photoreceptors in the eye Flashcards
what photoreceptors does the retina of the eye contain?
the retina of the eye contains two types of light sensitive photoreceptor cell, rods and cone
what are rods and cones used for?
rods are particularly useful for vision at low light intensities i.e. possess high visual sensitivity.
cones are important for good discrimination of detail i.e. possess high visual acuity and also enable colour vision.
draw a diagram of rods and cones
see diagram
what is visual sensitivity?
which photoreceptors have high visual sensitivity? why?
which photoreceptors have low visual sensitivity? why?
visual sensitivity refers to the ability to ‘see’ at low light intensities. rods have high visual sensitivity.
the main reason for the high visual sensitivity of rods is that several rods synapse with one bipolar neurone.
as many rods synapse to a single bipolar cell, then in dim light the stimulation of several rods next to each other has an ‘additive effect’ (spatial summation – see notes on synapses) in the bipolar neurone and impulses are transmitted to the brain. the combined stimulation of several rods enables the threshold value to be reached.
cones have low visual sensitivity as cones often have a 1:1 relationship with a bipolar neurone and at low light intensities do not generate impulses along the neurone.
consequently, in dim light, vision is poorest at the fovea of the retina, where cones are concentrated and best at the edges where there are more rods.
draw the distribution of rod cells and cone cells along the retina
see diagram
what is visual acuity?
which photoreceptors have high visual acuity? why?
which photoreceptors have low visual acuity? why?
acuity refers to the ability to discriminate detail i.e. the precision or sharpness of the image seen
thus if one part of the retina could distinguish between two separate, but closely placed points, and another part could not, the former part would have greater acuity.
cones have high visual acuity as most cones synapse with only one bipolar neurone.
therefore for each cone cell stimulated a separate set of impulses is transmitted to the brain. several stimulated cones will result in several separate sets of impulses to the brain.
this 1:1 relationship provides maximum possible acuity because each part of the image is being detected by a different cell and there is no ‘blurring’ or combining of information
acuity is greatest at the fovea, (normally at the centre of our field of vision), as it contains only cones (about 90% of the cones in the eye are tightly packed in the fovea).
rods have low visual acuity due to rods sharing a bipolar neurone.
due to rods sharing a bipolar neurone, several rods may be stimulated, but only one bipolar neurone will be stimulated so the brain only receives one set of impulses from a larger region of the retina.
the greater the number of rods sharing a bipolar neurone, the lower the visual acuity
what is the visual acuity and sensitivity of rods and cones related to?
the visual acuity and sensitivity of rods and cones is related to how they are connected to bipolar neurons and ganglion cells in the retina.
