littlebug wrote:
The attachment is a question from NBME. Could someone explain why optic tract is the right answer? I thought when the afferent neuron is damaged, the pupil should be big, since No light input due to neuron damage means dark. Moreover, optic tract conveys the signal from half retina of both sides. If half retina does not work, the other half should be able to elicit light reflex. If somebody knows, please explain.
thanks a lot!
Optic nerve is a pure sensory nerve. therefore compression of the optic tract should not cause any change in the diameter of pupil.
Pupil constriction is controlled by parasympathetic fibers relaying with the oculomotor nerve to the sphincter of the pupil via the ciliary ganglion. Even though an aneurysm of the anterior cerebral artery can compress the oculomotor nerve, it is not an option in the answer choices.
Cervical spi cord on the other hand carry sympathetic fibers to the radial dilator muscles of pupil, therefore a lesion may cause constricted pupil (aka horner syn). Please corect me if i am wrong
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| macintosh
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By the way here is the question again. (OCRed for easy viewing)
35 A 48-year-old man comes to his physician because his wife noticed that his right pupil is "small.'' Physical examination shows that the right pupil is constricted and does not react to light. His left pupil is normal. These findings are most likely due to a lesion Involving which of the following structures on the right?
A) Cervical spinal cord
B) Frontal eye fie ds
C) Lateral geniculate nucleus
D) Optic tract
E) Visual cortex
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