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.  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). 
[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]
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