Healthy Eye
Rods are cells in the eye that have the ability to detect light. They do not have the ability to see color and have low visual acuity, but are extremely sensitive. They are located in the outer segment of the eye and are used for peripheral vision.
Cones are cells in the eye that also have the ability to detect light. They have the ability to see color and have high visual acuity, but are less sensitive. They are located in the inner segment of the eye where the visual acuity is greatest which is the center of the field of vision.
The eye is the organ in the body that allows us to see. It has the ability to convert a light signal into an electrical signal for the brain to read.
Diseased Eye
Retinitis Pigmentosa
Retinitis pigmentosa is a group of inherited disorders that all lead to progressive loss of vision. Rods, which mediate peripheral vision are typically the first cells to be lost which leads to tunnel vision.
Color Blindness
Color blindness is an X-linked inherited trait where patients have difficulty distinguishing certain colors, most typically red/green and blue/purple. This is due to a genetic mutation that leads to a loss of function of a particular type of cone (red sensing cone or green sensing cone). 
Glaucoma is a condition where there is increased pressure inside the air. This is due to a buildup of liquid (aqueous humor) inside the eye and is damaging to the optic nerve. This leads to progressive loss of vision.
Eye Treatment
Retinitis Pigmentosa -> Rod
The use progressive loss of rods in retinitis pigmentosa can be treated with vitamin A, which is essential for the formation of the light-sensing protein, rhodopsin. Treatment with vitamin A is controversial, with some studies showing no benefit.
Color Blindness -> Cone
The lack of color-detecting cones leads to the inability to distinguish between certain colors (red/green, blue/purple). Enchroma glasses help to separate certain color wavelengths to help patients better distinguish those colors.
Glaucoma -> Eye
 The buildup of aqueous humor in the eye leads to increased intraocular pressure and eye damage. The prostaglandin analog, bimatoprost, can help relieve this pressure by activating the outflow pathways for the aqueous humor.

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