The human eye is often compared to a video recorder or camera. Although similarities certainly do exist, the eye is far more complex than either of these machines. Part of the reason is that the main job of the eye is to provide the brain with the vast majority of its sensory input. Compared to other sensory modalities, the importance of vision is difficult to overemphasize. Scientists think that over 40% of the neurons in the brain's cerebral cortex are involved in analyzing visual information.
Much of the anatomy of the human eye is designed to nourish and protect the retina. The photoreceptor cells of the retina (rods and cones) are the only cells in the human body capable of converting photons of light into neuronal signals; as such, the retina represents the very heart of the visual system. The retina is so important, in fact, that scientists consider it to be a part of the central nervous system.
The anterior surface of the eyeball is covered by a protective membrane called the conjunctiva, which also coats the inner lining of the eyelids. The tough, fibrous wall of the eyeball itself is called the sclera. Extraocular muscles attach to the sclera, making coordinated movements of the eyeballs possible. The cornea is continuous with the sclera and mainly serves to protect the eye from mechanical trauma. Corneal scarring or abnormal proliferation of blood vessels (corneal neovascularization) sometimes necessitates a corneal transplant. Transplanted corneas are generally obtained from cadavers.
The anterior chamber of the eye consists of the cornea, iris, ciliary body and muscle, as well as the lens. The iris consists of pigmented connective tissue lying in front of the lens. Contraction or relaxation of the ciliary muscle adjusts the size of the pupil, which is nothing more than a clear space within the iris that regulates the amount of light entering the eye. The lens consists of stacks of crystallin proteins that project incoming light rays on to the surface of the retina. The lens is held in place by suspensory ligaments. The ciliary body produces a fluid called the aqueous humor, which circulates throughout the anterior chamber. Most of the aqueous humor drains through an opening called the canal of Schlemm, eventually returning to the venous circulation.
The major disease processes affecting the anterior chamber of the eye are glaucoma and cataracts. Glaucoma results from partial or total obstruction of aqueous humor drainage through the canal of Schlemm. Fluid back up leads to elevated intraocular pressure. Over time, the increased intraocular pressure damages the optic nerve, resulting in irreversible vision loss. Because glaucoma tends to be insidious and asymptomatic, there is virtually nothing an ophthalmologist can do to reverse the course of this disease once vision loss has occurred. Intraocular pressure can be measured at regular intervals, however, and all adults, especially people with a family history of glaucoma, are urged to schedule an annual eye examination.
Cataracts tend to occur in later life. Approximately half of all Americans will develop a cataract by age 80. The main cause of cataracts seems to be a build up of calcium and protein deposits on the lens surface, clouding it and resulting in blindness if left untreated. Certain rare diseases, most notably galactosemia, can cause cataracts in newborns. A few drugs such as quetiapine (seroquel) raise a person's risk of developing cataracts. People who take these drugs on a long term basis are advised to be examined by an ophthalmologist at six month intervals.
The posterior chamber of the eye consists of the vitreous humor, choroid and retina. The vitreous humor is colorless and has the consistency of a thick gel. The vitreous humor is thought to lend mechanical support to the posterior chamber of the eye and help maintain the shape of the retina. The choroid is a pigmented layer of epithelial cells in which the outer segments of the retinal photoreceptors are anchored. The photoreceptors themselves cannot be replaced by mitosis, but as long as these cells remain viable, their parts can can be replaced throughout a person's life.
The retina itself consists of multiple cell types organized into three main layers: 1) Photoreceptor cells: Rods are responsible for black and white vision in dim light (scotopic vision). Cones come in three varieties and mediate bright light/color vision (photopic vision). 2) Bipolar cells - these cells transmit impulses from the rods, cones, and horizontal cells to the retinal ganglion cells. 3) Ganglion cells - these cells receive input from bipolar cells as well as amacrine cells, basically auxilliary cells that help integrate and fine tune the signal. The axons of the ganglion cells form the optic nerve, which wires the eyeball to the brain.
The optic nerves intersect in a structure located behind the eyes called the optic chiasm. Most optic nerve fibers cross over to the contralateral or opposite side of the brain from the eye of origin. 95% of each optic nerve's fibers innervate a group of thalamic neurons called the LGN (lateral geniculate nucleus), which in turn sends signals to the primary visual cortex in the brain's occipital lobe. Most of the remaining fibers innervate the suprachiasmatic nucleus (SCN), a small cluster of cells in the hypothalamus that regulate the body's circadian rhythms.
Several disease processes can affect the posterior chamber of the eye, almost all of which involve the retina. These include chronic hypertension, diabetic retinopathy, retinal detachment, macular degeneration; autoimmune diseases, especially lupus and multiple sclerosis (MS); uncommon
cancers such as retinoblastoma and ocular melanoma, as well as various mitochondrial disorders.
Longstanding high blood pressure and diabetes wreak havoc on the small blood vessels that nourish the retina. Over time, the retina's blood supply is compromised, starving the cells of oxygen and nutrients and eventually killing them. In diabetics, fragile blood vessels form in an effort to compensate for the inadequate blood supply. This condition is called proliferative retinopathy; unfortunately, these new vessels tend to leak and cause further damage to the beleaguered retina. Diabetic retinopathy, retinal detachments, and wet macular degeneration can be treated with laser therapy (photocoagulation) as well as injections of an antibody called bevacizumab (Avastin), which inhibits angiogenesis.
Flare ups of autoimmune diseases are usually managed with corticosteroids. Cancers arising from or involving the eye are generally treated by enucleation of the affected eyeball. No specific treatment exists for degenerative retinal diseases such dry macular degeneration, retinitis pigmentosa, or the mitochondrial disorders, e.g. Leber's Hereditary Optic Neuropathy. In the future, stem cell transplants or photosensitive electrode implants may partially restore vision in people with these conditions.