III. The mechanisms of action of neurotransmitters - Electrical changes underlying excitation and inhibition in intestinal and related smooth muscle

Abstract

Intestinal smooth muscle is normally spontaneously active and contraction is associated with spike activity. Stimulation of excitatory (cholinergic) nerves increases spike frequency while inhibitory (adrenergic) nerve activity reduces slow waves and spikes without necessarily producing hyperpolarization. Activity of intrinsic nerves produces inhibition with marked hyperpolarization. The anococcygeus muscle of the rat, a muscle associated with the alimentary canal, has a dense adrenergic innervation and has neither resting tone nor spontaneous activity. The mean resting potential is 58.4 mV. Field stimulation produces graded depolarization associated with contraction and abolished by phentolamine. The depolarization has an initial component of up to 10 mV followed by a response which can reach 50 mV, the largest sometimes having a single spike on the rising phase. Application of noradrenaline or guanethidine produces depolarization with oscillations at 1/s and maintained contraction. Field stimulation at low frequencies during this contraction causes relaxation and reduction in the membrane oscillations but no repolarization.