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Neuronal influence on the mechanical activity of the ciliary muscle

1 Neuronal effects and the pharmacological properties of the bovine ciliary muscle were investigated in vitro. The bovine ciliary muscle exhibited no spontaneous activity. 2 Electrical stimulation of an isolated short ciliary nerve produced distinct contractions. The minimal stimulus duration required to evoke a contraction was 0.2 ms and amplitude of the contraction was maximal at 2 ms. Twitch or incomplete tetanus reached a complete tetanus with 4 Hz stimulation. 3 Raising the external potassium concentration from 5.9 to 158.8 mm produced a contracture which consisted of an initial phasic and then tonic components. 4 The contractions generated by either electrical stimulation (0.2–100 ms) or high K were potentiated by physostigmine and completely inhibited by atropine. Neither adrenoceptor agonists nor blockers influenced these contractions. 5 Application of tetraethylammonium (TEA), potentiated the electrically‐induced ciliary muscle contraction, and the effect of TEA was not completely inhibited by high concentrations of either atropine or tetrodotoxin. Thus, TEA presumably acts both pre‐junctionally and post‐junctionally to increase the contractile development of ciliary muscle. 6 The ciliary contractile response is primarily mediated by acetylcholine released from nerves, and this response is accompanied by a negligible contribution from the sympathetic nerves. Depolarization induced by electrical currents or by high K was ineffective in evoking contraction of the ciliary muscle. 7 The results suggest that excitation of the ciliary muscle is probably mediated via junction potentials or by a direct transmitter action without any very great change in the potential. Action potentials are probably generated in the presence of TEA.