Consensual Pupillary Light Reflex
The pupillary light reflex is a reflex that constricts or dilates in response to increased or decreased illumination of the retina (the photosensitive lining of the posterior part of the eye). Most mammals experience consensual pupillary light reflex in which light directed at one eye causes increased illumination of the retina not only of the same eye, but also that of the other eye. Therefore, light directed at one eye causes the stimulated eye to constrict as well as the opposite pupil. Among vertebrates, there is a wide range in the size of consensual pupillary light reflex compared to direct pupillary light reflex. In humans, 100 percent of pupillary light reflex is consensual, while there is 0 percent in rabbits (Trejo et al., 1989). This variation may be related to the proportion of uncrossed fibers in the optic tract and the extent of binocularity in the mammal. However, some animals exhibit inconsistent observations and therefore further research must be conducted in order to solidify this theory.
The pupillary light reflex provides a useful diagnostic tool in testing the integrity of the sensory and motor functions of the eye. A lack of a consensual pupillary light reflex is often taken as a sign of serious neurological disorder involving the brainstem. In general, cranial nerve reflexes fall into the polysynaptic category due to interneurons that take sensory signals and transmit motor signals to bilateral sides. For instance, a bright penlight in one eye causes both pupils to constrict. If the light causes only the eye with the light to constrict, there is a problem with the motor neuron (oculomotor nerve) on the contralateral side. If there is no pupillary constriction when a penlight is shone, the problem likely exists with that ipsilateral (same side) optic nerve. This understanding can help identify problems with the nerves (such as inflammation or tumors) or with the brain itself (such as tumors or strokes).
Vivian Vu and Jennifer L. Hellier
See also: Blink Reflex; Brainstem; Central Nervous System; Cranial Nerves; Neurological Examination; Visual System
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Trejo, L. J., M. N. Rand, & C. M. Cicerone. (1989). Consensual pupillary light reflex in the pigmented rat. Vision Research, 29(3), 303—307.