Branches of the parasympathetic system play a role in lacrimal function, and pupil size is controlled by a balance between the innervation of the sympathetic fibers to the iris dilator muscles and of the parasympathetic fibers to the sphincter muscles. The accessory retractor muscles, including the Müller muscle in the upper eyelid, receive sympathetic innervation.
Sympathetic activity originates in the posterolateral region of the hypothalamus. Activity in the hypothalamus is influenced by signals in the frontal, sensorimotor, and occipital cortex and in the limbic system (cingulate gyrus). The course of sympathetic fibers destined for the orbit is divided into first-, second-, and third-order segments (Fig 1-37). Axons destined for the dilator muscles of the pupil and Müller muscle descend as the first-order segment, along with other sympathetic fibers, superficially in the anteromedial column through the brainstem to the spinal cord. Within the cervical spinal cord, the sympathetic fibers continue in the intermediolateral column. From levels C8 to T2, the sympathetic fibers destined for the orbit synapse in the ciliospinal center of Budge-Waller.
Figure 1-37 Anatomy of the sympathetic pathway showing first-order central neuron, second-order intermediate neuron, and third-order neuron pathways. Note the proximity of the pulmonary apex to the sympathetic chain. Note also the intimate relationship of the sympathetic fibers to CN VI within the cavernous sinus.
(Illustration by Christine Gralapp.)
The postsynaptic second-order fibers leave the spinal cord through the ventral rami of the cervical (C8) and upper thoracic (T1 and T2) levels before joining the paravertebral sympathetic plexus. Ascending rostrally, the sympathetic chain passes in the anterior loop of the ansa subclavia proximate to the innominate artery on the right and the subclavian artery on the left just above the lung apex. These fibers pass through the inferior and middle cervical ganglia to terminate in the superior cervical ganglion, at the level of the angle of the jaw (C2) and the carotid artery bifurcation.
The postganglionic third-order fibers continue in the wall of the bifurcated carotid. Sympathetic fibers innervating the sweat glands of the lower face follow the ECA.
The sympathetic fibers destined for the pupil continue along the ICA, entering the cranium through the carotid canal. Some sympathetic fibers leave the carotid artery as it exits the petrous bone, combining with the greater superficial petrosal nerve to form the vidian nerve. These sympathetic fibers parallel the parasympathetic fibers to the lacrimal gland. Within the cavernous sinus, sympathetic fibers destined for the dilator muscles leave the carotid in conjunction with CN VI for a few millimeters. Further anteriorly in the cavernous sinus, the sympathetic fibers join the nasociliary branch of CN V1. In the orbital apex, the fibers then pass through the ciliary ganglion (without synapsing). Along with the nasociliary branch, the sympathetic fibers reach the globe and travel with the long ciliary nerves to the dilator muscles of the pupil. The dilator muscle lies just superficial to the posterior pigment epithelium of the iris, which continues peripherally as the nonpigmented superficial layer of the ciliary body. The myoepithelial cells measure approximately 12.5 μm in thickness, with an apical epithelial portion and a basilar muscular portion that is oriented radially toward the pupillary opening. The muscular processes terminate peripheral to the sphincter muscle. Peripherally at the iris root, these cells are continuous with the pigmented epithelium of the ciliary body.
The fibers destined for the Müller muscle travel along the OphA and its subsequent frontal and lacrimal branches. The Müller muscle originates near the origin of the levator aponeurosis and inserts 10–12 mm inferiorly on the superior border of the tarsus. The superior orbital sympathetic fibers also innervate the sweat glands of the forehead. Thus, disruption of these sympathetic fibers is responsible for the mild ptosis and the frontal anhidrosis associated with distal Horner syndrome.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.