Reception Of Stimuli - Eyes Flashcards
Which muscle will cause the pupil to dilate?
Radial muscle
Which muscle will cause the pupil to constrict?
Circular muscle
Which system controls the size of the pupil?
Autonomic nervous system
What are the radial and circular muscles an example of?
Antagonist pair of muscles
What are circular muscles controlled by?
Parasympathetic reflex
What are circular muscles controlled by?
Parasympathetic reflex
What are the receptors in the retina?
Photoreceptors
What causes nerve impulses to be generated and passed along the optic nerve?
High light levels being detected by photoreceptors in the retina
Describe what happens when there is too much light entering the eye
- Photoreceptors in the retina detect high light levels
- The receptors send nerve impulses along the optic nerve to a group of coordinating cells in the midbrain
- The coordinating cells send impulses along parasympathetic motor neurons to the circular muscles, causing them to contract
- At the same time, radial muscles relax
- The pupil constricts and less light enters the eye
What is the purpose of the pupil reflex?
- To prevent damage to the retina from high-intensity light
- In dim light it ensures the maximum amount of light reaches the retina
The pupil reflex response to increased light is very rapid. Why does it need to be so?
-To protect the eye from sudden flashes of bright light
How are Na+ ions pumped out of a cell?
Using energy from the hydrolysis of ATP
What does a potential difference cause to happen to K+ ions?
K+ ions are pulled back into the cell
What two types of photoreceptors cells are found in the retina?
- Rod cells
- Cone cells
What do cone cells allow?
-Colour vision in bright light
What do rod cells allow?
-Black and white vision in both dim and bright light
Can you explain why some people describe the retina as functionally inside out?
- Expect photoreceptors to be on the surface of the retina
- Instead light has to travel through outer layers to reach the photoreceptors
What is the photochemical pigment found in rod cells?
Rhodopsin
Describe what happens to a rod cell in the dark
- Sodium ions flow into the outer segment of the rod cells through non-specific cation channels
- They then move down the concentration gradient into the inner segment, where pumps continuously transport them out of the cell
- This causes a slight depolarisation of the cell
- This triggers the release of the neurotransmitter, glutamate, from the rod cells, which is released constantly
- The neurotransmitter binds to the bipolar cell, stopping it depolarising
What happens when light falls on a rod cell?
- Rhodopsin breaks down into retinal and opsin
- Opsin activates a series of membrane-bound reactions that result in the closing of the cation channels
- Na+ ions moving into the cell decreases while they are continuously pumped out of the inner segment
- The inside of the cell becomes hyperpolarised
- Glutamate neurotransmitter is no longer released
- This causes a depolarisation of the neurons that make up the optic nerve which produces an action potential
What do you think a ‘non-specific’ cation channel is?
One that let’s positive ions through
eg Na+, Ca2+
Why does the rod cell membrane become hyperpolarised in the light?
- Na+ ions actively transported out of the cell
- Na+ ions cannot re-enter through the now closed cation channels
- This increases the potential difference across the membrane
What is dark adaption?
-The reforming of rhodopsin
Why must rhodopsin be reformed after being broken down?
To detect an increase in light when the next change happens
How does the retinal molecule exist in dark conditions?
In the cis form
What happens when a photon of light hits the retinal molecule?
- Its shape changes from cis to trans
- This causes it to break apart from opsin
- This is known as bleaching
What is bleaching?
-When opsin and retinal break apart after a photon of light hits it
Part of the retina of a young rat was removed and kept in the dark for two hours. Suggest what happens in the rod cells during this two hours of darkness
- Opsin detaches from rod cell surface membrane
- Trans retinal converts to cis retinal
- Rhodopsin is reformed from opsin and retinal
- This results in dark adaption
- Permeability of cell surface membrane to Na+ ions increases and cation channels open
- Hyperpolarisation of the cell decreases
- More neurotransmitter is released
