7: Physiology II

Question 1

What are the two types of photoreceptor found in the retina?

Answer 1

Rods (low-light vision)

Cones (acute colour vision)

Question 2

What happens to the membrane potential of a photoreceptor when light strikes it?

Answer 2

Hyperpolarises

Question 3

Which type of channel is open in the dark and closed in the light?

Answer 3

cGMP-gated Na+ channel

“dark current”

Question 4

Why do cells tend to have a negative resting potential?

Answer 4

Action of NaKATPase: Na+ actively transported out so resting membrane potential is closer to conductance of K+ (-90mV)

Question 5

What causes the photoreceptors to hyperpolarise when struck by light?

Answer 5

Closure of cGMP-Na+ channel

i.e “dark current” is turned off

Cell’s rmp was previously between sodium and potassium

Now potassium only > more negative

Question 6

Which protein is the pigment in photoreceptors?

Answer 6

Rhodopsin

Question 7

What are the rho and opsin parts of rhodopsin?

Answer 7

Rho = retinal (Vitamin A)

dopsin = opsin (G protein coupled receptor)

Question 8

What happens when rhodopsin is struck by light?

Answer 8

Opsin activates cGMP

cGMP activates Na+ channel, causing it to CLOSE

Hyperpolarisation due to reduced Na+

Nerve impulse

Question 9

What happens to glutamate release when photoreceptors are:

a) depolarised
b) hyperpolarised?

Answer 9

a) Higher glutamate secretion

b) Lower glutamate secretion

Question 10

What is visual acuity?

Answer 10

Ability to distinguish two nearby points

i.e how blurry your vision is

Question 11

What determines your visual acuity?

Answer 11

Photoreceptor spacing

Question 12

Which type of photoreceptor is concerned with

a) acute, colour vision
b) low light vision?

Answer 12

a) Cones

b) Rods

Question 13

Why is low-light vision less acute than colour vision?

Answer 13

Rods are more WIDELY spaced at the peripheries of the retina

Cones are more DENSELY packed at the fovea

Question 14

Why is low-light vision more sensitive than colour vision?

Answer 14

Ganglia pick up impulses from a GREATER number of rods than they do cones

Multiple rods to a ganglia, few cones to a ganglia

So all the information is added together - your vision is more sensitive but less acute

Question 15

Acute, colour vision depends on the ___ of cones.

Low light vision depends on the ___ of rods connected to each retinal ganglion cell.

Answer 15

density

convergence

Question 16

Why can we see more than one colour?

Answer 16

Different CONES for different wavelengths of light

Question 17

Nerve fibres from the (nasal / temporal) side of the each retina cross over where?

Answer 17

nasal retina

optic chiasm

Question 18

Optic ___ travel to the optic ___ and then continue on as optic ___.

Answer 18

Optic nerves

Optic chiasm

Optic tracts

Question 19

Because the (nasal / temporal) optic nerves cross over at the optic chiasm, the brain processes light from the left and right at the opposite visual cortex.

Answer 19

nasal crossover

LEFT VISUAL FIELD > RIGHT VISUAL CORTEX

and vice versa

Question 20

Information from the superior visual field is processed by the ___ visual cortex.

Answer 20

inferior visual cortex

and vice versa

Question 21

Which layer of the visual cortex receives all the information from the left and right visual fields?

Answer 21

4C

Question 22

In layer 4C of the visual cortex, light from each eye is processed (individually / collectively).

Answer 22

individually

Question 23

Past layer 4C of the visual cortex, information from each eye is processed (individually / collectively).

Answer 23

collectively

no distinction anymore

Question 24

What is Hebb’s postulate?

Answer 24

Cells that fire together, wire together

i.e input from right and left visual fields is required for optic pathway to develop properly, otherwise you get stunted development and vision problems