Parasympathetic Component of CN III: Oculomotor Nerve
Preganglionic parasympathetic axons extend from the Edinger-Westphal nucleus located in the periaqueductal grey matter of the midbrain. These fibres terminate within the intermediate ciliary ganglion located near to the orbital apex, with subsequent postganglionic fibres travelling to the sphincter pupillae muscle of the iris and the ciliary muscles of the lens, via the short ciliary nerves (figure 6).
Parasympathetic oculomotor fibres act to contract the sphincter pupillae muscles and the ciliary muscles, thus bringing about constriction of the pupil (figure 7) and accommodation of the lens when viewing in close range.
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Clinical Implications
Increased intra-cranial pressure resulting in herniation of the temporal uncus into the tentorium cerebellum, skull fractures disrupting the cavernous sinus and aneurysms can all damage this intricate neural circuitry by putting pressure on the brainstem. The parasympathetic implications, as a result of these lesions, present as permanent dilation of the pupil, with loss of ipsilateral pupillary reflex in response to changes in light source and a loss of accommodation with close range viewing.