HL7-9 Visual System

Question 1

Divergent pathway

Answer 1

One presynaptic neuron synapsing on many postsynaptic neurons

Question 2

Convergent pathway

Answer 2

Many presynaptic neurons synapsing on one postsynaptic neuron

Question 3

Optic tract is made of

Answer 3

Ganglion cell axons

Question 4

What are the local interneurons in the retina and what do they do?

Answer 4

1. Horizontal and amacrine cells

2. Modulate transmission onto bipolar and ganglion cells

Question 5

Three orders of cells in the retina

Answer 5

1. Rods & cones (photoreceptors)
2. Bipolar cells
3. Ganglion cells

Question 6

Cones are for….

Answer 6

colour vision

Question 7

Rods are for…

Answer 7

black & white

Question 8

Cones are mostly tightly packed in the …. and sparsest in the ….

Answer 8

1. fovea

2. periphery

Question 9

Rods have …. spatial resolution but are … sensitive to light

Answer 9

low

very

Question 10

Cones have …. spatial resolution but are … sensitive to light

Answer 10

high

insensitive

Question 11

Rods contain

Answer 11

rhodopsin

Question 12

Cones contain

Answer 12

various opsins

Question 13

Number of rods

Answer 13

100 million

Question 14

Number of cones

Answer 14

6 million

Question 15

Three segments of photoreceptors

Answer 15

1. Outer - discs of photopigments
2. Inner - nucleus & organelles
3. Synaptic - output to bipolar cells

Question 16

How does light affect photoreceptors?

Answer 16

• Photons of light are absorbed by the opsin protein which undergoes conformational change to trans config
• the opsin then activates transducin (G protein) which causes cGMP breakdown by activating cGMP phosphodiesterase
• cGMP sodium channels close and the cell is hyperpolarised

Question 17

How do photoreceptors react to darkness

Answer 17

• Darkness = no photons to trigger breakdown of cGMP

- open cGMP sodium channels = depolarisation = Glu release onto bipolar cells

Question 18

How is rhodopsin recycled

Answer 18

-arrestin facilitates conversion of all-trans retinol back to 11-cis retinal

Question 19

How does light affect on-centre ganglions

Answer 19

increases firing

Question 20

How does light affect off-centre ganglions

Answer 20

decreases firing

Question 21

How does a decrease in Glu affect the bipolar cell

Answer 21

• Glu binds to mGluR6 resulting in closure of cGMP-gated sodium channels
• decreased Glu = open Na+ channels = depolarisation of bipolar cells

Question 22

The optic nerve innervates…

Answer 22

1. Superior colliculus - controls head and eye movements
2. Midbrain pretectal area - controls the pupil reflex
3. Thalamus - relays to the visual cortex

Question 23

Receptive field of the retina

Answer 23

area of the retina that will change the membrane potential of the bipolar cell when stimulated by light

Question 24

Receptive field centre (retina)

Answer 24

direct input from the photoreceptor

Question 25

Receptive field surround (retina)

Answer 25

input from horizontal cell

Question 26

Under what conditions does the ganglion cell fire the most?

Answer 26

When there is greatest contrast between light illuminating the centre and surround

Question 27

Layers 1, 4, 6 of the LGN receive innervation from the ... side of the retina

Answer 27

contralateral

Question 28

Layers 2, 3, 5 of the LGN receive innervation from the ... side of the retina

Answer 28

ipsilateral

Question 29

Three inputs to the superior colliculi

Answer 29

1. RGCs 2. Visual cortex 3. Other cortical areas (auditory, somatosensory)

Question 30

Three functions of the superior colliculi

Answer 30

1. Integrate visual and movement input 2. Rapid changes in visual field (detect & alter gaze) 3. Saccades

Question 31

Retinohypothalamic Pathway

Answer 31

Projections from the retina to the pretectum and the superchiasmatic nucleus of the hypothalamus via specialised RGCs which react to light via melanopsin (cf photoreceptors)

Question 32

Three thalamic nuclei

Answer 32

1. Relay 2. Accessory 3. Reticular

Question 33

Four ways that info is processed in the thalamus

Answer 33

1. Local - in thalamic nucleus 2. Brainstem modulation - NA, 5HT 3. Inhibitory modulation - thalamic reticular nucleus 4. Excitatory feedback - from cortex

Question 34

Two pathways from the retina to the LGN

Answer 34

1. Magnocellular | 2. Parvocellular

Question 35

P cells respond to ....

Answer 35

colour change

Question 36

M cells respond to ....

Answer 36

luminance contrast

Question 37

Retinotopic map

Answer 37

Spatial arrangement of retinal projections is preserved in the thalamus i.e. neighbouring cells in the retina project onto neighbouring areas of the LGN.

Question 38

4 properties V1 neurons respond to

Answer 38

1. Colour 2. Depth 3. Motion 4. Form

Question 39

Simple cells

Answer 39

V1 cells that respond best to stimuli with a specific orientation

Question 40

Complex cells

Answer 40

V1 cells that are responsive mostly to movement | Have larger RFs than simple cells

Question 41

V1 neurons are mainly concerned with...

Answer 41

object edges

Question 42

Two V1 visual streams

Answer 42

1. Dorsal - where i.e. motion | 2. Ventral - what i.e. colour & form

Question 43

Which layers generally contain simple/complex cells?

Answer 43

Simple cells - layers that receive direct thalamic input (i.e. 4 and 6) Complex cells - layers that mostly receive cortical input i.e. 2 and 3 (input from 4) and 5 (input from 2 and 3)