This pathway bypasses the pretectal nuclei. The afferent aspect of the pupillary near response follows the afferent visual pathway to higher cortical centers in the striate cortex that information is relayed to the frontal eye fields then to the oculomotor and Edinger-Westphal nuclei. The neural pathway for this triad of responses is not completely understood, but the pupillary response is dependent on a supranuclear (frontal and occipital lobe influence) connection between the pupillary sphincter, ciliary body muscles and medial recti neurons. When focus is changed from far to near, a triad of responses occurs: convergence, accommodation and pupillary constriction. The eyelids’ smooth levator and Mueller muscles are also innervated by this route, but exit the carotid plexus and follow the oculomotor nerve (CN III) into the orbit. The postganglionic (third-order) neuron’s fibers then travels through the carotid plexus and cavernous sinus where they join the ophthalmic division of the trigeminal cranial nerve (CN 5) they follow the nasociliary branch then the long ciliary nerves to the radial iris dilator muscle. The preganglionic (second-order) neuron then travels up the sympathetic chain, over the lung apex and through several ganglia to synapse at the superior cervical ganglion, located where the carotid bifurcates. The sympathetic central (first-order) neuron begins in the dorsolateral hypothalamus and travels, uncrossed, through the brainstem to the ciliospinal center of Budge at the level of C8 to T2 in the cervical spinal cord. Sympathetic innervation to the eye involves a three-neuron chain.
Postganglionic parasympathetic pupillary fibers travel along the short ciliary nerves to the ciliary body muscle (93% to 97% of fibers) and to the circumferential iris sphincter (3% to 7% of fibers). The fibers then travel in the superficial layer of the third cranial nerve to the cavernous sinus, follow the cranial nerve’s inferior division through the superior orbital fissure and synapse in the ciliary ganglion, which is posterior to the globe, within the muscle cone.
The parasympathetic efferent pupillary pathway begins where the afferent pathway left off: the Edinger-Westphal nucleus. The efferent (motor) pupillary pathway has both parasympathetic and sympathetic nervous system actions. The consensual light reflex occurs because both the optic and tectotegmental tracts carry fibers from both eyes. They then leave the pretectal nucleus and distribute about equally to the two Edinger-Westphal nuclei via the tectotegmental tract. From there, they travel to the midbrain and synapse at the pretectal nucleus, at the level of the superior colliculus. The neural fibers separate from the tract just anterior to the lateral geniculate body. The afferent (sensory) pupillary pathway begins with the retinal photoreceptors, passes through the optic nerve and optic chiasm and travels along the posterior third of the optic tract. Having an understanding of the neural pathways that control the normal afferent and efferent pupillary responses is foundational, so we will review this before discussing abnormalities of the pupillary response. The pupils will offer substantial insight into one’s ocular health and the state of the visual pathways before even looking into a patient’s eyes. The keen practitioner can gain helpful information simply by assessing pupillary responses.
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