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From where do rod cells derive their name?
The rhodopsin pigment embedded into disks of its membrane known as lamellae
What other organelle is required by the rod cells?
What occurs after the rhodopsin is stuck by light?
- It becomes bleached
- It breaks down into retinal and opsin
What occurs after the pigment is broken down?
- Sodium channels are opened resulting in the production of a generator potential
- If the generator potential is great enough an action potential is sent down sensory neurones to the brain
What happens to the rhodopsin after it is bleached?
- It is resynthesised back into its unbleached form
- The energy for this reaction is provided by the many mitochondria present in the rod cell
Why are rod cells useless in bright light conditions?
- Rhodopsin is broken down more quickly than it can be resynthesised
- Rod cells are used instead to see in low light intensities
Where are rod cells positioned?
All over the retina except at the fovea and the blind spot
What pigment is contained within cone cells?
Why are cone cells useless at night?
Iodopsin is only broken down in high light intensities
Why are cones much better than rod cells for daylight viewing?
- Iodopsin is broken down in high light intensities
- It is resynthesised to its unbleached form much more quickly than rhodopsin
Where are cone cells found in high concentrations?
What is sensitivity?
The ability to see at low light intensities
How do rod cells allow us to see at low light intensities?
- Many rod cells (up to 45) are connected to single bipolar cell
- Dim light only breaks down a small amount of rhodopsin in each cell
- The combined rhodopsin breakdown of each rod cell connected to the bipolar cell is enough to create a generator potential big enough to exceed threshold value and initiate an action potential
- This is an example of spatial summation
Why do cone cells have little sensitivity?
They are connected to a single bipolar cell
What is visual acuity?
The ability of the eye to resolve two stimuli separately
How do the properties of cone cells allow them to possess a high visual acuity?
- Each cone cell is connected to a single bipolar cell
- This means that each cone generates a separate action potential
What requirements must be met in order for an object to be resolved?
The light from the object must land on two cones separated by at least one unstimulated cone
Why do rod cells have poor visual acuity?
- Many rod cells converge on a single bipolar cell
- This means that light from two objects may strike two rod cells separated by unstimulated rod cells but only a single action potential would be sent to the brain and one object seen
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