the fovea: located in the center. This is a very small area (< 1 square millimeter) that contains three types of cone photoreceptors (for red, green and blue). This small area provides for sharp and colour vision. (memory trick; cone = colour)
the peripheral retina which contains only rod photoreceptors. These rods only sense black and white but are more sensitive than the cones.
There is also a blind spot, located in the inferior and nasal quadrant of the eye where the optical nerves exit the eye on their way to the brain.
B. The Photoreceptors
Both types of photoreceptors share the same plan:
1. At one end is a stack of "shelves" which are really infoldings of the plasma membrane. These shelves contain millions of the photo pigment molecules (such as rhodopsin).
2. The second part (or segment) contains the molecular machinery for the cell (mitochondriae etc).
3. The third part contains the nucleus
4. At the other end is the synapse that connects the receptor cell to other nervous cells in the retina.
The difference between the two cells is that in the rods, the shelves are of the same size whereas in the cones, the shelves diminish in size further away from the cell body, hence its shape and its name.
C. The Rhodopsin cycle:
Rhodopsin: this the molecule that is waiting on the shelves to be excited by a passing photon. In a normal situation, there will be millions of rhodopsin waiting.
Excitation: Once excited, the collision with the photon will cause one chemical bond in rhodopsin to change from a -trans to a -cis configuration. This is very fast. This molecule is now called bathorhodopsin (the names are not really important here).
Unstable: This new molecule is very unstable and changes spontaneoulsy into the next molecule, lumirhodopsin, which is still unstable and changes into the next (metarhodopsin I) which finally stabilizes as metarhodopsin II.
Delay: All these spontaneous transformation also take sequentially more time (from pico- to micro- to milliseconds). This is necessary for the metabolic processes in the cell (which takes milliseconds) to react to this excitation
Restoration of Rhodopsin: The final stable meta-rhodopsin II is converted through scotopsin back into rhodopsin. This takes time (minutes) and energy (ATP).
Vitamine A: The rhodopsin molecules are derived from vitamin A. If there is not enough vitamin A (deficient diet) then the person becomes gradually less sensitive to light (night blindness).
D. The Retina B
The retina has two layers:
a. the photoreceptor layer: which consists of rods (in the diagram) or cones.
b. the nervous layer: the receptor cells do not connect immediately to the brain cells. Instead they interconnect with other nerve cells. These intermediary cells already process the signals before communicating with the ganglion cells. The axon of the ganglion cells then combine to form the optic nerve.
Note that the direction of light is opposite what you would expect. The light rays have to go through the (thin) nervous layer before reaching the photoreceptors.
E. The Blind Spot
The blind spot:
The blind spot is located where the axons from the ganglion cells leave the eye ball. Because the nervous layer is on top of the photo receptor layer, the nerve has to go through the photo receptor layer to reach the sclera and leave the eye. Therefore, at that site, the photoreceptor cell layer is interrupted, hence the '"blind" spot.
Note that if the photo receptor layer had been at the other side of the nervous layer, and in front of the light rays, that then, there would have been no blind spot!
F. The Iris and the Pupil:
A. The ciliary muscle also controls the iris and therefore the size of the pupil. It actually consists of two muscles; one outer muscle which is oriented in the radial direction and a second inner muscle that is oriented in the circular direction.
B. Miosis: When the circular muscle contracts, the hole (= pupil) within the muscle becomes smaller. This works like a sphincter that you can see in other parts of the body (in the gut or the blood vessels for example).
This is actually a famous reflex (pupillary light reflex) that doctors often use when shining a bright light into the eye to check whether the patient is still alive. This reflex is controlled by the parasympathetic nervous system.
C. Mydriasis: is the opposite action (dilatation of the pupil) which is caused by contraction of the radial muscle. This happens in dim light allowing more light into the eye.
This contractionis controlled by the sympathetic nervous system.
G. Adaptation of the eye to light and dark:
very slow but very strong
very fast but not so strong
When you step from a bright room into a dark one, then initially, sensitivity to vision is much reduced. But with time, the eyes become more sensitive and one sees better in the dark.
This is because, when you stepped from the bright room, a lot of the rhodopsin had been activated and were being used in the cycle. These molecules were therefore not available to pick up the few photons in the dark room.
But with time (up to minutes), the molecules reverted back to the rhodopsin shape and the amount of 'waiting' rhodopsin molecules increased. This makes the eye more sensitive.
The pupil dilates (with the radial fibers) to allow more light into the eye (sympathetic reflex).
is the opposite of dark adaption. When you step from a dark to a bright room, then all the rhodopsin molecules will be activated.
In fact, one may be even very insensitive to light (=blind) as all the rhodopsin have suddenly become unavailable (= refractory) but in time, one can see again, but at a reduced sensitivity.
The pupil constricts (with the circular fibers) to areduce the amount of light into the eye (parasympathetic reflex