Autonomic Regulation of the Eye

The functions of the eye are regulated by and dependent upon the autonomic nervous system. The parasympathetic nervous system controls constriction of the iris and accommodation of the lens via a pathway with preganglionic motor neurons in the Edinger-Westphal nucleus and postganglionic motor neurons in the ciliary ganglion. The parasympathetic nervous system regulates choroidal blood flow and the production of aqueous humor through a pathway with preganglionic motor neurons in the superior salivatory nucleus and postganglionic motor neurons in the pterygopalatine (sphenopalatine) ganglion. The sympathetic nervous system controls dilation of the iris and may modulate the outflow of aqueous humor from the eye. The sympathetic preganglionic motor neurons lie in the intermediolateral cell column at the first level of the thoracic cord, and the postganglionic motor neurons are found in the superior cervical ganglion. The central pathways controlling different autonomic functions in the eye are found in a variety of locations within the central nervous system. The reflex response of the iris to changes in luminance levels begins with melanopsin-containing retinal ganglion cells in the retina that project to the olivary pretectal nucleus. This nucleus then projects upon the Edinger-Westphal preganglionic motoneurons. The dark response that produces maximal pupillary dilation involves the sympathetic pathways to the iris. Pupil size is also regulated by many other factors, but the pathways to the parasympathetic and sympathetic preganglionic motoneurons that underlie this are not well understood. Lens accommodation is controlled by premotor neurons located in the supraoculomotor area. These also regulate the pupil, and control vergence angle by modulating the activity of medial rectus, and presumably lateral rectus, motoneurons. Pathways from the frontal eye fields and cerebellum help regulate their activity. Blood flow in the choroid is regulated with respect to systemic blood pressure through pathways through the nucleus of the tractus solitarius. It is also regulated with respect to luminance levels, which likely involves the suprachiasmatic nucleus, which receives inputs from melanopsin-containing retinal ganglion cells, and other areas of the hypothalamus that project upon the parasympathetic preganglionic neurons of the superior salivatory nucleus that mediate choroidal vasodilation.