Autonomic Nervous System and Control of Visual Function.

Corresponding author: T. R. Raju, Department of Neurophysiology, National Institute of Mental Health and Neurosciences, Bangalore, Karnataka 560029, India. E-mails: trraju.nimhans@gmail.com or director.research@sankaraeye.com The peripheral nervous system element known as the autonomic nervous system regulates involuntary neurobiological processes such as heart rate, blood pressure, and breathing rate as well as ocular changes such as accommodation, intraocular pressure (IOP) regulation, and pupil size. Autonomic afferents are found in almost all ocular parts and regulate local balance in the body via antagonistic and synergistic interactions. The current review illustrates the critical role played by ANS in regulating visual functions. Any imbalance in the functioning of ANS can lead to pathological changes such as myopia and hyperopic defocus. The sympathetic, parasympathetic, and enteric nervous systems comprise the autonomic nervous system. Both peripheral nerve fibers and innervation are found in the sympathetic and parasympathetic nervous systems, which receive sensory input from the central nervous system (CNS) and transmit motor output from it. A preganglionic neuron with a cell in the CNS and a postganglionic neuron with a cell body in the periphery are typically found in SNS and PNS pathways.1 Numerous studies have found that the ANS accurately controls the physiological functions of the eye, including the regulation of IOP, pupil dimensions, lens accommodation, and circulation within the eyes. Nearly, all ocular components contain autonomic innervation for regulating local homeostasis through antagonistic and synergistic interactions.2 Numerous ocular functions are influenced by the ocular projections of ANS. There are two of them: (1) pupillary diameter and ocular accommodation, each controlled by the eye’s muscles in the iris and ciliary body; these structures are innervated by the ciliary (parasympathetic) and postganglionic fibers of the upper cervical (sympathetic) ganglia and (2) ocular blood flow controlled by the vascular system of the optic nerve, retina, choroid, and iris. The production and excretion of the aqueous humor are primarily regulated to control IOP. Aqueous humor formation is significantly influenced by the autoregulation of the ciliary epithelium and its blood vessels, so even though intracellular outflow is influenced by the control of the episcleral vascular system and trabecular meshwork. Postganglionic fibers from the superior cervical (sympathetic) and pterygoid (parasympathetic) ganglia innervate these tissues (Figures 1 and 2).3 The ANS primarily regulates the function of the majority of the intraocular muscles in the eyes. Additionally, the pupillary light reflex (PLR) is frequently utilized to identify ANS dysfunction.4 Nerve endings are capable of secreting solubilized trophic materials that promote lacrimal gland production, elicit eye movement’s reaction times, and maintain the integrity of the corneal surface.5 The PNS forms the major innervation of ciliary muscles, and its action is mediated via acetylcholine's interaction with muscarinic (largely m3 subtype) receptors. Positive accommodation is regulated by PNS input to the ciliary muscle, which also generates the demand for quick focus shifts, due to its quick onset effect. Further, it is known that because of the sympathetic innervation to the ciliary muscle, noradrenaline acts on two subclasses of postsynaptic receptors and the inhibitory α1 and β2 adrenoceptors.6 Ocular accommodation happens when the lens “bulges” (increases in convexity) as a result of ciliary muscle contraction leading to the reduction of zonular fiber and the increase in the lens’ refractive power. The parasympathetic afferents of the ciliary body are predominantly responsible for controlling the dynamic behavior of amicable responses. In contrast to sympathetic afferents, which act for a bit longer period of time (10−40 seconds) to cause hypermetropia of almost 1.5 dioptres, parasympathetic innervation acts rapidly one second to generate a positive accommodation up to 20 dioptres.6 Prolonged near work can increase accommodation