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What is a photoreceptor?
A receptor cell that is sensitive to light
What are the two forms of photoreceptor in a human and what type of vision do they provide?
Rods - black & white vision, cones - colour vision
Where are they located?
Within the retina of the eye
Label the diagram
Name the photochemical pigment present in rod cells
Rhodopsin
Describe where it is located
Within the flattened vesicles of the outer segment
Describe what happens to a rod cell in the dark:
Na+ is actively pumped out of the inner segment of the rod cell.
This requires the hydrolysis of ATP.
Na+ in the outer segment diffuses down its concentration gradient into the inner segment.
Na+ flows into the outer segment through open non-specific cation channels by facilitated diffusion down its concentration gradient.
This influx causes a slight depolarisation of -40mV.
This slight depolarisation causes the release of the neurotransmitter glutamate from the rod cell.
Binding of the glutamate to the bipolar cell hyperpolarises it and prevents it from depolarising (inhibitory synapse).
The bipolar cell does not release its neurotransmitter to depolarise the ganglion neurone.
Describe what happens to a rod cell in the light:
When light (photons) hits the rod, rhodopsin (formed of retinal and opsin) absorbs the light.
The retinal (non-protein) converts from its cis form to the trans form and breaks apart from the opsin (protein).
This is called bleaching.
The opsin activates a series of membrane-bound reactions ending in the hydrolysis of a cyclic nucleotide molecule.
This hydrolysis results in the closure of the non-specific cation channels in the outer segment.
The influx of Na+ into the rod cell decreases.
Na+ is still pumped out of the inner segment.
This results in the hyperpolarisation of the rod cell causing the release of glutamate to stop.
The lack of glutamate results in the depolarisation of the bipolar cell.
This causes the bipolar cell to release its neurotransmitter and depolarise the ganglion neurone.
An action potential is generated and impulses are sent along the optic nerve to the brain.
What occurs during dark adaptation?
The trans retinal converts back to cis retinal.
The retinal and opsin recombine to form rhodopsin.
This requires ATP.
The process can take up to 50 minutes.
This enables the eye to transition from a high light setting to a low light setting and restore retinal sensitivity i.e. the eye adapts its definition of what is black.
