Structure: < Click on image >


Light Reflex. The diagram on the left shows the visual pathway from the eyes to the superior colliculi in the midbrain, and the pathways back out to the eye from the midbrain nucleus (the Edinger-Westphal nucleus) important in light reflexes. Note that light projected in only one eye will cause constriction of both pupils: the ipsilateral, because of the direct (same side) light reflex, and the contralateral, because of the consensual (crossed) light reflex. The consensual light reflex is possible because of the crossing fibers in the optic chiasm and in the posterior commissure: visual input from one eye thus reaches the Edinger-Westphal (EW) nucleus on both sides. The outflow from the EW nucleus travels with the oculomotor nerve (III) to reach the ciliary ganglion. Postganglionic fibers go to the pupillary constrictor muscles in the iris. (Note: the ciliary muscles also receive output from the EW nucleus. During accommodation for near vision the ciliary muscles contract, relaxing the tension of the zonule fibers and allowing the lens to round up, thus increasing its focusing power). The insert at the right is an MRI axial (horizontal) scan at about the level of the drawing.



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