Physiology of the visual system

Question 1

Describe the steps involved in perception of light

Answer 1

1- The retina contains a sheet of light-responsive photoreceptor cells
2- Each cell has a receptive field that generates an electrical response when stimulated by light
3- Signal is transmitted across the retina and terminates in the cortex

Question 2

What cells do the visual signal pass through on its way to the geniculate nucleus?

Answer 2

Photoreceptors
Bipolar cells
Ganglion cells

Question 3

Does light perception cause excitation or inhibition of a membrane potential?

Answer 3

It inhibits membrane potential leading to hyperpolarization

Question 4

What voltage is the membrane potential at rest in the dark?

Answer 4

-40mV

Question 5

In the dark are sodium channels open or closed?

Answer 5

Open
- This allows influx of sodium and the maintained depolarisation

Question 6

What is the mechanism to change membrane potential when light is perceived?

Answer 6

• Light stimulation of rhodopsin leads to g-protein activation
• G-protein activates cGMP phosphodiesterase
• PDE hydrolyses cGMP reducing its concentration and closes Na+ channel

Question 7

Are rods or cones more numerous?

Answer 7

Rods

Question 8

Where are cones most concentrated?

Answer 8

Fovea

Question 9

What does convergence mean?

Answer 9

Convergence is the number of rods/cones sending their signal to a ganglion cell
- The lower the convergence the more sensitive the ganglion cell is to change in light

Question 10

Is there higher convergence in the rod or cone system?

Answer 10

Rod system
- More rods per ganglion than cones

Question 11

What are cones meant for?

Answer 11

Vision in daylight
- Responsible for colour vision

Question 12

What are rods meant for?

Answer 12

Vision in dim light

Question 13

What aspect of cones are responsible for colour vision?

Answer 13

Short-wave, Middle-wave and long-wave cones containing different opsins.

Question 14

Is blue short or long wavelength?

Answer 14

Short

Question 15

Is red short or long wavelength?

Answer 15

Long

Question 16

What does lateral inhibition do?

Answer 16

emphasizes contrast

Question 17

What is the difference between the ON and OFF pathway?

Answer 17

The bipolar cell contains a inotropic receptor in the OFF pathway and a metabotropic receptor in the ON pathway

Question 18

How do bipolar cells react to light in the OFF pathway?

Answer 18

There is less receptors activated and therefore the bipolar cell is more negative and sends less signals to the ganglion cells

Question 19

How do bipolar cells react to light in the ON pathway?

Answer 19

Light stops inhibition of influx of Ca+ and inhibits outflowing K+, this means the bipolar cell is more positive and therefore sends more signals to stimulate the ganglion cells.

Question 20

How is the receptive field of a ganglion cell?

Answer 20

Center-surround organisation
- The center must be dark and light on the outer circle to fire most AP’s
- Looks like a doughnut

Question 21

What cells are responsible for center-surround organisation of receptive field?

Answer 21

Horizontal cell
Amacrine cell

Question 22

What are the 3 types of retinal ganglion?

Answer 22

M- (movement)
P- (form & colour)
nonM-nonP- (colour)

Question 23

What layers of the geniculate nucleus does info from the M-type ganglion cells go to?

Answer 23

1&2

Question 24

What layers of the geniculate nucleus does info from the P-type ganglion cells go to?

Answer 24

3,4,5,6

Question 25

What is the competition hypothesis?

Answer 25

During a critical period of development there is a mononuclear deprivation of one eye at the visual cortex and therefore one eye becomes dominant