Photoreceptor - Eye

The eye is the organ of vision, sitting in the bony orbit of the skull and held there by six extrinsic muscles — the superior, inferior, lateral and medial rectus muscles plus the superior and inferior oblique muscles — which receive their nerve supply from cranial nerves III, IV and VI and together give the eyeball its range of movement. Accessory structures protect the eye itself: the eyelids shield it from excessive light and foreign particles while spreading lubricating secretions; the eyelashes and eyebrows guard against foreign objects, sweat and direct sunlight; sebaceous (ciliary) glands at the base of the eyelashes lubricate the follicles; and paired lacrimal glands, sitting in the upper outer region of each orbit, secrete tears — about 1 mL a day — containing salts, mucus and the antibacterial enzyme lysozyme. A thin protective mucous membrane, the conjunctiva, lines the outer surface of the eyeball itself.

The eyeball wall has three layers. The outer sclera is dense, non-vascular connective tissue with two regions: the transparent cornea at the front, made of stratified squamous epithelium that constantly renews itself because it is so exposed to damage, and the opaque sclera proper (the 'white of the eye') behind it, innervated posteriorly by the optic nerve; where cornea meets sclera, a small channel called the canal of Schlemm continuously drains excess aqueous humor. The middle choroid layer is heavily vascularised and pigmented, nourishing the other eye layers while its pigment absorbs stray light to stop internal reflection; toward the front it thickens into the ciliary body and the iris, the coloured ring around the central pupil. The iris carries two opposing muscle types — the radial dilator pupillae and the circular sphincter pupillae — so that in bright light the circular muscle contracts to shrink the pupil, while in dim light the radial muscle contracts to enlarge it, controlling how much light enters. The ciliary body's smooth ciliary muscle adjusts the lens's curvature for near or far focus (accommodation), aided by the suspensory ligament that holds the lens in place, and its capillaries secrete the watery aqueous humor that fills the front chambers of the eye; the rear compartment, between lens and retina, instead holds the thicker, jelly-like vitreous humor, which helps the eyeball keep its spherical shape.

The inner retina has a non-visual pigmented epithelial layer and a neural layer containing three cell types: photoreceptor cells (rods and cones), bipolar cells, and ganglion cells. At the centre of the retina's posterior region sits the macula lutea ('yellow spot'), responsible for the sharpest, most detailed vision, and within it a small depression, the fovea centralis, contains cones exclusively. Where the optic nerve and retinal blood vessels enter the eye — slightly below the posterior pole — there are no photoreceptors at all, which is why this spot is called the blind spot.

Vision itself works like this: light is refracted (bent) as it passes through the cornea, aqueous humor and lens, and is focused onto the retina, where it excites the rod and cone cells. Their photopigment is built from opsin (protein) bound to retinal, a derivative of vitamin A; light causes retinal to dissociate from opsin, changing opsin's shape and generating an action potential in the photoreceptor, which then travels via bipolar cells and ganglion cells to the optic nerve and on to the visual cortex of the brain for interpretation. …