Pupillary light reflexes are measured based on a 0 to 4+ gradient that considers the magnitude and speed of the light response. Of note, the pupillary dark reflex involves a separate pathway, which ends with sympathetic fibers from long ciliary nerves innervating the dilator pupillae muscle. And, because of the crossing fibers, there is not only a direct pupillary reflex but also a consensual pupillary light reflex. This extensive pathway is being tested when a light is shined in the eyes. The contraction of the iris sphincter muscles leads to pupillary constriction (miosis). Efferent parasympathetic preganglionic fibers travel on the oculomotor nerve and synapse with the ciliary ganglion, which sends postganglionic axons to directly innervate the iris sphincter muscles. There are a minority of axons that go to the hypothalamus and the olivary pretectal nucleus (OPN).
Each pretectal area sends bilateral signals to the preganglionic parasympathetic nuclei in the midbrain called Edinger-Westphal nuclei. The optic tracts join the brachium of the superior colliculus, and then signals travel to the pretectal area of the midbrain. Thus, the right optic tract will contain temporal retinal fibers from the right eye, as well as nasal retinal fibers from the left eye. At the optic chiasm, nasal retinal fibers will cross to the contralateral side of the optic tract, and the temporal retinal fibers continue on the ipsilateral side. The optic nerve then forms the optic chiasm, which diverges into a left and right optic tract. These are the first steps of the pupillary light reflex afferent pathway. The optic nerve sends impulses to the brain for further processing and image recognition. These signals are then relayed to the bipolar cells, which interact with ganglion cells, which in turn coalesce to form the optic disc and optic nerve (CN II). Photoreceptor cells in the outer layers of the retina, which are called rods and cones, convert light stimuli into neuronal impulses. Light travels through the cornea, anterior chamber, pupil, lens, and the posterior chamber, eventually reaching the retina.
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