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The pupil refers to the central opening in the iris. It acts as an aper-ture to improve the quality of the resulting image by controlling the amount of light that enters the eye.
This reflex arc consists of an afferent path that detectsand transmits the light stimulus and an efferent path that supplies the muscles of the iris (Fig. 9.1).
Afferent path.This path begins at the light receptors of the retina (Fig. 9.1, A),continues along the optic nerve (B), the optic chiasma (C) where some of the fibers cross to the opposite side. The path continues along the optical tracts until shortly before the lateral geniculate body (E).
There the afferentreflex path separates from the visual pathway and continues to the pretectalnuclei (F) and from there to both Edinger-Westphal nuclei (G). Each of the two pretectal nuclei conduct impulses to both Edinger-Westphal nuclei. This bilateral connection has several consequences:
❖Both pupils will normally be the same size (isocoria) even when one eye is blind. Deviations up to 1 mm are normal
❖Both pupils will narrow even when only one eye is illuminated (consensuallight reflex).
Efferent parasympathetic path.This path begins in theEdinger-Westphalnucleus (G). Its nerve fibers form the parasympathetic part of the oculomotornerve (H) and travel to the ciliary ganglion (I) in the orbit. Postganglionic nerve fibers pass through the short ciliary nerves to the effector organ, the sphincter pupillae muscle (J).
Perlia’s nucleus and the Edinger-Westphal nuclei are also responsible for the near reflex, which consists of accommodation, convergence, and miosis.
Efferent sympathetic nerve supply to the pupil.Three neurons connected bysynapses supply the pupil (Fig. 9.2):
❖Thecentral first neuron begins in the posterior hypothalamus (A), passes the brain stem and the medulla oblongata to the ciliospinal center (Budge’s center; B) in the cervical spinal cord (C8 –T2).
❖ Thepreganglionic second neuron extends from the ciliospinal center through the white rami communicantes and sympathetic trunk (C) to the superior cervical ganglion (D). It is vulnerable to certain lesions such as Pancoast tumors because it is immediately adjacent to the tip of the lung.
❖Thepostganglionic third neuron extends from the superior cervical gan-glion as a neural plexus along the internal carotid artery, ophthalmic artery, and long ciliary nerves to the effector organ, the dilator pupillaemuscle (E).
Pupil size ranges from approximately 1 mm (miosis) toapproximately 8 mm (mydriasis).
❖ Pupils tend to be wider in teenagers and in darkness. They are also wider with joy, fear, or surprise due to increased sympathetic tone, and when the person inhales deeply.
❖ Pupils tend to be narrower in the newborn due to parasympathetic tone, in the elderly due to decreased mesencephalic inhibition and sympathetic diencephalic activity, in light, during sleep, and when the person is fatigued (due to decreased sympathetic activity).
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