Topic 9.7 Detection of light by mammals Flashcards
Describe the order of the structure of the human retina, in order of direction of light
- Ganglionic cells from optic nerve fibre
- Bipolar neurons
- Photoreceptors
Name two types of photoreceptor cell located in the retina
- Cone cells
- Rod cells
Where are rod and cone cells located in the retina?
Rod: evenly distributed around the periphery but NOT in central forvea.
Cone: mainly in central forvea.
No photoreceptors at blind spot where ganglion axon fibres form optic nerve.
Explain why rod cells do not generate action potentials in the dark
- Na+ enters outer segment of rod via non-specific cation channels. Active transport of Na+ out of inner segment = rod cell is slightly depolarised.
- Action potential = voltage-gated channel Ca2+ channels open. Triggers exocytosis of glutemate to hyperpolarise the bipolarneuron.
- Glutamate acts as an inhibitory transmitter to hyperpolarise bipolar neuron.
Explain how rod cells generate an action potential in the light
- Rhodopsin pigment bleaches when it absorbs light and breaks down into opsin and retinal.
- Opsin closes cation channels via a hydrolysis reaction. Activate transport of Na+ out of inner segment continues.
- Rod cell becomes hyperpolarised. No glutamate is released, so no inhibitory signal.
- Bipolarneuron depolarises.
Describe the pigments in the rod and cone cells.
Rod: rhodopsin absorbs all wavelengths of light = monochromatic vision.
Cone: 3 types of iodopsin which absorb red, blue or green wavelength of light = tricolour vision.
Describe the visual acuity of rod and cone cells
Rod: many rod cells synapse with 1 bipolar neuron = low resolution.
Cone: 1 cone cell synapses with 1 bipolar neuron so there is no retinal convergence = high resolution.
Describe the light sensitivity of rod and cone cells.
Rod: very sensitive due to spatial summation of subthreshold impulses = vision in low light conditions.
Cone: less sensitive = vision in bright light.
