![]() Afferent light stimulus is received by retinal ganglion cells whose axons traverse as the optic nerve, optic chiasm and optic tract that eventually synapses at the pretectal nuclei of the dorsal midbrain at the level of the superior colliculi. The parasympathetic pathway is a four-order neuron pathway that controls pupillary constriction and accommodation. Sudomotor fibers course with the external carotid artery to innervate sweat glands of the face. The third-order neuron ascends along the carotid artery into the cavernous sinus to ultimately synapse at the pupillary dilator muscle, the Muller muscle of the upper eyelid, and the smooth muscle of the lower eyelid. The second-order neuron continues over the lung apex and synapses at the superior cervical ganglion located at the carotid bifurcation. The first-order neuron originates from the posterolateral hypothalamus and travels caudally down the brainstem to the ciliospinal center of Budge (C8-T2). The sympathetic pathway is a three-order neuron pathway that mediates pupillary dilation. Pupillary function is autonomic, largely occurring in response to light stimulation and adrenergic tone. Physiologic control of pupillary function is dictated by sympathetic and parasympathetic innervation to the pupillary dilator and the pupillary sphincter muscles, respectively.
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