hello i'm james and welcome to another sutton brain hub video where we'll be covering the pupillary light reflex but what is the pupillary light reflex the pupillary light reflex regulates the amount of light entering the eye by adjusting the diameter of the pupil according to the intensity of light this allows the eye to see in a wide range of light levels the components of the pupillary light reflex firstly we have the eye with the photosensitive retinal ganglion cells then the optic nerve which is part of the visual pathway the pretectal nucleus where a minority of optic nerve synapse the edding of s-vowel nucleus which is a parasympathetic nucleus responsible for mediating the constriction of the pupil then the ciliary ganglion where the cell bodies of the short ciliary nerves are and finally the sphincter papillae muscles which act to constrict the pupil i'm now going to talk through the pathway of the pupillary light reflex we start in the eye with the retinal ganglion cells of the retina receiving visual information from photoreceptors in the retina and conveying sensory information via the optic nerve a proportion of axons deviate from synapsing at the lateral genetic nucleus and instead project to synapse at the pre-tectal nucleus the pretectal nucleus then bilaterally projects there's an adding of s-found nuclei parasympathetic fibers from the adding of vespal nuclei then travel with the oculomotor nerve to innervate the ciliary ganglia finally short ciliary nerves project to innervate the sphincter papillae muscle of the iris to contract it this results in constriction of both pupils the eye that the light was shown into is said to have a direct reflex whereas the opposite eye is said to have a consensual reflex let's take a closer look at the eye to see how this appears clinically again it is normal on examination to see constriction of the pupil you're shining the light into in the direct reflex and constriction of the opposite eye in a consensual reflex pupil constriction is due to parasympathetic action of the pupillary sphincter muscle of each eye let's finally take a look at the muscles controlling pupil size there are two muscles found in the iris that control the diameter of the pupil the constrictor or sphincter and the dilator pupillae the constricted pupillae has a circular arrangement in the iris and is innovated by the parasympathetic nervous system to constrict the pupils in contrast the dilated pupillae runs readily in the iris and is innovated by the sympathetic nervous system to dilate the pupil so that concludes this video on the pupillary light reflex thank you for your attention during this video i hope it's been helpful please make sure to check out more sun brain hub content on our youtube channel for more new anatomy help find us on facebook instagram and subscribe to our youtube channel to help explain the mysteries of the brain