The pupillary light reflex consists of bilateral pupil constriction, which is also called miosis, to light shone in either eye. Light shone in one eye is detected by the retina, which projects this information through axons in the ipsilateral optic nerve to the optic chiasm. At the optic chiasm these axons split so that half project through the ipsilateral optic tract and half project through the contralateral optic tract. Some of these axons enter the upper brainstem to synapse in bilateral pretectal nuclei, each of which receives light information from both eyes.
Axons from each pretectal nucleus then project a short distance anteriorly to synapse in both Edinger-Westphal nuclei, which contains preganglionic parasympathetic neurons. Axons from these neurons then project through the ipsilateral oculomotor nerve to the ipsilateral orbit where they synapse in the ciliary ganglion, which contains postganlglionic parasympathetic neurons. Axons of these neurons then project to the iris to innervate the iris sphincter muscle, which is smooth muscle, causing pupil constriction. Therefore, light shone in one eye causes constriction of the ipsilateral pupil, called the direct pupillary light response, as well as causing constriction of the contralateral pupil, called the consensual pupillary light response.
An afferent pupillary defect is caused by dysfunction of the ipsilateral eye, retina, or optic nerve, so that both pupils constrict equally with light shone in the normal side, but neither pupil constricts normally when light is shone in the abnormal side. With an afferent pupillary defect the pupils remain symmetric in size in the light or dark because the efferent limbs of the pupillary light reflex are normal. Unilateral dysfunction of the efferent limb of the pupillary light reflex causes asymmetric pupil sizes, which is called anisocoria. The anisocoria becomes worse in the light (or with light shone in either eye), because the afferent limbs are normal bilaterally, and pupillary constriction is normal on the normal side. The anisocoria becomes better in the dark, because pupillary dilation is normal bilaterally. Many people have the normal variant called physiologic anisocoria, where the pupils are slightly different sizes, but the difference is small, and both pupils are normally reactive to light (the size difference is about the same in the light or dark).
The parasympathetic neurons that control iris constriction also control smooth muscle around the lens called the ciliary muscle. The ciliary muscle controls the shape of the lens for accommodation, which refers to focusing the image on the retina for visual objects at different distances from the eye. Abnormalities of this pathway may cause blurry vision for objects are certain distances from inability to accommodate normally.