2013 Vision

There are two fundamental protective mechanisms for the eye. Regulation of eyelid position (including BLINKING) involves striated (ACh; nicotinic) and smooth (NE; α₁ adrenergic) muscles.  TEAR PRODUCTION occurs spontaneously (basal), reflexly or in response to emotional stimuli, and is partially regulated by the parasympathetic nervous system (ACh; muscarinic).  EPIPHORA (overflow of tears) can be due to either overproduction or blocked drainage.

The cornea and lens focus light on the retina; the cornea has greater refractive power but the focusing power of the lens can be adjusted to allow near vision (accomodation).  Refractive errors include cataracts, hyperopia, myopia, presbyopia and astigmatism.

Light intensity is regulated by the PUPILLARY LIGHT REFLEX, which causes MIOSIS as a result of parasympathetic stimulation of the sphincter pupillae muscles (muscarinic receptors).  MYDRIASIS results from sympathetic stimulation (α₁ receptors) that activates the dilator pupillae muscles.

Increased intraocular pressure causes loss of vision (potentially permanent). Open angle glaucoma (the most common form) results from overproduction of the aqueous humor. Closed angle glaucoma (typically the most rapidly evolving form) is caused by blockage of fluid outflow.

Rods are responsible for SCOTOPIC vision (the monochromatic vision that occurs in low light).  The three types of cones (blue, green and red; or Short, Medium and Long wavelength) have better temporal and spatial resolution than rods, making PHOTOPIC VISION better for discrimination of surfaces and movement under bright light conditions.

The ability to discriminate fine details of the visual scene is termed VISUAL ACUITY.  Three types are recognized:  SPATIAL, TEMPORAL and SPECTRAL.  Visual acuity is primarily a function of the cone system.

PHOTOTRANSDUCTION occurs via a 4 step process that uses a 2nd messenger cascade to amplify the signal.  In rods, activation of rhodopsin ultimately results in the closure of cyclic nucleotide gated Na⁺ channels, and hyperpolarization of the photoreceptor.

The VISUAL CYCLE consists of bleaching and recycling of 11-cis-retinol between the photoreceptors and the retinal pigment epithelium (RPE).  It is a key component of dark adaptation in rods and is disrupted in vitamin A deficiency, and macular degeneration.