Eye Physiology
Kyle M. Keenan
Product of an Independent Study @ SHU.EDU
Eye Physiology Terms
- Accommodation
- A function of the lens in which it adjusts to keeps near and far objects in focus on the retina
- Astigmatism
- A condition of asymmetric focusing, usually due to a malformation of the cornea.
- Aqueous Humor
- A fluid found in the front chamber, between the cornea and the lens. This fluid is formed continuously and provides nutrients to the cornea and the lens.
- Bipolar Neuron
- Receive impulses directly from the photoreceptors and then propagates them to the ganglion cells.
- Blind Spot
- The area of the retina where the optic nerve exits. It is called the blind spot because there are no receptors located in this area.
- Cerebral Cortex
- The outer layer of the brain, organized into four regions/lobes: Occipital, temporal, frontal and parietal. As is it pertains to sight, it is responsible for the perception of information received from the eyes.
- Choroid
- The is the middle layer in the eyes coat. It is found between the retina (inside) and the Sclera (outside). Its main function is to reduce reflection of stray light. The iris and the ciliary body is part of the choroid layer.
- Ciliary Body
- Produces aqueous humor and contains the ciliary muscle.
- Ciliary Muscle
- Works in conjunction with the zonule fibers to adjust the lens when focusing on an object.
- Color Blindness
- Any number of disorders of the eye in which parts of the visible spectrum can not be observed.
- Deuteranopia
- Disorder where the medium wavelength system does not function properly.
- Dichromats
- A disorder in which only two types of cones are functioning properly. Individuals, usually cannot distinguish red from green.
- Monochromats
- A disorder in which all three cone types are not functioning.
- Protanopia
- Disorder where the long wavelength system does not function properly.
- Trichromats
- The normal condition of eyesight. All three types of functional cones work properly and the full visible spectrum is observed.
- Tritanopia
- Disorder where the short wavelength systems does not function properly.
- Cones
These are the color receptors for the eye. Unlike rods, cones work best in lighted conditions. They are responsible for the eyes
high acuity. Through processes from these three cones, red, blue and green the entire visible spectrum can be seen.
- Conjunctiva
- The mucous membrane of the eye.
- Cornea
- The outermost layer of the eye through which light must pass through before entering the pupil. It is the main refractive surface of the eye.
- Diopter
- The unit of refractive power.
- Fovea
- This is the retina center where cones are most densely populated. This is the point of highest acuity in the eye.
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- Ganglion Cells
- Carry impulses from the bipolar cells to the thalmus. The optic nerve is composed of axons of the ganglion cells.
- Geniculate Bodies
- "relay stations" deep within the brain, between the optic chiasm and the visual cortex. It is here that some scientists believe visual information is coordinated with impulses sent by other sensory organs
- Horizontal Cells
- Also translate impulses from the photoreceptors to the ganglion cells.
- Hypermetropia
- (see hyperopia)
- Hyperopia
- Shortening of the optical lobe resulting in farsightedness. A far image has a focal point in front of the retina where the retina is normally located. A near object, however, can be focused on the retina without accommodation.
This disorder can be fixed with the use of a concave lens. A far image can now be focused on the retina without accommodation. A near image is focused on the retina through accommodation.
(Image is focused behind the retina) 
(Corrected with concave lens)
- Iris
- This is the colored part of the eye, which lies just in front of the lens and just behind the aqueous humor. It adjusts the diameter of the pupil in response to light.
- Lateral Geniculate Nucleus - LGN
- It is believed that this is the processing area for information coming in from both visual fields. There is a LGN located in each hemisphere of the brain. From here
the processed information is projected to the visual cortex located in the occipital lobe of the cerebral cortex.
- Lens
- Varies its refractive ability, accommodation, in order to focus an object on the retina.
- Macula Lutea
- This is the center of the retina, a region of high acuity. The fovea is the depression at its center.
- Myopia
- Elongation of the optical lobe resulting in Nearsightedness. A far image has a focal point in front of the retina where the retina is normally located. A near object, however, can be focused on the retina without accommodation.
This disorder can be fixed with the use of a concave lens. A far image can now be focused on the retina without accommodation. A near image is focused on the retina through accommodation.
(Image is focused in front of retina) 
(Corrected with convex lens)
- Optic Chiasm
- The area in the optic tract where information from each eye crisscrosses. Fibers from each eye cross over to the opposite side, while other fibers remain on their respective side. It is in this way that information from each eye is received and processed on both sides of the brain.
- Optic Nerve
- Carries the impulses from the retina's rods and cones to the brain where they can be translated into images.
- Optic Tracts
The organized bundles of nerve fibers leaving the optic chiasm ending in one of the lateral geniculate nuclei.
Each optic tract carries information from each eye. The lateral half of one eye and the medial half of the other.
- Photopic Vision
- The ability to see in lighted conditions
- Photoreceptors
- The functional receptor for light in the eye. Rods and cones are the receptors.
- Pigment
- Any one of a number of colored substances.
- Presbyopia
- A disorder that occurs with age. It is when the lens loses its elasticity and therefore its ability to accommodate.
- Pupil
- The pupil is at the center of the iris, it adjusts its diameter in relation to light. It is under control of the autonomic nervous system.
- Refractive Power
- Measured in diopters, it is the measure of how much a substance can bend light. The cornea is the main refractive surface of the eye.
- Retina
- The inner most layer of the eye where the light receptors, rods and cones, are located.
- Retinal
- The light absorbing photopigment, a derivative of Vitamin A, found in the eyes light receptors.
- Retinal Interneurons
- Functional neurons within the retina that process incoming information
- Rhodopsin
- This is the light sensitive receptor protein found in the disk membranes of rod cells.
- Rods
These are the black and blue light receptors for the eye. The rods work best under low light conditions.
- Sclera
- The outermost layer of the eye. It is a hard, fibrous membrane that provides protection to the eye. It also functions to maintain the shape of the eye. The cornea is the forward most part of the sclera.
- Scotoma
- Disorder where the short wavelength systems does not function properly.
- Scotopic Vision
- The ability to see under low lighting conditions
- Thalamus
- It is here that the Lateral Geniculate Nuclei are located. The thalmus is considered the gateway to the cortex.
- Transducin
- A messenger protein in the light pathways located in the retina.
- Trichromatic Color Vision
- This is the result of the three cones working together.
- Visible Light
- These are electromagnetic waves found between 400 and 700 nm in length
- Vitreous Humor
- A jelly like fluid found in the rear chamber, area between the lens and the retina.
- Zonule Fibers (Suspensory Ligaments)
- These are the fibers attached to the edge of the lens with the other end attached to the ciliary body. When the fibers are most taunt the lens is at it flattest, which allows for focusing on far objects.
Mechanics of Lens Movement
See Critical Eye Anatomy
See Retina
Visual Fields and Binocular Vision
The Need for Eye Glasses
Some Eye Tricks with Explanations:
See Optical Pathway
Eye Physiology Glossary
Other Eye Physiology Links
Great Links for Eye Illusions
For more information of Eye Disorders:
References
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