Ocular Autonomic Nervous System
Introduction[edit | edit source]
The Ocular Autonomic Nervous System (OANS) is responsible for the autonomic physiological functions of the eye. These are:
- Pupil size
- Lens accommodation
- Ocular circulation
- Intraocular pressure regulation 
The eyes are innervated by the sympathetic, parasympathetic and trigeminal sensory nerve fibers. Knowledge of these autonomic neurons and their functions may provide insights into the response of the eye under different physiological and pathological circumstances.  This may point to visual challenges contributing to motor and gait abnormalities that need to be assessed and addressed when managing patients (e.g. Parkinson's disease) in physiotherapy practice. Finally, such knowledge may highlight unknown potentials of the ANS in the restoration of function after injury. 
Autonomic control of the eye[edit | edit source]
The autonomic functions of the eye include the control of the following structures:
- Cornea: one of the most densely innervated structures of the human body. Innervation plays a key role in ensuring optimal health of the surface of the eye. Corneal nerves regulate the release of soluble trophic substances that enhance lacrimal gland secretion, the function of blinking reflexes and ocular surface integrity.  Several neuropeptides and transmitters in the cornea have been suggested to influence epithelial renewal, protection from oxidative stress, wound healing and cornea homeostasis. 
- Iris: a circular pigmented membrane in front of the lens that divides the eye cavity into anterior and posterior chambers. It controls pupil size through the pupillary sphincter and as a result, the amount of light entering the eye. Innervation not only controls pupillary constriction and dilation, but also may mediate protective reflexes,  smooth muscle response, intraocular blood vessels, and immune function. 
- Lacrimal glands: Secretions from the lacrimal glands are essential for the integrity of the cells on the ocular surface (conjunctiva, corneal epithelium). Their regulation involves stimulation of the sensory nerve on the ocular surface as well as parasympathetic and sympathetic activation of the lacrimal secretory cells.  Mechanonociceptors, polymodal nociceptors, and cold receptor fibers are distributed on the conjunctiva and cornea. Stimulation of the corneal polymodal nociceptors causes reflex tear secretion, while mechanonociceptors and cold receptors are less effective in mediating this effect. Tear production is regulated by both the sympathetic and parasympathetic nerves. Generally, sympathetic nerves affect tear secretion via the following two methods: (1) alteration of blood flow and (2) via increased secretion of sympathetic neurotransmitters. However, the role of sympathetic nerves in the lacrimal gland remains uncertain. Tear secretion is mainly controlled by the parasympathetic nerves and this is why parasympathetic nerve lesions may exhibit such decreases. 
- Retina: The neural component of the retina is a layered structure that converts light into visual information that ends up in the brain.  Connections of sensory neurons inside the retina with structures in the visual cortex and midbrain (superior colliculus) allow for visual perception and visually guided reflexive behaviours respectively.  Retinal circulation is considered autoregulatory by local and chemical stimuli getting a sense of oxygen levels and not heavily influenced by the OANS.  However, it is worth mentioning that, due to the loss of sympathetic innervation, significant loss of photoreceptor cells and increased reactivity of the glial cells may be noticed with with increasing age. 
- Choroid: This is the posterior part of the uvea, between the retina and sclera. The choroid is full of blood vessels which are regulated by choroidal circulation (i.e. choroidal ganglion cells or intrinsic choroidal neurons). 
Clinical bottom line[edit | edit source]
The Ocular Autonomic Nervous System (OANS), which is responsible for the autonomic physiological functions of the eye, may provide insights into the response of the eye under different physiological and pathological circumstances but also point to visual challenges contributing to motor and gait abnormalities that need to be addressed when managing patients in physiotherapy practice.
References[edit | edit source]
- ↑ 1.0 1.1 1.2 Wu F., Zhao Y., Zhang H. Ocular Autonomic Nervous System: An Update from Anatomy to Physiological Functions. Vision 2022; 6:6.
- ↑ Stuart S., Lord S., Hill E., Rochester L. Gait in Parkinson’s disease: A visuo-cognitive challenge. Neuroscience & Biobehavioral Reviews 2016; 62: 76-88.
- ↑ Tereshenko V., Dotzauer D., Luft M., Ortmayr J., Maierhofer U., Schmoll M., Festin C., Carrero Rojas G., Klepetko J., Laengle G., Politikou O., Farina D., Blumer R., Bergmeister K., Aszmann O. Autonomic Nerve Fibers Aberrantly Reinnervate Denervated Facial Muscles and Alter Muscle Fiber Population. Journal of Neuroscience 2 November 2022, 42 (44) 8297-8307.
- ↑ Stern M.M., Beuerman R.W., Fox R.I., Gao J., Mircheff A.K., Pflugfelder S.C. The pathology of dry eye: the interaction between the ocular surface and lacrimal glands. Cornea. 1998 Nov;17(6):584-9.
- ↑ Ghiasi Z., Gray T., Tran P., Dubielzig R., Murphy C., McCartney D.L., Reid T.W. The Effect of Topical Substance-P Plus Insulin-like Growth Factor-1 (IGF-1) on Epithelial Healing After Photorefractive Keratectomy in Rabbits. Transl Vis Sci Technol. 2018 Jan 23;7(1):12.
- ↑ Suvas S. Role of substance P neuropeptide in Inflammation, wound healing and tissue homeostasis. J Immunol 2017; Sep 1: 199(5): 1543-52.
- ↑ Fujiwara M.,Hayashi h., Muramatsu I., Ueda N.Supersensitivity of the rabbit iris sphincter muscle induced by trigeminal denervation: the role of substance P. The Journal of Physiology 1984; 350(1): 583-597.
- ↑ Neuhuber W., Schrödl F. Autonomic control of the eye and the iris. Auton. Neurosci. 2011; 165: 67–79.
- ↑ Dartt D. Neural regulation of lacrimal gland secretory processes: Relevance in dry eye diseases. Prog. Retin. Eye Res. 2009; 28: 155–177.
- ↑ Toshida H., Nguyen A., Beuerman R., Murakami A. Evaluation of Novel Dry Eye Model: Preganglionic Parasympathetic Denervation in Rabbit. Investig. Opthalmol. Vis. Sci. 2007; 48: 4468–4475.
- ↑ Sibille J., Gehr C., Benichov J., Balasubramanian H., Lun Teh K., Lupashina T., Vallentin D., Kremkow J. High-density electrode recordings reveal strong and specific connections between retinal ganglion cells and midbrain neurons. Nat Commun 2022; 13: 5218
- ↑ Delaey C., Van De Voorde J. Regulatory Mechanisms in the Retinal and Choroidal Circulation. Ophthalmic Res. 2000; 32: 249–256.
- ↑ Laties AM. Central Retinal Artery Innervation. Absence of Adrenergic Innervation to the Intraocular Branches. Arch. Ophthalmol. 1967; 77: 405–409.
- ↑ Steinle J., Lindsay N., Lashbrook B. Cervical sympathectomy causes photoreceptor-specific cell death in the rat retina. Auton. Neurosci. 2005; 120:46–51.
- ↑ May C., Neuhuber W., Lütjen-Drecoll E. Immunohistochemical Classification and Functional Morphology of Human Choroidal Ganglion Cells. Investig. Opthalmol. Vis. Sci. 2004; 45: 361–367.