated with action potential firing. When the tissue was anoxic action potentials were not seen, but spontaneous membrane depolarizations resembling E.J.P.s were observed. Spontaneous miniature excitatory junction potentials (M.E.J.P.s) were observed only infrequently whether the tissue was well oxygenated or not.6. Single stimulating pulses applied to extrinsic or intrinsic nerves frequently evoked inhibitory junction potentials (I.J.P.s) in smooth muscle cells of the gizzards of both birds. I.J.P.s were blocked by tetrodotoxin (10(-7) g/ml.). Repetitive stimulation of inhibitory nerves gave rise to a membrane hyperpolarization; when stimulation was stopped the membrane potential showed rebound depolarization.7. These results are discussed in terms of the autonomic innervation of the avian gizzard, and possible explanations of the various types of activity observed are considered.