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

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.