eye 👁️

• Photoreceptors are found in the retina, there are two types: rods and cones. 
• Rods and cones are transducers, they convert light energy into electrical energy of a nerve impulse. 
• Optical pigments absorb light and break down - resulting in the production of a generator potential. 
• Rhodopsin is the only pigment in rod cells - broken down in dim light.  
• Iodopsins are found in cone cells - there are three different types, each sensitive to a different wavelength of light (bright light). 
• Rod cells cannot distinguish between different wavelengths of light - so only produce black and white images. 
• There are more rod cells than cone cells. 
• Many rod cells share a single bipolar cell, this is called a retinal convergence and the process is summation. There is a much greater chance that the threshold will be reached. This means you can see in low light intensity. 
• Because many rod cells are joined to one bipolar cell, only a single impulse will be produced. This means rod cells cannot distinguish between separate sources of light that stimulated them, 2 dots close together will appear as a single blob - rod cells have a low visual acuity. 
• ATP is used to resynthesise the rhodopsin. 
• Each cone cell responds to a different wavelength of light. Depending on the proportion of each type stimulated, we perceive images in full colour.
• Cones that can detect blue light are not coloured blue, they contain a blue pigment.  
• Each photoreceptor has a different form of iodopsin. 
• Each cone cells has its own bipolar cell connected to a sensory neurone, so often a generator potential is not reached. This means cone cells only respond in high light intensities. If 2 adjacent cone cells are stimulated the brain receives 2 impulses. 
• Distribution:

  Light is focused by the lens on the fovea - fovea receives the highest light intensity. Only cone cells are found at the fovea. Concentration of cone cells reduces with distance from the fovea. Furthest from the fovea (periphery) only rod cells are found.