A study of excitatory neuromuscular transmission in the bovine trachea

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Abstract
The excitatory innervation of bovine tracheal smooth muscle was studied with the sucrose-gap apparatus. Single 2 ms electrical stimuli applied to the whole tissue excited intrinsic nerves, and produced a small transient depolarization of the smooth muscle, the excitatory junction potential (ejp). The ejp caused a twitch-type contraction; twitches and ejp summated during repetitive stimulation but facilitation was not observed, and action potentials were never elicited. The effects of electrical stimulation could be abolished by atropine (5 .times. 10-7 mol/l) and augmented by neostigmine (4 .times. 10-6 mol/l), and were mimicked by exogenous acetylcholine (1.0 .mu.g/ml). EM showed the density of innervation to be low (1 axon/90 smooth muscle cells). Axons were found in small groups in the clefts between bundles of cells, but no axons penetrated within the muscle bundles. Naked axon varicosities containing agranular vesicles were seen, but no axon approached within 200 nm of a smooth muscle cell. It is difficult to reconcile the sparsity of innervation with the dependence of the tissue on nerve excitation to initiate activity.