Retinal Function and Visual Pathways

Question 1

Describe a photoreceptor at rest (in the dark)

Answer 1

Become depolarised (Na+ diffusing in, Ca2+ pumped out, cyclicGMP keeps Na+ gate open)

Producing Glutamate

Question 2

Describe photoreceptors in light?

Answer 2

Cell hyperpolarises i.e. become more -ve than outside

Reduction in glutamate production

Question 3

How do photoreceptor cells hyperpolarise in light?

Answer 3

Photon hits rhodopsin molecule causing retinal dissociation, i.e. small retinal molecule to change shape and dissociate from large opsin molecule

Retinal molecule inactivates cGMP so that Na+ gates close, i.e. Na+ can’t get in but Ca2+ still pumped out

Results in cells becoming negative

Question 4

What occurs in the neural processing of diffuse light?

Answer 4

Lateral inhibition by horizonal and amacrine cells limits depolarisation of photoreceptors and impulses sent if diffuse light hitting large area of retina

Question 5

What is the nerve fibre layer made up of and where in the eye will you find it?

Answer 5

Nervous ganglia nuclei

Overlying the retina

Question 6

Explain the role of ganglia in the nerve fibre layer

Answer 6

Nervous nuclei and fibres which lead to the optic disc

Approx. 2% are photosensitive to blue light

They also regulate melatonin production for circadian rhythms

Question 7

What is the role of the inner plexiform layer?

Answer 7

Synapses between ganglia and bipolar/amacrine cells

Question 8

What processor and supportive cells make up the inner nuclear layer?

Answer 8

Processor cells - horizontal and amacrine cells

Supportive cells - Muller cells

Question 9

What is the role of bipolar cells in the nuclear layer?

Answer 9

Relay impulses from photoreceptors to ganglia

Question 10

What is the role of amacrine cells in the nuclear layer?

Answer 10

Laterally inhibit photoreceptors and ganglia

Mechanism of action is unknown

Question 11

What is the role of muller cells in the inner nuclear layer?

Answer 11

Muller cells are glial cells (nervous connective tissue cells) therefore act as support cells.

Also shown to differentiate into progenitor cells in response to retinal damage

Question 12

What is the role of the outer plexiform layer?

Answer 12

Synapses between photoreceptors and processor cells

Question 13

Where is the photoreceptor layer and what is it composed of?

Answer 13

Outer nuclear layer, made up of rods (~120m) and cones (~7m) which are concentrated in the fovea

Question 14

What are the roles of rods and cones in the photoreceptor layer?

Answer 14

Rods - sense light/darkness

Cones - sense red and green or blue

Question 15

How is light transferred into a nervous impulse?

Answer 15

Rods and cones contain rhodopsin, light (photons) causes hyperpolarisation by break down of rhodopsin into large opsin and small retinal molecules, retinal molecule inactivates cGMP molecule and closes NA gate, Ca2+ pumped out Na+ cant get in, cell becomes -ve and reduced glutamate production, this signal reaches ganglia and is then transmitted to the optic nerve.

Question 16

What is the role of the retinal pigment epithelium?

Answer 16

Prevents reflection of light and phagocytoses cell debris from inner layers

Question 17

What is the choroid?

Answer 17

Vascular supply of the retina but on a seperate layer

Question 18

How does the action of bipolar centres affect vision?

Answer 18

One bipolar cell connects to many rods.
If centred on a hyperpolarising rod, the bipolar cell will depolarise.

However this is inhibited by horizontal and amacrine cells when a bipolar cell is neighboured by a non hyperpolarising rod.

This allows for increased edge/contour definition.

Question 19

How do bipolar cells affect foveal acuity?

Answer 19

One cone is connected to one bipolar cell therefore acuity is increased?

Question 20

How does colour blindeness occur?

Answer 20

Red, blue and green cone cells each have different rhodopsin molecules.
Colour blindness occurs when the eye is unable to produce one of these rhodopsin molecules.
Most commonly red and green, this is a sex linked condition therefore most common in boys

Question 21

What stimuli will make the pupils change in size?

Answer 21

Change in light/dark and close/distant objects

Question 22

Describe the pathway of the pupillary light reflex?

Answer 22

Optic nerve -> 
optic chiasm -> 
pretectal nuclei -> 
erdinger westphal nuclei ->
occulomotor nerve/ciliary ganglia/constrictor muscles of iris

Question 23

Why do both eyes respond to a light stimulus in one eye?

Answer 23

In the optic chiasm, fibres from the medial half of the retina decussate whilst fibres from the lateral half remain ipsilateral (same side)

Question 24

What would a lesion to the optic nerve anterior to the optic chiasm result in?

Answer 24

No sensory information passing from that eye

Question 25

What pupillary effect would a lesion to the optic nerve begind the optic chiasm result in?

Answer 25

Will not affect the pupillary light reflex as fibres from the other eye will pass to both erdinger westphal nuclei

Question 26

What pupillary effect would a lesion to an occulomotor nerve result in?

Answer 26

Prevent any response in that eye but have no effect on the other

Question 27

What are the three actions of the pupillary near response?

Answer 27

Convergence, accomodation, and constriction

Question 28

Describe the sensory pathway?

Answer 28

``` Optic nerve -> optic chiasm -> lateral geniculate bodies -> optic radiations -> primary visual cortex ```

Question 29

What occurs in the sensory pathway at the optic chiasm?

Answer 29

Fibres from medial half of retina (or lateral visual field) decussate fibres from lateral half remain on ipsilateral field

Question 30

Where are the lateral geniculate bodies found?

Answer 30

Thalamus

Question 31

What occurs in the sensory pathway at the lateral geniculate bodies?

Answer 31

Fibres synapse and split into those from top and bottom of retina

Question 32

What does the superior optic radiation carry and to where?

Answer 32

Carry fibres from upper retina (lower field of vision) through the parietal lobes

Question 33

What does the inferior optic radiation carry and to where?

Answer 33

Carries fibres from the lower retina (upper field of vision) through the temporal lobes

Question 34

What occurs at the visual cortex and where is it?

Answer 34

Neurons from radiations synapse with corticol cells In the occipital lobe

Question 35

What is macular sparing and why does it occur?

Answer 35

Poor blood supply to the cortex makes it vulnerable to CVA | However, cells dealing with macula located at the occipital pole dual blood supply, therefore can be spared in CVA

Question 36

Describe the motor pathway

Answer 36

Visual cortex-> frontal eye field -> erdinger westphal nuclei -> occulomotor nerve

Question 37

What would a central lesion in the optic chiasm result in?

Answer 37

Bitemporal hemianopsia (blindness over half vision field)