Материал: Kaplan USMLE-1 (2013) - Anatomy

Section IV • Neuroscience

Open-Angle Glaucoma

A chronic condition (often with increased intraocular pressure [IOP]) due to de­ creased reabsorption of aqueous humor, leading to progressive (painless) visual loss and, ifleft untreated, blindness. IOP is a balance between fluid formation and its drainage from the globe.

Narrow-Angle Glaucoma

An acute (painful) or chronic (genetic) conditionwith increased IOP due to block­ ade ofthe canal ofSchlemm. Emergencytreatmentprior to surgeryoften involves cholinomimetics, carbonic anhydrase inhibitors, and/or mannitol.

VISUAL REFLEXES

Pupillary Light Reflex

When light is directed into an eye, it stimulates retinal photoreceptors and results in impulses carried in the optic nerve to the pretectal area. Cells in the pretectal area send axons to the Edinger-Westphal nuclei on both sides.

The Edinger-Westphal nucleus is the parasympathetic nucleus ofthe oculomotor nerve and gives rise to preganglionic parasympathetic fibers thatpass in the third cranial nerve to the ciliary ganglion. Because cells in the pretectal area supply both Edinger-Westphal nuclei, shining light into one eye results in constriction of both the ipsilateral pupil (direct light reflex) and contralateral pupil (consensual light reflex).

Accommodation-Convergence Reaction

This reaction occurs when an individual attempts to focus on a nearbyobject after looking at a distant object. The oculomotor nerve carries the efferent fibers from the accommodation-convergence reaction, which consists of3 components: ac­ commodation, convergence, and pupillary constriction.

Accommodation refers to the reflex that increasesthe curvature ofthe lens needed for nearvision. Preganglionic parasympathetic fibers arise in the Edinger-Westphal nucleus and pass via the oculomotor nerve to the ciliary ganglion. Postganglionic parasympathetic fibers from the ciliaryganglionsupplythe ciliarymuscle. Contrac­ tion ofthis muscle relaxes the suspensory ligaments and allows the lens to increase its convexity (become more round). This increases the refractive index ofthe lens, permittingtheimage ofa nearbyobject to focus on the retina.

Convergence results from contraction ofboth medial rectus muscles, which pull the eyes to look toward the nose. This allows the image ofthe near object to focus on the same part ofthe retina in each eye.

Pupillaryconstriction (miosis) results from contraction ofthe constrictor mus­ cle ofthe iris. A smalleraperturegives the optic apparatus agreater depth offield. With Argyll Robertson pupils, both direct and consensual light reflexes are lost, but the accommodation-convergence reaction remains intact. This type ofpupil is often seen in cases ofneurosyphilis; however, it is sometimes seen in patients with multiple sclerosis, pineal tumors, or tabes dorsalis. The lesion site is believed to occur near the pretectal nuclei just rostral to the superior colliculi.

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