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Kufa college of medicine, Department of medical physiology Dr.Zaid Yousif Shukur M.B.Ch.B , M.Sc. , ICO , D.O PUPILLARY LIGHT REFLEX PATHWAY Kufa college of medicine Department of medical physiology By Dr.Zaid Yousif Shukur M.B.Ch.B , M.Sc. , ICO , D.O The pupil is round, regular shape and nearly equal in size. There may rarely be a physiological difference in pupil size (physiological anisocoria). Each pupil is located with its center a little below and slightly to the nasal side of the center of the cornea. The pupils are dilated in excitement, in the dark, and with mydriatic agents like atropine, conversely they are constricted in neonates, during sleep, with ocular convergence as part of the near reflex, with use of the miotic agents like acetylcholine and pilocarpine, and also they are constricted with light directed toward the eye. The pupils reach their maximum diameter at about the age of 21 years of age and constricted thereafter to become small in old age (involutional miosis). The pupil is regarded miotic if its diameter is less than 2 mm and mydriatic if they are more than 6 mm. Light reflex The light reflex consists of a simultaneous and equal constriction of the pupils in response to illumination of one or the other eye. The afferent pupillary pathway originate in the retina , the axons of retinal ganglion cells pass into the optic nerve and decussate in the chiasm and pass with the optic tract to the midbrain, the pupillary fibers do not synapse with the visual fibers in the lateral geniculate body(LGB)but pass to the pretectal nuclei (PTN) at the level of the superior colliculus with intercalated fibers that pass as the efferent pupillary pathway to the Edinger-Westphal nucleus (EWN)of the oculomotor nerve (CN III)on each side (both sides) Preganglionic parasympathetic fibers run in the oculomotor nerve as it leaves the brain stem. The fibers pass downward to lie inferiorly in the inferior division of the third nerve as it enters the orbit. These fibers synapse in the ciliary ganglion and give rise to postganglionic parasympathetic myelinated short ciliary nerves, about 3%-5% of which are pupillomotor. The rest are designated for the ciliary muscle and are concerned with the near reflex. Baseline Pupil Size The most important influence on pupillary size is the amount of ambient light. The status of retinal adaptation also affects pupil size. Sleep results in decreased sympathetic activity and therefore somewhat smaller pupils. Pupils also, overall, become smaller with age. Arousal, excitement, or startle increases pupil size. Elevated intraocular pressure may result in enlargement of the pupil, possibly by inducing ischemia in the iris. Anoxic brain injury often results in pupillary dilatation, probably from midbrain ischemia. Anisocoria: Efferent disturbances of the pupil size are usually unilateral, which produces inequality in the diameters of the two pupils. This condition is called anisocoria. Although it is important for the clinician to look for anisocoria when assessing the pupil, the presence of anisocoria does not necessarily represent an abnormality. In many individuals, anisocoria is physiologic. Physiologic Anisocoria Physiologic anisocoria (also known as simple or essential anisocoria) is the most common cause of a difference in pupillary size. About 20% of individuals have noticeably different diameters in their two pupils. A pupillary inequality of 0.2 mm can be detected clinically, and anisocoria of 0.4 mm is readily visible. Anisocoria is more common in older adults. The amount of anisocoria in an individual can vary from day to day. The Pupillary light reflex Pupillary light reflex should be studied in illumination just bright enough to permit observation of the pupils, the subject is directed to gaze in a distance, attention should be directed to any difference in size(anisocoria), shape(dyscoria), or position(corectopia). When the amount of light falling on the eye is increased, the pupil will constrict, there is a latent period of(0.18) second then constriction begins, and maximum contraction of the sphincter pupillae occur about 1 second after the beginning of the stimulation. Light falling on one eye will cause pupillary constriction of the other eye simultaneously and to a similar degree (Indirect or consensual pupillary light reflex)