Stimulation of the parasympathetic nerves also excites the pupillary sphincter muscle, thereby decreasing the pupillary aperture; this process is called miosis. Conversely, stimulation of the sympathetic nerves excites the radial fibers of the iris and causes pupillary dilation, which is called mydriasis.
Pupillary Light Reflex. When light is shone into the eyes, the pupils constrict, a reaction called the pupillary light reflex. The neuronal pathway for this reflex is demonstrated by the upper two black arrows in Figure 1. When light impinges on the retina, a few of the resulting impulses pass from the optic nerves to the pretectal nuclei. From here, secondary impulses pass to the Edinger-Westphal nucleus and, finally, back through para sympathetic nerves to constrict the sphincter of the iris. Conversely, in darkness, the reflex becomes inhibited, which results in dilation of the pupil.
Fig1. Autonomic innervation of the eye, showing also the reflex arc of the light reflex. N., nerve. (Modified from Ranson SW, Clark SL: Anatomy of the Nervous System: Its Development and Function, 10th ed. Philadelphia: WB Saunders, 1959.)
The function of the light reflex is to help the eye adapt extremely rapidly to changing light conditions, as explained in Chapter 51. The limits of pupillary diameter are about 1.5 millimeters on the small side and 8 millimeters on the large side. Therefore, because light brightness on the retina increases with the square of pupillary diameter, the range of light and dark adaptation that can be brought about by the pupillary reflex is about 30 to 1— that is, up to as much as 30 times change in the amount of light entering the eye.
Pupillary Reflexes or Reactions in Central Nervous System Disease. A few central nervous system diseases damage nerve transmission of visual signals from the retinas to the Edinger-Westphal nucleus, thus sometimes blocking the pupillary reflexes. Such blocks may occur as a result of central nervous system syphilis, alcoholism, encephalitis, and so forth. The block usually occurs in the pretectal region of the brain stem, although it can result from destruction of some small fibers in the optic nerves.
The final nerve fibers in the pathway through the pretectal area to the Edinger-Westphal nucleus are mostly of the inhibitory type. When their inhibitory effect is lost, the nucleus becomes chronically active, causing the pupils to remain mostly constricted, in addition to their failure to respond to light.
Yet the pupils can constrict a little more if the Edinger Westphal nucleus is stimulated through some other path way. For instance, when the eyes fixate on a near object, the signals that cause accommodation of the lens and those that cause convergence of the two eyes cause a mild degree of pupillary constriction at the same time. This phenome non is called the pupillary reaction to accommodation. A pupil that fails to respond to light but does respond to accommodation and is also very small (an Argyll Robertson pupil) is an important diagnostic sign of a central nervous system disease such as syphilis.
Horner’s Syndrome. The sympathetic nerves to the eye are occasionally interrupted. Interruption frequently occurs in the cervical sympathetic chain, which causes the clinical condition called Horner’s syndrome. This syndrome consists of the following effects: First, because of interruption of sympathetic nerve fibers to the pupillary dilator muscle, the pupil remains persistently constricted to a smaller diameter than the pupil of the opposite eye. Second, the superior eyelid droops because it is normally maintained in an open position during waking hours partly by contraction of smooth muscle fibers embedded in the superior eyelid and innervated by the sympathetics. Therefore, destruction of the sympathetic nerves makes it impossible to open the superior eyelid as widely as normally. Third, the blood vessels on the corresponding side of the face and head become persistently dilated. Fourth, sweating (which requires sympathetic nerve signals) cannot occur on the side of the face and head affected by Horner’s syndrome.
