The actions of the Edinger-Westphal nucleus are important in focusing the object of interest on the fovea. Recall from Fig. 1 that the lens separates the eyeball into two major chambers: The anterior chamber lies between the cornea and the lens and is filled with aqueous humor; the vitreal chamber is located between the lens and the retina and is filled with a highly viscous fluid, the vitreous humor, which provides pressure to keep the globe spherical. Light rays are refracted as they pass through these transparent components in the anterior of the eye, particularly the cornea. The curvature of the normal cornea will cause parallel light rays from distant objects to converge at a point roughly 2.4 cm behind it, which is about the distance from the cornea to the retina. A relatively small amount of additional focusing power (approximately 25%) is provided by the lens for bringing distant objects into proper focus. This assumes that the jelly-like lens is being held in a relatively flattened shape by tension on the ligaments that keep it suspended within the circular ciliary muscle of the eye. If the object of interest is closer than about 9 m, additional focusing power must be provided by the lens. This is accomplished through the accommodation reflex.

Accommodation Reflex (Near Reflex)

Adaptation of the visual apparatus of the eye for near vision occurs as a reflex action involving cortical, brain stem, and eye structures. The reflex is initiated when visual attention is directed to a nearby object. The initial image of the near object (unfocused) is sent to the occipital cortex via the primary visual pathway. Information is then sent from the visual association areas back to the upper brain stem regions and cranial nerve III. Stimulation of the Edinger-Westphal nucleus causes contraction of ciliary muscle, releasing tension on the suspensory ligaments of the lens and allowing the lens to assume a more spherical shape. The increased curvature of the lens brings the object into focus. Activation of the Edinger-Westphal nucleus also contracts the pupillary constrictor muscle and decreases pupil size, thus blocking certain types of optical aberrations. Stimulation of specific somatic nuclei of cranial III causes both medial rectus muscles to converge the eyes, so that the image falls on the foveal region of both eyes.

Pupillary Light Reflex

When light is directed into the pupil of one eye, both pupils constrict. The response of the illuminated pupil is called the direct pupillary light reflex; that of the other pupil is called the consensual pupillary light reflex. Light activation of the retina is transmitted, via the optic nerve and tract, to nuclei in the midbrain both on the ipsilateral side, to elicit a direct reflex, and via crossing fibers in a structure called the posterior commissure, on the contralateral side, to elicit a consensual reflex. These midbrain areas project to the Edinger-Westphal nuclei, which in turn cause constriction of the pupil. A test of the pupillary light reflex is usually included in standard neurological exams. An altered consensual reflex usually denotes damage to structures localized within the upper

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