The human eye is a complex organ, made of many different types of cells. It's main purpose is to focus and detect light, sending nerve impulses to the brain, which then constructs a mental picture of the surroundings.
The eye has to types of photosensitive sensitive cells. Rod cells and Cone cells.
- Rod cells are the most numerous type of receptor in the eye, with around 120 million in each eye
- They cannot distinguish between different colours of light; the images they form in the brain are in black and white.
Multiple rod cells can join to one sensory neurone, via one bipolar neurone. This has two implications:
- Rod cells can detect very low intensities of light. This is because for the sensory neurone to be triggered, a certain value, known as the threshold value must be achieved. One rod cell may not be stimulated enough to exceed the threshold value, but if linked with other rod cells, it can sum up its individual action potentials to exceed it. Another reason for rod cells ability to detect low levels of light is due to the pigment they contain. Rhodopsin is broken…