The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict. First described by Rhazes of Baghdad in the ninth century, the PLR has since become a standard examination tool for all healthcare professionals.
Terminology The pupil is the dark circular opening in the center of the iris and is where light enters the eye. By analogy with a camera, the pupil is equivalent to aperture, whereas the iris is equivalent to the diaphragm. It may be helpful to consider the Pupillary reflex as an 'Iris' reflex, as the iris sphincter and dilator muscles are what can be seen responding to ambient light, whereas the pupil is the passive opening formed by the active iris. Pupillary reflex is synonymous with pupillary response, which may be pupillary constriction or dilation. Pupillary reflex is conceptually linked to the side (left or right) of the reacting pupil, and not to the side from which light stimulation originates. Left pupillary reflex refers to the response of the left pupil to light, regardless of which eye is exposed to a light source. Right pupillary reflex means reaction of the right pupil, whether light is shone into the left eye, right eye, or both eyes. When light is shone into only one eye and not the other, it is normal for both pupils to constrict simultaneously. The terms direct and consensual refers to the side where the light source comes from, relative to the side of the reacting pupil. A direct pupillary reflex is pupillary response to light that enters the ipsilateral (same) eye. A consensual pupillary reflex is response of a pupil to light that enters the contralateral (opposite) eye. Thus there are four types of pupillary light reflexes, based on this terminology of absolute laterality of the reacting pupil (left versus right) and relative laterality of the light source (same side versus opposite side, ipsilateral versus contralateral, direct versus consensual):
Left direct pupillary reflex is the left pupil's response to light entering the left eye, the ipsilateral eye. Left consensual pupillary reflex is the left pupil's indirect response to light entering the right eye, the contralateral eye. Right direct pupillary reflex is the right pupil's response to light entering the right eye, the ipsilateral eye. Right consensual pupillary reflex is the right pupil's indirect response to light entering the left eye, the contralateral eye.
Neural pathway anatomy The pupillary light reflex neural pathway on each side has an afferent limb and two efferent limbs. The afferent limb has nerve fibers running within the optic nerve (CN II). Each efferent limb has parasympathetic nerve fibers traveling along the periphery of the oculomotor nerve (CN III). The afferent limb carries sensory input. Anatomically, the afferent limb consists of the retina, the optic nerve, and the pretectal nucleus in the midbrain, at level of superior colliculus. Ganglion cells of the retina project fibers through the optic nerve to the ipsilateral pretectal nucleus. The efferent limb is the pupillary motor output from the pretectal nucleus to the pupillary sphincter of the iris. The pretectal nucleus projects nerve fibers to the ipsilateral and contralateral Edinger-Westphal nuclei, which are also located in the midbrain. Each Edinger-Westphal nucleus gives rise to preganglionic parasympathetic fibers which exit with CN III and synapse with postganglionic parasympathetic neurons in the ciliary ganglion. Postganglionic nerve fibers leave the ciliary ganglion to innervate the pupillary sphincter. Each afferent limb has two efferent limbs, one ipsilateral and one contralateral. The ipsilateral efferent limb transmits nerve signals for direct light reflex of the ipsilateral pupil. The contralateral efferent limb causes consensual light reflex of the contralateral pupil. Sympathetic nervous system plays a role in dilating the pupils in low light conditions.
Types of neurons The optic nerve, or more precisely, the photosensitive ganglion cells through the retinohypothalamic tract, is responsible for the afferent limb of the pupillary reflex; it senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex; it drives the iris muscles that constrict the pupil.
Retina: The pupillary reflex pathway begins with the photosensitive retinal ganglion cells, which convey information via the optic nerve, the most peripheral, distal, portion of which is the optic disc. Some axons of the optic nerve connect to the pretectal nucleus of the upper midbrain instead of the cells of the lateral geniculate nucleus (which project to the primary visual cortex). These intrinsic photosensitive ganglion cells are also referred to as melanopsin-containing cells, and they influence circadian rhythms as well as the pupillary light reflex. Pretectal nuclei: From the neuronal cell bodies in some of the pretectal nuclei, axons synapse on (connect to) neurons in the Edinger-Westphal nucleus. Those neurons are the preganglionic cells with axons that run in the oculomotor nerves to the ciliary ganglia. Edinger-Westphal nuclei: Parasympathetic neuronal axons in the oculomotor nerve synapse on ciliary ganglion neurons. Ciliary ganglia: Short post-ganglionic ciliary nerves leave the ciliary ganglion to innervate the Iris sphincter muscle of the iris.
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