Sensitivity in the Rods and Cones Flashcards
What is retinal convergence?
Many photoreceptor cells linked to one bipolar neurone.
Rods show retinal convergence, producing what?
This produces sensitivity in the rods.
Sensitivity is the ability to detect light at a low intensity, i.e. dim light.
Rods:
- Dim light falls on ____
“several rods at the same time.”
Rods:
- Each rod produces a small ____
“generator potential”, which on its own would be insufficient to produce an action potential in the bipolar neurone.
Rods:
- The generator potential from each rod is added together and is ____
“sufficient to produce an action potential in the bipolar neurone”.
This is called spatial summation, allowing dim light to be detected by the brain due to the sensitivity of the rods.
Rods:
- The action potential is passed on to the ____
“ganglion cell”.
Why do rods lack visual acuity?
Light can fall anywhere on the collection of rods (~105)
The brain cannot detect exactly where the light fell.
Therefore the rods cannot give vision in detail.
What if rods were linked to their own bipolar neurone?
Dim light would not have enough energy to produce a generator potential.
This means there would be no action potential in the bipolar neurone.
Therefore, light would not be detected by the brain.
Each cone is linked to its own bipolar neurone.
Cones do not show retinal convergence.
This gives them good visual acuity but poor sensitivity.
Cones:
- Light falls on two cones side by side.
Each one produces a generator potential so two action potentials arrive side by side at the brain.
The brain cannot distinguish whether the light is from one large object or two smaller ones.
2 AP arrive together
Cones:
- Light falls on two cones from separate sources with one cone in between.
Each cone produces a generator potential but two separate action potentials arrive at the brain so two separate objects are detected.
2 separated AP
The one to one relationship between the cones and the bipolar neurones means that ____
“each part of an image is detected by a different cell and there is no combining information”.
This gives good visual acuity.
What is visual acuity?
It is the ability of the eye to resolve 2 or more stimuli which are spatially separate and refers to the precision or sharpness of the image produced.
What does the level of acuity depend on?
How close the cones are; the closer the cones the more detailed the image detected by the brain.
Why are cones not as sensitive as rods?
They have a different pigment which requires light so they cannot detect dim light.
