Retinal circuitry

Question 1

what types of bipolar cells does a photoreceptor connect to

Answer 1

on and off centre

Question 2

what type of receptors do off centre bipolar cells have and what does it mean

Answer 2

“normal” glutamate receptors
- respond to darkness on the centre
glutamate = excitatory

Question 3

what type of receptors do on centre bipolar cells have and what does it mean

Answer 3

metabotropic glutamate receptors (mGluR6)
- respond to light on the centre
glutamate = inhibitory
(close when glutamate is present)

Question 4

what types of cells do bipolar cells connect to

Answer 4

on centre bipolar —> on centre ganglion cell
off centre bipolar —> off centre ganglion cell

Question 5

what types of receptors do on/off centre ganglion cells have and what does it mean

Answer 5

both have normal glutamate receptors
glutamate = excitatory

Question 6

what is the relationship between amount of light and glutamate release

Answer 6

dark = lots of glutamate released
light = very little glutamate released

Question 7

why do off centres have a higher firing rate in the dark

Answer 7

• glutamate = excitatory
• lots of glutamate released in the dark
• higher firing rate

Question 8

why do on centres have a higher firing rate in the light

Answer 8

• glutamate is inhibitory (MGluR6)
• less glutamate released in the dark
• therefore less inhibition
• higher firing rate

Question 9

what will happen when the centre is illuminated (general)

Answer 9

cone will hyperpolarise
- results in LESS glutamate being released from the cone onto the bipolar cells

Question 10

what will happen when the centre is darkened (general)

Answer 10

cone will depolarise
- results in MORE glutamate being released from the cone onto bipolar cells

Question 11

what do off centre bipolar cells do in the case of light on the centre

Answer 11

hyperpolarise due to LESS glutamate
- shuts down off centre bipolar and ganglion cells
(prefers light OFF)

Question 12

what do on centre bipolar cells do in the case of light on the centre

Answer 12

depolarise due to LESS glutamate (less inhibition)
- activates on centre bipolar and ganglion cells
(prefers light ON)

Question 13

what do off centre bipolar cells do in the case of dark on the centre

Answer 13

depolarise due to MORE glutamate
- activates the off centre bipolar and ganglion cells
(prefers light OFF)

Question 14

what do on centre bipolar cells do in the case of dark on the centre

Answer 14

hyperpolarise due to MORE glutamate (more inhibition)
- shuts down the on centre bipolar and ganglion cells
(prefers light ON)

Question 15

what do amacrine cells contain

Answer 15

GABA (inhibitory)

Question 16

what do photoreceptors release and have receptors for

Answer 16

release - glutamate
have receptors for - GABA

Question 17

what type of receptors do horizontal cells (inhibitory neurons) have

Answer 17

normal glutamate receptors
- release GABA onto neighbouring photoreceptors

Question 18

what occurs when just the centre region is lit up

Answer 18

• hyperpolarises the cone in the centre
• reduces its glutamate release
• depolarises on centre bipolar cell
• increases firing rate of on centre ganglion cell

Question 19

what occurs when the surround cones are also lit up

Answer 19

• hyperpolarises the cones in the surround
• reduces their glutamate release
• hyperpolarises the horizontal cells
• LESS GABA released onto centre cone

Question 20

what occurs when less GABA is released on to the centre cone when the surround is lit up

Answer 20

• depolarises the centre cone
• increasing its glutamate release (more inhibition)
• hyperpolarises the on centre bipolar cell
• reduces firing rate of on centre ganglion cell

Question 21

what is the difference in firing rate when different parts of the centre and surround are lit up

Answer 21

centre lit, surround dark = highest firing rate
centre lit, surround lit = lowest firing rate