Pupillary Light Reflex Pathway

Pupillary Light Reflex Pathway, is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina of the eye, thereby assisting in adaptation to various levels of darkness and light, in addition to retinal sensitivity. Greater intensity light causes the pupil to become smaller “Miosis” (allowing less light in), whereas lower intensity light causes the pupil to become larger “Mydriasis“ (allowing more light in). Thus, the Pupillary Light Reflex Pathway regulates the intensity of light entering the eye by constricting or dilating the pupils. 

Pupillary Light Reflex Pathway, Part 1:

Pupillary Light Reflex Pathway, Part 2:

Mechanism of  Pupillary Light Reflex Pathway:

The optic nerve, or more precisely, the photosensitive ganglion cells through the retinohypothalamic tract, is responsible for the afferent limb of the Pupillary Light Reflex Pathway – it senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex – it drives the muscles that constrict the pupil.

Neuron 1:

The Pupillary Light Reflex Pathway begins with the photosensitive retinal ganglion cells, which convey information to the optic nerve (via the optic disc). The optic nerve connects to the pretectal nucleus of the upper midbrain, bypassing the lateral geniculate nucleus and the primary visual cortex. These “intrinsic photosensitive ganglion cells” are also referred to as “melanopsin-containing” cells, and they influence the circadian rhythms and the pupillary light reflex.

Neuron 2:

From the pretectal nucleus, axons connect to neurons in the Edinger-Westphal nucleus, whose axons run along both the left and right oculomotor nerves.

Neuron 3:

Parasympathetic neurons from the oculomotor nerve synapse on ciliary ganglion neurons.

Neuron 4:

Short ciliary nerves leave the ciliary ganglion to innervate the constrictor muscle of the iris.