The contraction of the extrinsic muscles of the eye by choline and nicotine

Abstract

It has long been known that choline, acetylcholine (Reisser, 1921), and nicotine (Langley, 1906-14) contract the normal striped muscle of Sauropsidæ (frog, fowl, etc.), and it has been recently demonstrated that a similar reaction occurs in the fœtal muscle of mammals (Rückert, 1930), but hitherto it has never been shown to occur in the muscles of the fully-developed mammalian. It has also been well established that the voluntary muscles of mammals, after degeneration of the motor nerves, exhibit a "pseudo-motor" contraction on the injection of choline. (Frank, Northmann and Hirsch-Kauffmann, 1922-23), a contraction which appears analogous in origin and in nature to that first described in the tongue by Vulpian and Phillipeaux (1863), and in the muscles of the limbs by Sherrington (1894), which occurs on stimulation of the sensory roots after the motor roots have degenerated. Recent writers in discussing the theoretical basis of these phenomena have stressed the point that this type of contraction occurs in mammals only after degeneration of the motor nerves, and have based some of their conclusions upon this assumption. The following experiments show, however, that this statement is no universally true, and that the extrinsic muscles of the eye form an exception to the general rule. The matter arose as a side-issue during an extended research on the mechanism controlling the intra-ocular pressure, when anomalous changes were noted while investigating the effect of choline and acetylcholine upon the pressure in the eye. These experiments are recorded in a separate publication (Duke-Elder, 1930): it is sufficient for the present purpose to say that in experiments upon anæthetised dogs, while small doses of choline such as produce a depressor effect when injected intra-venously (0·2 c. c. of a 1 in 20 solution) give rise to a fall in the intra-ocular pressure of the order which would be expected from the events in the vascular circulation, larger doses, on the other hand, lead to an increase in the intra-ocular pressure much larger than could be explained by any vascular events. In order, therefore, to reduce the number of variables with which we were dealing, the technique was extended to the perfusion of the eye with an artificial circulation, whereby the conditions in the general circulation were kept constant (Duke-Elder, 1930); and fig. 1 shows that in these circumstances, even when we would have expected a fall in the intra-ocular pressure owing to a local vasoconstriction when the pressor component of choline was elicited, a rise was obtained. The most significant feature was that this rise was accompanied by a movement of the base-line in the tracing registered by the optical manometer denoting a movement of the eye in the direction of enophthalmos.