9.7

• The human retina consists of two types of photoreceptor cells: cone cells and rod cells.
• Rod cells are evenly distributed around the periphery of the retina but are not located in the central fovea.
• Cone cells are mainly located in the central fovea of the retina.
• There are no photoreceptors at the blind spot where ganglion axon fibres form the optic nerve.
• Rod cells do not generate action potentials in the dark due to the following process: Na+ enters the outer segment of the rod cell via non-specific cation channels.
• Active transport of Na+ out of the inner segment of the rod cell slightly depolarises the cell.
• When a certain threshold of depolarisation is reached, voltage-gated Ca2+ channels open, triggering the exocytosis of glutamate.
• Glutamate acts as an inhibitory neurotransmitter to hyperpolarise a bipolar neuron.
• Rod cell becomes hyperpolarised.
• Rod cells are very sensitive due to spatial summation of subthreshold impulses, which allows them to see in low-light conditions.
• Bipolar neuron depolarises.
• Rod cells generate an action potential in the light.
• Cone cells contain three types of iodopsin which absorb red, blue or green wavelengths of light, giving them tricolour vision.
• No glutamate is released, so no inhibitory signal.
• Cone cells have 1 cone cell synapse with 1 bipolar neuron, which results in no retinal convergence and high resolution.
• Rod cells have many rod cells synapse with 1 bipolar neuron, which results in low resolution.
• Cone cells are less sensitive, which affects their vision in bright light.
• Opsin closes cation channels via a hydrolysis reaction.
• Rhodopsin absorbs all wavelengths of light, which gives rod cells monochromatic vision.
• Rhodopsin pigment bleaches when it absorbs light and breaks down into opsin and retinal.
• Active transport of Na+ out of the inner segment continues.