Control of Pupil Size

Our pupils become larger when we move to a darker environment, and they constrict again when we move back into the light. This light reflex is mediated via contraction and relaxation of the sphincter pupillae muscle, innervated by parasympathetic final motorneurons located in the ciliary ganglia behind the eyeballs. In turn, the final motoneurons are regulated via inputs from brain stem parasympathetic moto-neurons in the Edinger-Westfal subdivision of the oculomotor (cranial III) nucleus located in the mid-brain, ventral to the aqueduct. The afferent limb of the light reflex involves activity in optic nerve (cranial II) fibers, with a branch to the pretectal nucleus in the superior colliculus of the midbrain and from there to the parasympathetic motorneurons in the Edinger-Westfal nucleus.

The pupils also dilate when strong emotions are experienced, such as when we are surprised, afraid, or sexually aroused. This active dilatation is mediated by sympathetic innervation of the dilator pupillae muscle. The final cell bodies are located in the superior cervical ganglion, near the bifurcation of the common carotid artery. The preganglionic cell bodies are in the upper thoracic spinal cord. The pathway descending from the brain is a principally unilateral direct projection from the paraventricular nucleus of the hypothalamus. Interruption of the pupillodilator pathway in the brain stem, the spinal cord, or in the periphery causes an ipsilateral Horner's syndrome, consisting of a small pupil (meiosis) and slight retraction of the eyeball associated with partial closure of the eyelids. The drooping upper eyelid is referred to as a ptosis.

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