Detection of light by mammals

In a nutshell

The human eye contains many light-sensitive cells called rods and cones that enable you to see. These transmit signals to bipolar neurones which relay the signal to the optic nerve, which then carries the signal to the visual cortex in the brain.

Retinal structure

The retina contains two types of light-sensitive cells called rods and cones. These cells have an inner segment which contains lots of mitochondria and outer segment that is made up of flattened membrane-bound sacs that contain rhodopsin or iodopsin. The retina is described as 'inverted' because light passes through the neurones, passes the synapse with the rods and cones and then reaches the outer segment.

The fovea is the region of the retina with the most photoreceptors, this is the region with the greatest visual acuity. The visual acuity is how accurate the images are, therefore it is a measure of how well the eye can distinguish between two objects that are close together. The cones have a greater visual acuity than the rods.

1. Light rays, 2. Ganglion cell body, 3. Synapse, 4. Bipolar neurone, 5. Rod cell, 6. Cone cell, 7. Inner segment, 8. Outer segment, 9. Pigment cells, 10. Blind spot, 11. Optic nerve fibres, 12. Membranes of the outer segment that contain photosensitive pigments.

Rods

The retina of a human eye contains approximately $120\ million$ rods. They are distributed evenly throughout the retina and they are very sensitive to light. This means they can still be active in lower light intensities, so they are most important in dim light and in the dark. However, they have a lower visual acuity and are not able to distinguish between colours.

Rods use a photochemical pigment called rhodopsin which is made up of the protein opsin and retinal, which absorbs light. Rod cells use convergence, this means that many rod cells are connected to one optic nerve.

Curiosity: Retinal is derived from vitamin A which is why a vitamin A deficiency can lead to blindness.

An advantage of this is that many weaker generator potentials can combine to reach the threshold and trigger an action potential in rod cells, hence explaining why they can still produce an action potential in dim light. However, this also reduces the visual acuity of the rods as they may share the same optic nerve. Therefore, the light may stimulate two rods as there are two points, but only one signal will be sent to the brain which means that it will interpret only one point.

Biology; Control systems; KS5 Year 12; Detection of light by mammals — A. Dim light, B. Dark, C. Ganglion cell, D. Bipolar cell, E. Pigment in the inner segment, F. Outer segment with rhodopsin.

In the light

1a.	When light hits the retina, the rhodopsin molecule undergoes bleaching. This means that it breaks down into its constituent elements; retinal and opsin. $rhodopsin \xrightleftharpoons[darkness]{light}opsin+retinal$
2a.	Opsin activates a series of membrane-bound reactions which results in the hydrolysis of a molecule attached to the cation channel in the outer segment. This causes the cation channels to close which decreases the influx of $Na^+$ into the rod.
3a.	However, $Na^+$ ions are still being pumped out of the inner segment. This results in hyperpolarisation of the rod cell as the inside of the cell is more negative.
4a.	No neurotransmitter (glutamate) is released.
5a.	As there is no inhibition of the post synaptic membrane, $Na^+$ ions enter the bipolar cell and causes membrane depolarisation.
6a.	An action potential is generated in the ganglion neurone of the optic nerve connected to the rod cell.
7a.	The signal is then transmitted to the visual cortex of the brain. *Note:* Rhodopsin is then reconstructed using ATP that the mitochondria in the inner segment produces. In very bright light, all of your rhodopsin is being bleached so you rely on your cones to see.

Curiosity: When you move from a very bright location to a dimmer location, you may be temporarily blinded as your rhodopsin is reforming to allow you to see again.

In the dark

1b.	In dark conditions, $Na^+$ diffuses into the outer segment through non-specific cation channels. The ions move down their concentration gradient into the inner segment where they are actively pumped out of the rod cell using ATP that is produced by the mitochondria.
2b.	The influx of $Na^+$ ions causes a slight depolarisation of the cell membrane and a potential difference of $-40\ mV$ . This triggers the release of glutamate which prevents the depolarisation of the bipolar cell.
3b.	This prevents the release of any neurotransmitter between the bipolar cell and the ganglion cell.
4b.	Therefore, no action potential is generated and no signals are send via the optic nerve to the brain.

Cones

There are approximately $6\ million$ cones in the human retina and they are primarily located in the fovea. The cones allow you to distinguish between different colours and they have a high visual acuity as they are found in a $1:1$ ratio with bipolar neurones. Therefore, stimulation of two cone cells will send two impulses to the brain.

Cones use a different photochemical pigment called iodopsin which is not broken down as easily as rhodopsin. This explains why cones can only work in higher light intensities and why you can't see colours in dim light (as only your rods are working which can't distinguish between colours).

There are three types of cone cells in the human retina and each one has a different type of the iodopsin pigment. This is known as the trichromatic theory of colour vision where each iodopsin pigment absorbs a different wavelength of light which allows you to see different colours.

Processing images

Action potentials are transmitted through bipolar neurones to the optic nerves and into the visual cortex of the brain. Through a process known as contralateral processing, the brain interprets the action potentials from the left eye on the right side of the visual cortex and action potentials from the right eye on the left side of the visual cortex. These are then combined to allow you to see the full image.

Biology; Control systems; KS5 Year 12; Detection of light by mammals — 1. Left visual field, 2. Right visual field, 3. Left eye, 4. Right eye, 5. Optic chiasma, 6. Region of synapses, 7. Visual cortex, 7a. Left side, 7b. Right side.