Dark Adaptation

Question 1

What is Photopic vision?

Answer 1

During brighter conditions with just the cones functioning (luminance about 0.03 candela/m2)

Question 2

What is Mesopic vision?

Answer 2

During twilight conditions (both rods and cones functioning)

Question 3

What is Scotopic vision?

Answer 3

During darker conditions (just rods functioning)

Question 4

What is candela?

Answer 4

Unit of luminous intensity in the International System of Units

Question 5

What is EPSP?

Answer 5

Excitatory post-synaptic potential
Allows for glutamate to be released more from photoreceptors when the channels are open

Question 6

What is IPSP?

Answer 6

Inhibitory post-synaptic potential
This is when channels are closed in the light so the cell interior becomes more negative to below RMP (-40 in this case) which reduces the glutamate release

Question 7

What is the RMP of photoreceptor cells?

Answer 7

Resting membrane potential (-40)

Question 8

What is TP of photoreceptor cells?

Answer 8

Threshold potential, becomes more positive so reach threshold potential which is an excitatory post-synaptic potential (EPSP) but when it becomes more negative it becomes an inhibitory post-synaptic potential (IPSP)

Question 9

What happens in the light to cis-retinal?

Answer 9

Transformed to trans retinal and opsin in the Rhodopsin housing (this uses up the cis retinal molecule until it can be regenerated). Regeneration occurs in the dark, and the period where rhodopsin is essentially useless, is called ’bleaching’ period. During this period, cones are doing the majority of work.

Question 10

What does Opsin do in photoreceptor cells?

Answer 10

Opsin = kicked off of the cis retinal when it’s converted to trans retinal. This Opsin activates transducin that activates phosphodiesterase that then hydrolyses cGMP so can’t keep ion channels open so they close so less positive ions come in so the interior of the cell becomes more negative (below RMP) which causes an IPSP to occur and so glutamate excretion reduces.

Question 11

What are the types of BP cells?

Answer 11

Rod, Flat, Midget

Question 12

What cells, and how many of them, do rod BP cells synapse with?

Answer 12

ROD BP cells – synapse several rod cells to 1-4 ganglion cells

Question 13

What cells, and how many of them, do flat BP cells synapse with?

Answer 13

FLAT BP cells – synapse many cone cells with many ganglion cells

Question 14

What cells, and how many of them, do midget BP cells synapse with?

Answer 14

MIDGET BP cells – single cone to single ganglion

Question 15

What are On-centre BP cells?

Answer 15

BP cells in the centre of the radial orientation which activate when photoreceptors are ‘OFF’ in the light

Question 16

What are Off-centre BP cells?

Answer 16

OFF centre = BP cells in the periphery of the radial orientation which activate when photoreceptors are ‘ON’ in the dark

Question 17

What receptors do on-centre BP cells have?

Answer 17

Mesobotropic

Question 18

What receptors do off-centre BP cells have?

Answer 18

Ionotropic

Question 19

In the dark, what happens to on-centre BP cells?

Answer 19

They become hyperpolarised and release LESS glutamate

Question 20

In the dark, what happens to off-centre BP cells?

Answer 20

In the dark, OFF centre BP cells have more interaction with glutamate which react with ionoptropic receptors. This causes potassium to leave the BP cell resulting in a more positive cell = depolarisation = more glutamate release

Question 21

In the light, what happens to on-centre BP cells?

Answer 21

On-centre cells have less interaction with glutamate as less is being released by the photoreceptors in the light. This allows them to be in a DEPOLARISED state and thus release more glutamate

Question 22

In the light what happens to the off-centre BP cells?

Answer 22

OFF centre BP cells have ionotropic receptors. These bind to whatever glutamate there is and this HYPERPOLARISES the cell

Inside of the cell becomes more negative which decreases glutamate release from OFF centre BP cells

This makes sense because in the light, you want ON centre BP cells to be working more than OFF centre BP cells

Question 23

What do horizontal cells release when the photoreceptors release glutamate?

Answer 23

GABA alongside stimulating the BP cells that inhibit the photoreceptors and tell them to relax until stimulated again.

Question 24

What does GABA from horizontal cells do?

Answer 24

This will reduce the amount of glutamate being released by the photoreceptor in the light (not in the dark as we want the retina to be sensitive in the light which is good because mesobotropic receptors (central fovea) work better with less glutamate (where they’re by releasing MORE glutamate for the ganglion cells to fire MORE as the BP cells are hyperpolarised with LESS glutamate). Regulate the photoreceptors release of glutamate via GABA as the day goes on so that we know that it has gotten brighter as the day goes on

Question 25

What do Amacrine cells do?

Answer 25

They do a similar job to the horizontal cells. The ganglion cells are receiving glutamate from the ganglion cells above them that are giving action potentials and form the optic nerve, Glutamate hits the ganglion cells from the BP cells and the amacrine cells also release GABA and other stuff too like dopamine and ACh which inhibit the ganglion cells as they’re saying there is enough going to the brain in the centre so tell the peripheral cells to fire less action potentials until they are stimulated by a further amount of glutamate.

Question 26

What unit of measurement is luminance in?

Answer 26

Candelas

Question 27

What is illuminance?

Answer 27

The measure of how bright the reflection from the emitted light is.

Question 28

What is dark adaptation?

Answer 28

Dark adaptation refers to how the retina recovers its sensitivity in the dark after exposure to various intensities of light

Question 29

How does dark adaptation occur?

Answer 29

It occurs when going from a well light area to a dark area. Initially blackness is seen because our cones cease functioning in low intensity light. Also, all the rod pigments have been bleached out due to the bright light and the rods are initially non-functional. Once in the dark, rhodopsin regenerates and the sensitivity of the retina increases over time (this can take approximately one hour).  During these adaptation process reflexive changes occur in the pupil size.

Question 30

What is the dark adaptation curve?

Answer 30

Cones reach maximum sensitivity within 5-8 minutes – so now shine low light in the eye in the dark after this period, this is the time when most likely to pick up a light as at max luminance. Rods take around 30 minutes for your rods to recover to see light in the dark.

Question 31

What affects dark adaptation?

Answer 31

Intensity and duration of pre-adaptation light - the brighter the light, the longer it takes to adapt when going from light to dark Size and position of the retina – faster adaptation when light only on rods Wavelength of light – longer wavelengths take longer to adapt from Rate of rhodopsin regeneration (vitamin A deficiency the Rods take longer to adapt)

Question 32

How do you measure someone's dark adaptation?

Answer 32

'Bleach’ the patient (the time for the photoreceptors to be able to take off signals as unable to start the cascade because the rhodopsin in the cell has all be used up; the only way to reset this is to go into the dark so that opsin can bind back to trans retinal to become cis retinal which lives in the rhodopsin). Unlikely to bleach every rhodopsin cell in the photoreceptors due to how many cells and how much rhodopsin there is.

Question 33

What happens in the dark to the photoreceptors starting at the open channels?

Answer 33

Guanylyl cylase converts GTP to cGMP This binds to various channels in the outer membrane Positive ions come in and keep the cell depolarised = generate an EPSP = increased glutamate