Reception Of Stimuli - Eyes

Question 1

Which muscle will cause the pupil to dilate?

Answer 1

Radial muscle

Question 2

Which muscle will cause the pupil to constrict?

Answer 2

Circular muscle

Question 3

Which system controls the size of the pupil?

Answer 3

Autonomic nervous system

Question 4

What are the radial and circular muscles an example of?

Answer 4

Antagonist pair of muscles

Question 5

What are circular muscles controlled by?

Answer 5

Parasympathetic reflex

Question 6

What are circular muscles controlled by?

Answer 6

Parasympathetic reflex

Question 7

What are the receptors in the retina?

Answer 7

Photoreceptors

Question 8

What causes nerve impulses to be generated and passed along the optic nerve?

Answer 8

High light levels being detected by photoreceptors in the retina

Question 9

Describe what happens when there is too much light entering the eye

Answer 9

• 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

Question 10

What is the purpose of the pupil reflex?

Answer 10

• To prevent damage to the retina from high-intensity light

- In dim light it ensures the maximum amount of light reaches the retina

Question 11

The pupil reflex response to increased light is very rapid. Why does it need to be so?

Answer 11

-To protect the eye from sudden flashes of bright light

Question 12

How are Na+ ions pumped out of a cell?

Answer 12

Using energy from the hydrolysis of ATP

Question 13

What does a potential difference cause to happen to K+ ions?

Answer 13

K+ ions are pulled back into the cell

Question 14

What two types of photoreceptors cells are found in the retina?

Answer 14

• Rod cells

- Cone cells

Question 15

What do cone cells allow?

Answer 15

-Colour vision in bright light

Question 16

What do rod cells allow?

Answer 16

-Black and white vision in both dim and bright light

Question 17

Can you explain why some people describe the retina as functionally inside out?

Answer 17

• Expect photoreceptors to be on the surface of the retina

- Instead light has to travel through outer layers to reach the photoreceptors

Question 18

What is the photochemical pigment found in rod cells?

Answer 18

Rhodopsin

Question 19

Describe what happens to a rod cell in the dark

Answer 19

• 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

Question 20

What happens when light falls on a rod cell?

Answer 20

• 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

Question 21

What do you think a ‘non-specific’ cation channel is?

Answer 21

One that let’s positive ions through

eg Na+, Ca2+

Question 22

Why does the rod cell membrane become hyperpolarised in the light?

Answer 22

• 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

Question 23

What is dark adaption?

Answer 23

-The reforming of rhodopsin

Question 24

Why must rhodopsin be reformed after being broken down?

Answer 24

To detect an increase in light when the next change happens

Question 25

How does the retinal molecule exist in dark conditions?

Answer 25

In the cis form

Question 26

What happens when a photon of light hits the retinal molecule?

Answer 26

• Its shape changes from cis to trans
• This causes it to break apart from opsin
• This is known as bleaching

Question 27

What is bleaching?

Answer 27

-When opsin and retinal break apart after a photon of light hits it

Question 28

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

Answer 28

• 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