The Pupillary Light Reflex

Clinical Exam

At the time of the pupillary light reflex, the pupils should be examined for symmetry. Resting pupils should be equal in diameter, although slight differences are insignificant. Unequal pupil size is called ansiocoria. When performing the PLR examination, a bright light is shone into one eye at a time. Both pupils should constrict, with a direct response in the tested eye being slightly stronger than a consensual response in the opposite eye. When a bright light is initially flashed into the eye, a blink response may occur. This is referred to as the dazzle reflex and probably does not involve the forebrain but rather thalamic nuclei. If a poor quality light source is used, the stimulated pupil may dilate slightly with continued stimulation. This is referred to as pupillary escape.

Anatomic Pathway

Light penetrates the globe of the eye to the retina. Electrochemical receptors in the retina are excited and the generated impulses are projected into the optic nerve. Light is next projected from the retina through the optic nerve to the optic chiasm where, in dogs, 75% of the optic nerve fibers cross over to enter the contralateral optic tract (65% in cats). After crossing at the chiasm, the impulse is projected through the optic tract to synapse on the pretectal nucleus. Pretectal neurons then predominantly synapse on the opposite oculomotor nuclei (parasympathetic portion). Stimulation is relayed along the axons of the parasympathetic portion of CN III out of the brainstem and to the ciliary ganglion. Post-ganglionic fibers in the short ciliary nerves innervate the ciliary muscles and the pupillary constrictor muscles.

Overview of Anatomy

The initial part of the afferent or sensory visual pathway (optic nerve (CN II), the optic chiasm, and the optic tract,) is evaluated in conjunction with the pathway responsible for constriction of the pupil (the rostral mesencephalon or midbrain, specifically the parasympathetic component of the CN III nucleus; the oculomotor nerve; the ciliary ganglion; the short ciliary nerves and the ciliary muscle of the pupil).

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Anatomic Sections

Light penetrates globe of the eye to the retina. Electrochemical receptors in the retina are excited and the generated impulses are projected into the optic nerve. Light is next projected from the retina through the optic nerve to the optic chiasm where, in dogs, 75% of the optic nerve fibers cross over to enter the contralateral optic tract (65% in cats). After crossing at the chiasm, light is projected through the optic tract to synapse on the pretectal nucleus. Pretectal neurons then predominantly synapse on the opposite oculomotor nuclei (parasympathetic portion). Stimulation is relayed along the axons of the parasympathetic portion of CN III out of the brainstem and to the ciliary ganglion. Post-ganglionic fibers in the short ciliary nerves innervate the ciliary muscles and the pupillary constrictor muscles.