The Pupil in Clinical Diagnosis

The muscle fibers in the iris which regulate pupil size are innervated by sympathetic and parasympathetic divisions of the autonomic nervous system. The sphinctel; a 1 mm circular band at the pupillary margin, is under parasympathetic control and is activated in response to light stimuli or during accommodation (near response). Afferent fibers for redex pupil constriction ~ light originate in the retina and run parallel to visual afferents in the optic nerve (Figure 1). Approximately 53% of light reflex afferents cross the optic chiasm to travel in the contralateral optic tract, whereas 47% remain ipsilateral. The pupil afferents leave the optic tract in its posterior one-third to enter the midbrain and synapse in the pretectal nucleus. Fibers then hemidecussate around the sylvian aqueduct and innervate each Edinger-Westphal nucleus. This hemidecussation provides essentially equal afferent drive from each eye to each Edinget Westphal nucleus.