DETECTION OF LIGHT BY MAMMALS Flashcards
(21 cards)
What is the function of the retinal cells? (1)
Initiate action potentials if they are to communicate via the nervous system
What are the types light-sensitive cell in the retina? (1)
They are called rods and cells, these ver elongated cells have an outer part called the outer segment. This consists of flattened membranous vesicles housing a light-sensitive pigment. An inner segment contains many mitochondria. Rods are distributed evenly throughout the retina while cones are concentrated at and around a region called the fovea.
What is the fovea? (1)
This is an area where vision is most accurate, here is the greatest density of photoreceptors (the light passes through the neurones, synapsing with the rod and cone cells before reaching the outer segments of these cells. Because of this feature the retina is described as ‘inverted)
What are the differences between the rod and cone cells in terms of light? (1)
Rod cells are extremely sensitive to light more so than cones, but rod cells do not discrimate colour. Since they respond to lower light intensities than cones, they are principally used for dim light and night vision.
What type of visual pigment molecule housed in rod cells? (1)
Called ‘visual purple’ or rhodopsin, this molecule is a combination of a protein (opsin) and a light-absorbing compound derived from a vitamin A called retinal.
Describe the phenomenon known as convergence. (1)
Each cone cells synapses with only one optic nerve fibre whereas a single optic nerve fibre has synapses connecting it to several rod cells, this means that in bright light cone cells canon not only provide a coloured image but also a much more accurate image.
What is visual acuity? (1)
This is the ability of the eye to distinguish between two points close together
Describe the visual acuity of rods and cone cells. (1)
The visual acuity of the cones is much higher than that of rods, as points become increasingly closer and closer, there comes a time when they are so close that despite stimulating two different rods, these rods share the same optic nerve fibre so only one signal is sent to the brain.
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How do rod cells respond to stimulation in dim light? (1)
1) light absorption changes rhodopsin
2) Na+ ion channels closed
3) Na+ ions continue to be pumped out; rod cell becomes more negative (hyper-polarised)
4) no transmitter substance released
5) bipolar cell becomes depolarised
6) release of transmitter substance here
7) action potential generated - passes along optic nerve
How do rod cells respond to stimulation in the dark? (1)
1) flow of sodium ions (outer segment of rod cell with rhodopsin)
2) released of transmitter substance here (pigment in inner segment of rod cell with mitochondria)
3) no release of transmitter substance here (bipolar cell)
4) no action potential generated (ganglion cell)
What occurs at the first stage of a rod cell response in the dark? (1)
At the same time as the sodium ion flow, the cell surface membrane of the inner segment reverses this movement of ions - sodium ions are continuously pumped out of the cell at this point. This efflux occurs at the expense of ATP formed in the mitochondria of the inner segment. A concentration gradient forms between the two segments
What occurs because of the dark conditions for the rod cells? (1)
The rod cells release a neurotransmitter substance (glutamate) that binds to the bipolar cell and prevents it depolarisation, the consequence of that no action potential is generated in the optic nerve that synapses with that bipolar cell.
What happens when light falls on the retina? (1)
It’s causes reversible structural change in rhodopsin (called bleaching) breaking it down into retinal and opsin. Opsin now functions as an enzyme that activates a series of reaction resulting in the closing of the cation channels of the outer segment and so the influx of sodium ions is blocked.
What causes the interior rod cell to become negative during presence in the light? (1)
A state described as depolarisation, in this condition no neurotransmitter is released by the rod cell, and the bipolar cell becomes polarised.
What is the role of cone cells? (1)
Come cells are able to distinguish colour, they operate the same as rod cells but with a different pigment called iodopsin, this is less readily broken down; it needs more light energy. Cones work only in high light intensities, we cannot see colours in dim light.
What is the trichromatic theory? (1)
There are three types of cone cell present in the retina, each with a different form of iodopsin. These absorb different wavelengths of light - in the blue, green and red regions of the spectrum. White light stimulates all three types equally, but different colours are produced by the relative degree of stimulation of the three types of cone.
What happens as a consequence of a reduction of images in the retina? (1)
Action potentials generated in the rods and cones are carried by neurones of the optic nerves to the visual cortex of the brain.
What is the contralateral processing? (1)
While each eye views left and right side of the visual field, the brain receives and interprets actions potentials from the right and left visual fields on the opposite side of the visual cortex.
What is perception? (1)
The messages from interpretation of these actions potentials are combined by the brain to produce a single impression - your right. ‘Seeing’ therefore occurs largely in the brain, and the seeing process.
