The Retina is the innermost layer of the eye, an outgrowth of the Central Nervous System (CNS), made up of nerve cells and covering most of the inner wall of the eyeball. When we look directly at an object, its image falls on the Retina at a fixed spot, along the visual axis, that lies in a slight depression: the Fovea, this tiny area is responsible for our central, sharpest vision.

• Rods: See in black, white, and shades of gray and tell us the form or shape that something has. They are super-sensitive, allowing us to see when it's very dark.

• Cones: Sense color and need more light than Rods to work well. Cones are most helpful in normal or bright light. There are 3 types of cones - red, green, and blue - to help you see different ranges of color. Together, these Cones sense combinations of light waves that enable our eyes to see millions of colors.

Several layers of the Retina correspond to different parts of cell groups. Horizontal Cells connect Photoreceptors together laterally, their nuclei lies among the Bipolar cells. The synaptic layer between Bipolar, Horizontal cells and Photoreceptors forms the outer plexiform layer of the Retina. Amacrine cells connect laterally between Ganglion cells, their nuclei also lies amongst those of the Bipolar cells. These interactions are important for determining the ways in which patterns of light affect the activity in Ganglion cells. The Axons of Ganglion Cells sweep across the Retina and meet at the Optic Disc and leave the eye as the Optic Nerve at the Blind Spot, an area absent of Photoreceptors.

Photoreceptors are composed of an inner segment and an outer segment, as well as a cell body and synaptic terminal. The main difference between Rods and Cones is in the outer segment, where the visual pigment is located:

• Pigment in Rods: On flattened, internalised discs.
• Pigment in Cones: On a region of infoldings of the membrane.

A cycle of events takes place in rhodopsin when light strikes on Rod-cells:

1. In the dark, opsin is bound tightly to retinene.
2. When light intensity is increased, retinene changes shape.

(This is a structural change from cis- to trans- form.)

3. Opsin cannot hold onto retinene, retinene comes off (this process is called bleaching).

4. Generator potential is produced when the membrane of a Rod is depolarized.

5. Generator potentials summate (add together) to form an action potential.
6. A nerve impulse is fired off to brain via the Optic Nerve.
7. Rhodopsin is reformed when retinene resumes its original shape using the energy in mitochondria (the power-house of every cell). This is light independent (does not need light).
8. Rhodopsin is ready to be bleached again.

Photoreceptors -> Bipolar Cells: The receptors release glutamate on to the Bipolar- and Horizontal cells, causing either excitation via ionotropic receptors or inhibition via metabotropic receptors.

• IF the Photoreceptor inhibits the Bipolar cell then the Bipolar cell would be excited (disinhibited) when the photoreceptor is hyperpolarized: "ON" response.
• OR the Photoreceptor excites the Bipolar cell, then light would produce an "OFF" response.

Photoreceptors are therefore unusual in the fact that their stimulus causes a hyperpolarisation, not a depolarisation.

Rhodopsin vs. Iodopsin: The light-sensitive photopigments are made of the protein opsin and the chromophore retinene. Bleaching of iodopsin in Cones is similar to rhodopsin in Rods, only Rods contain a higher concentration of visual pigment than Cones, so more light is needed to cause an action potential to be fired in Cones (threshold for Cones is higher than Rods). In other words, Rods are mainly used for dim light vision, Cones for bright light vision. A period of time is needed before our eye can adapt to new light conditions so we can see properly again, this is called "Adaptation":

• Dark Adaptation: When we move from a lit room to a dark room, we cannot see clearly, because not enough stimulated rhodopsin (peripheral): rhodopsin is bleached faster than it is reformed in strong light, insufficient rhodopsin reformed instantaneouslycones are not stimulated: light intensity too low. It takes about 20 minutes for enough rhodopsin to reform for us to see properly.

• Light Adaptation: When we move from a dark room to a brightly lit room, we feel uncomfortable from the glare. But after some time, the visual threshold in Cones (foveal) increases relative to the generator potential. Cones is less stimulated, and we will see better. This takes about 5 minutes.

3 types of Cones: Blue, Green and Red Sensitive.

This is what allows us to distinguish between different colours, the iodopsin molecules in these 3 types of Cones are slightly different from each other.

Overview: