Lecture 1 - The retina

Question 1

the retina

Answer 1

• the main cavity behind the lens is filled with vitreous humour. Gives the eyeball its shape and keeps the retina pinned to the back of the eye.
• considered part of the brain

Question 2

what are the 5 main cell types in the retina:

Answer 2

• photoreceptors (rods and cones)
• bipolar cells
• ganglion cells
• horizontal cells
• amacrine cells

Question 3

what do we call our vision when only rods are active

Answer 3

scotopic

Question 4

what is the photopigment in rods called and when do rods respond best

Answer 4

photopigment is called rhodopsin

rods respond very well to extremely dim light and are therefore very useful at night.

Question 5

when it is so bright that the rods cannot function we call our vision….

Answer 5

photopic

cones are less sensitive to light than rods and are responsible for most of our daytime vision

Question 6

where are rods and cones located in the retina

Answer 6

• Cones are heavily concentrated in the fovea.
• Rods are completely absent from the central fovea and are most densely packed some 12–15° into the periphery.

Question 7

What are the 5 main layers in the retina and what are they responsible for

Answer 7

1) Outer nuclear layer - photoreceptor cell bodies
2) outer plexiform layer - synapse between photoreceptors and bipolar and horizontal cells
3) inner nuclear layer - cell bodies of bipolar and horizontal and amacrine cells
4) inner plexiform layer - synapses between bipolar, ganglion and amacrine cells
5) ganglion cell layer - ganglion cell bodies - provides output that form the optic nerve.

Question 8

The visual receptive field of a neuron is…

Answer 8

the region of the visual field in which a stimulus can modulate the firing of the neuron.

Question 9

where are bipolar cell bodies located

Answer 9

in inner nuclear layer

Question 10

what do ON bipolar cells do

Answer 10

ON bipolar cells have inhibitory glutamate receptors therefore responding to light images against a dark background.

Question 11

what do OFF bipolar cells do

Answer 11

OFF bipolar cells have excitatory glutamate receptors therefore respond to dark images on a bright background.

Question 12

what do ON and OFF bipolar cells connect with

Answer 12

ON bipolar cells connect with ON ganglion cells
OFF bipolar cells connect with OFF ganglion cells

Question 13

what do ON and OFF ganglion cells do

Answer 13

they form the output signals from the retina

Question 14

what is the role of horizontal cells

Answer 14

• receives input from many cones, so its receptive field is large.
• add an opponent signal that is spatially constrictive, giving the bipolar cell what is known as a centre-surround organization

Question 15

what do amacrine cells do?

Answer 15

conveys additional information to the ganglion cell–possibly sharpening the centre-surround antagonism.

Question 16

what do ganglion cells do

Answer 16

• output neurons of the retina
• only cells in retina that generate action potentials
• intensity is coded by firing rate

Question 17

what are center surround receptive fields

Answer 17

the type of receptive field in a visual system where a specific region known as the center responds more strongly to a stimulus while the surrounding region provides inhibitory input to supress activity.
- this helps to enhance detection of edges, boundaries and spatial contrast of stimuli

Question 18

brief summary of how center-surround receptive fields are generated

Answer 18

1) photoreceptors detect light and transmit signals to bipolar cells
2) bipolar cells receive this input and transmit signals to ganglion cells and amacrine cells
3) amacrine cells receive input and modulate signals transmitted between bipolar cells and ganglion cells
4) Bipolar cells provide excitatory input to ganglion cells in the center of receptive field
5) amacrine cells provide inhibitory input to ganglion cells in the surrounding region
- this creates a contrast between the center and the surround.