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Both these reflexes affect both eyes, even if only one eye is stimulated.
The pattern of pupillary response to light can help determine which of the cranial nerves is damaged. There are two types of response assessed for each eye:
The reflex pathway consists of retinal ganglion cells, which convey information from the photoreceptors to the optic nerve which connects to the pretectal nucleus of the high midbrain. It bypasses the lateral geniculate nucleus and the primary visual cortex. From the pretectal nucleus neurons send axons to neurons of the Edinger-Westphal nucleus whose axons run along both the left and right oculomotor nerves. Oculomotor nerve axons synapse on ciliary ganglion neurons whose axons innervate the constrictor muscle of the iris.
Test for nerve damage
Emergency room physicians often assess the pupillary reflex because it is useful for gauging brain stem function. Normally, pupils react (constrict) equally. Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain death and depressant drugs, such as barbiturates. The optic nerve is responsible for the afferent limb of the pupillary reflex, or in other words, senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex; in other words, it drives the muscles that constrict the pupil.
Normally, each pupil should constrict with light shone into either eye. On testing each reflex for each eye, several patterns are possible.
Pupillary Accommodation Reflex
The pupillary accommodation reflex reduces the size of the pupil when an object is close to the eye. A smaller pupil produces a sharper image on the retina. There is also a separate accommodation reflex which changes the shape of the lens so as to focus the image on the retina.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pupillary_reflex". A list of authors is available in Wikipedia.|