rod cells and cone cells

light receptors are found at the back of the eye in a layer called the retina, there are two types of light receptor cells: rod cells and cone cells

and they both convert light energy into an electrical nerve impulse - act as transducers

nerve impulses from the retina pass down the optic nerve to the brain

rod cells function with a bipolar neurone

retinal convergence is the idea that several rod cells signal to one bipolar neurone

• each rod cell contains a light-sensitive pigment called rhodopsin

• when light hits the rod cells, this causes rhodopsin molecules to be broken down

• the rod cells then cause the bipolar neurone to depolarise

rod cells are very sensitive to low light intensity

→ rhodopsin does not need a lot of energy to break down

→ because several rod cells are connected to the same bipolar neurone the signals from these rod cells add together = summation (increases chances that the bipolar neurone will exceed the threshold value and produce a generator potential)

however because several rod cells signal to a single bipolar neurone the brain cannot distinguish which individual rod cell absorbed light - so in low light intensity the brain perceives a low resolution image = low visual acuity

cone cells only respond to high intensity light

• each cone cell connects to an individual bipolar neurone - so they must trigger the generator potential in its bipolar neurone

• cone cells contain the light sensitive pigment iodopsin - iodopsin requires high light intensity to break down

the human eye contains three different types of cone cells which each contain a different type of iodopsin that responds to different wavelengths of light