RESPONSES OF THE IRIS TO PROLONGED STIMULATION OF ITS PARASYMPATHETIC NERVE SUPPLY

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Abstract
The optimum frequency for brief periods of stimulation of the parasympathetic nerve supply to the cat''s iris is 25 per sec. Prolonged stimulation of the post-ganglionic parasympathetic fibers at frequencies of 50-150 per sec. resulted in a contraction of the sphincter of the iris which was sustained for 1-2 hrs. and which thereafter gradually declined. There was no evidence of 4th stage followed by a 5th stage of increased contraction. Similar prolonged stimulation of the preganglionic fibers in the 111d cranial nerve resulted in a marked contraction followed by a fall (4th stage) and later by an increase of response (5th stage). The appearance of the 5th stage is evidently not a property of all cholinergic nerves, but only of preganglionic and motor fibers.

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