This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
The iris consists of two sets of muscles - the circular, supplied by the third nerve, and the radial, supplied by the sympathetic fibers from the superior cervical ganglion. These two sets of muscles are in constant action, and by opposing each other constitute an exceedingly sensitive balanced mechanism for the regulation of the size of the pupil. Dilatation of the pupil may result from circular depression or radial stimulation; contraction of the pupil from circular stimulation or radial depression, and these stimulations or depressions may be of center, ganglia, nerve-endings, or muscle-fibers.
Fig. 50. - Increased convexity of the lens during accommodation. The solid white outline of the lens, l, shows its form when relaxed. The dotted line shows the increased curvature of the anterior surface during accommodation, and its advancement forward into the anterior chamber, a. z is the suspensory ligament; m, the ciliary muscle; and i, the iris (Landolt).
When a 1 per cent. aqueous solution of atropine sulphate is dropped in a man's eye, the pupil dilates in about fifteen or twenty minutes, but takes two hours more to reach the maximum dilatation. There is no effect on the other eye. If atropine is injected into an excised mammal eye, the pupil dilates, and if an animal is atropinized, stimulation of the third nerve, either central or peripheral to the ciliary ganglia, is without effect on the pupil. The action is, therefore, a purely peripheral one. But it is not a direct effect upon the muscle, for in the atropinized animal direct stimulation of the circular muscle results in contraction; therefore the site of the paralyzing action of the drug must be confined to the third-nerve endings or the neuromuscular junction.
The dilatation from atropine is, therefore, the result of the unopposed action of the radial muscles. It is, however, frequently strong enough to break weak adhesions between cornea and iris, or to make an iris which is strongly attached at two points bow out between the points of attachment. The pupil gradually regains its power, but is not fully restored to normal for one or two weeks. That there is no stimulation of the radial mechanism is evident, for, after atropine, stimulation of the cervical sympathetic results in a still greater dilatation.
A drug which causes dilatation of the pupil is called a mydriatic. Belladonna gets its name from this mydriatic action (bella, beautiful; donna, lady), which makes the eye seem bright and sparkling.
(b) Accommodation depends essentially on the curvature of the crystalline lens, and this curvature is regulated by the ciliary muscle. When the ciliary muscle contracts, the capsule of the lens relaxes, and the elastic lens bulges forward and becomes more convex, i. e., accommodates for near objects. But when this muscle is paralyzed, the capsule of the lens is drawn, the lens is more flattened, and it is impossible to focus the sight on near objects. A drug that paralyzes accommodation in this manner is a cycloplegic. Atropine is strongly cycloplegic. This effect on accommodation does not take place until some time after the pupil has begun to dilate, and it wears off more quickly than the effect on the pupil; but until the power of accommodation is nearly restored, the patient cannot read or see near objects clearly.
In fitting glasses paralysis of accommodation is necessary. A 1: 200 solution of atropine sulphate usually paralyzes accommodation in one hour, but restoration does not take place for several days.
The normal eyeball tension depends chiefly on two factors, viz.: (1) The amount of intra-ocular secretion, and (2) the freedom with which fluids may escape through the efferent lymph-channels, i. e., through the spaces of Fontana at the margin of the pupil, into the canal of Schlemm. The tension may be raised either by extra secretion or by dilatation of the pupil which results in shutting off the spaces of Fontana. It is by dilatation of the pupil that atropine causes the increase of tension. In glaucoma, a disease in which the tension is already high, atropine may produce a dangerous condition; and even when there is merely a glaucomatous tendency, it may precipitate an attack of glaucoma.
Atropine gives moderate relief from the pains of iritis and other intra-ocular inflammations.
Since atropine is highly selective, the same ocular effects may be seen after the internal administration of large doses. An antagonist of atropine is physostigmine, which stimulates the ends of the third nerve. It is not powerful enough to remove the effects of atropine at once, but greatly lessens the time which the eye takes to return to normal.
Probably no direct action. The smooth muscle of the viscera is weakened by the depression of motor nerve-endings mentioned above.
Stimulation of the sciatic nerve of a normal cat regularly induces sweating of the foot. In an atropinized animal sweating cannot be induced. The profuse sweating of pilocarpine is checked by atropine, also the sweating from certain other drugs, such as aspirin and phenacetin; also the night-sweats of tuberculosis.
After all the nervous connections are severed, the breasts still have the power to secrete milk, though the secretion is less in amount. Hence atropine, which merely cuts off the nervous influences, tends to reduce the milk secretion very little, and cannot cause the complete stoppage of the secretion. The drug acts when applied to the breasts, as well as when taken by mouth.