The retina in detail Flashcards
What are the two specialised photoreceptors in the retina?
-rods and cones
What is the light sensitive pigment in rod cells and where is it found?
-rhodopsin
-packed into an array of membranes in the outer part of the rod cell
How is rhodopsin made?
-from the protein opsin combined with the light absorbing compound retinal
What happens to the rhodopsin when light hits it?
-it breaks down into its retinal and opsin compounds
Once the rhodopsin is broken down, what does this cause to happen?
-the membrane potential changes to create a generator potential
-if a threshold level is reached this can cause the adjacent neurone to become depolarised and so causes an action potential
What is a generator potential?
-the degree of depolarisation a stimulated receptor can produce
What cell organelles does the inner part of the rod cell contain? What is ATP used for in rod cells?
-contains a nucleus and mitochondria
-used to produce ATP for the re synthesises of rhodopsin from opsin and retinal
What type of light intensity does rod cells work in and why?
-low light intensities as they are highly sensitive to light
What happens in rods cells when exposed to low light intensities?
-rhodopsin will break down readily in low light
-to opsin and retinal
What is the dark adaptation mean in terms of rod cells?
-in bright light all the rhodopsin is broken down (bleached) and it takes time for it to all be resynthesised
-this explains why when we move from a dark area to a well lit area our vision is poor but then gradually improves
Look at diagram of rod cell on page 73
What is the pigment found in cone cells and where is it found?
-iodopsin
-found in membranes in the outer segment
When do cone cells produce a generator potential and why?
-iodopsin is less readily broken down and so only produces a generator potential in bright light
What type of vision do cone cells produce?
-colour vision
Why does iodopsin provide colour vision?
-because it exists in 3 forms each with different wavelengths
-the absorption peaks of each correspond to different colours, red, green and blue
What is the trichromatic theory of colour vision?
-blue light will only be broken down by the corresponding blue absorption peak of iodopsin
Apart from their structure and sensitivities, what else is important in the functioning of rods and cones?
-their distribution across the retina
Where are the rods and cone layer found in the eye?
-immediately inside the choroid
What type of neurones are the rods and cones linked to? What are these neurones then linked to?
-bipolar neurones
-then they are linked to a layer of sensory neurones known as ganglion cells
What part of the ganglion cells make up the optic nerve, what is the function of this nerve?
-the axons
-carries impulses from the retina to the brain
Look at diagram on page 74
How many come cells are connected to each bipolar neurone and what is the effect of this?
-one cone can synapse individually to one bipolar neurone
-this means that each can provide its own discrete image
What is the visual acuity like in cone cells? What does high visual acuity mean?
-cones have high visual acuity
-the ability of cones to provide highly precise colour vision with high resolution
How many rods are synapsed to each bipolar neurone and what is the term to describe this?
-multiple rods share a common bipolar neurone which is known as retinal convergence
How does retinal convergence allow an action potential to be reached?
-it allows the generator potential in individual rods to combine together reach a threshold to produce an action potential
What is the effect of retinal convergence in terms of light sensitivity and break down?
-light energy reaching one rod is not enough to stimulate the bipolar neurone but a group of rods provides enough stimulation to reach a generator potential to produce an impulse
What type of visual acuity do rod cells provide and why?
-they provide low acuity as the rods converge into one bipolar neurone
Look at the graph on page 75
What type of vision does the presence of two eyes in mammals produce?
-binocular vision
What is the definition of binocular vision?
-the ability to form 3 dimensional images
Why are predator species eyes at the front of their head?
-allows excellent judgement of distance and 3D vision
Why are prey’s eyes at the side of their head?
-provided a wider field of view
