Cornea: clear, light passes through easily, innervated by free nerve endings, sensitive to pain, temperature, and touch

Sclera: outer protective area, white fibrous material

Found at the junction of the cornea and sclera.  Muscles increase focusing power by reducing tension on the suspensory ligaments (fibers of zen) attached to the lens.

The ciliary process is a continuous fluid source that makes up the vitreous humor and posterior chamber of the eye.  Continuous circulation of the fluid provides nutrients to the cornea and removes debri and waste products. Drains into the venous system at the canal of Schlemm. Problems with circulation causes glaucoma (increased pressure and cloudiness).

When light passes from one media to another, from something less dense to something more dense, the light is slowed down.  When this happens the light bends.  This bending is called the refractive index.

The bending power of a lens is called the refractive power and is measured in diopters.

Refractive power increases with increased curvature and density.

1) Diopters can be calculated using the refractive index of the two mediums using the following equation:

D=(n₂-n₁)/r

Where n is the refractive index and r is the radius in meters.

2)  Diopters are also related to the focal distance.  More diopters are required to see something up close.

D=1/f  or f=1/D

When light comes into the eye, it goes through the various parts and is bent.  Ultimately the total diopters focuses the light onto the retina. 

If you bring something close to you, the focal distance is small and more diopters are required.  For every mm you move closer, you need additional diopters. 

The lens provides these diopters by increasing its curvature.  The tension from the fibers is releases causing the lens to become shorter and therefore more curved.  This increases the diopters.

As you age, your lens looses its elasticity.  This means it is no longer capable of providing as many additional diopters.

One diopter allows you to focus one meter.

Farsightedness can be due to lens power, the eye being too short, or the muscles can be messed up.

The eye focuses behind the retina.  So, at the retina the image is fuzy.

To correct hyperopia, you need to add curvature in front of the eye in order to add diopters.  This is achieved by using biconvex lens.

Nearsighted: able to see pretty well up close because you have too much focusing power.  Beyond 5 feet or so, the image is blury because it is focused in front of the retina.

To correct you need to take away curvature or reverse the curve.  This is achieved by using biconcave lenses.

The lens is essentially squishy.  The curvature is not consistent; you may have more curvature at one angle than another. 

Corrected with cylindrical lenses.

-Photopigments are manufactured in the mitochondria and sit with in the outer segment of the membrane

-Light must come through the layers of the epithelium before it can hit the pigments and be absorbed.

-Rods and cones are under continuous dynamis growth- innersegment (membrane synthesis and pigment insertion) then the membrane flows up and folds over itself creating the outer segment

-For the rods, the folds break and free discs are created.  The rods also shed membrane pieces that are no longer needed.

-For cones, the discs are all connected on one side

-Density of photopigment is much higher.  So, they are more sensitive to light.

-Outer disc contains so many photoreceptors that they are able to detect a single photon of light.

-Good for nighttime vision

-Rod spectrum- 500nm- much wider spectrum, interpreted as black and white

-Require much more light and are useful for daytime vision

- 3 variants of photopigment that optimally absorb light at different frequencies

-blue cones/short wave length/ 437nm/ s-cells

-green cones/medium wavelength/ 533 nm/ m-cells

-red cones/ long wavelength/ 564 nm/ l-cells

-There is overlap with less energy/absorption

1)  Light is absorbed by the pigments in the outer segment.

2)  That causes a change in the stereoconfiguration from cis-rodobsin to trans-rodobsin

3) Produces meterodobsin which activates a g-protein transducent

4)  Cascade processes- cGMP converted to GMP by activation of phosphodiasterase

5) Since GMP causes a decrease in cGMP available, Na channels close and the cell is hyperpolarized.

6)  Hyperpolarizing response causes a decrease in neurotransmitter release.