lecture 6

Question 1

Describe Joan’s case.

Answer 1

• 45 year old woman
• has trouble seeing at night
• trips over her children’s toys
• has had many car accidents over the last 2 years
• can read OK
• black crud in peripheral retina - indicative of a disease process in the retina
• pigment that has come from underneath
• macula is fine so has no central vision problems
• has tunnel vision

Question 2

What is the anatomy of the eyeball?

Answer 2

• cornea at the front - clear surface that light passes through
• lens bends the light
• focussed on the retina which occupies the back sort of 5/6 of the eyeball (inside surface of the eyeball)
• outer layer is there for strength
• middle layer with lots of blood vessels for nutrition
• inner layer = retina
• optic nerve contains the axons of the ganglion cells as they go off to the brain
• fovea is the most important part of the eye - allows us to see centrally

Question 3

What fundamentally limits visual acuity?

Answer 3

1. neural factors

2. optical factors

Question 4

What are optical factors affecting visual acuity?

Answer 4

• pupil size
• clarity of optical media: cataracts, corneal opacities etc
• refractive errors –> blur: myopia, hypermetropia, astigmatism, presbyopia

Question 5

Describe the neural retina

Answer 5

• a series of neurons and neuronal layers
• light has to go all the way through the retina to the photoreceptors which sit very deep in the wall of the eyeball
• the photoreceptors are the cells which ‘see’ the light and stimulate a neural response
• pigmented epithelium lay behind the photoreceptor cells and help keep the photoreceptors alive
• if the retina was layered the other way around the pigmented cells would be the first thing the light reached: these cells prevent light from passing through

Question 6

What are the photoreceptor cells?

Answer 6

Rods

• night vision
• “scotopic”
• very sensitive
• one type only
• no colour vision
• 100 million
• absent from fovea

cones

• day vision
• “photopic”
• less sensitive
• three types
• allow colour vision
• 5 million
• densest in fovea

Question 7

How do we optimise our ability to see?

Answer 7

• cone and rod density changes
• cones are extremely dense in the middle of the fovea whereas rods are lowest
• fine detail during the day is defined by cones
• rods are more dense in the periphery

Question 8

How do photoreceptors function?

Answer 8

• two important things needed for phototransduction: photopigment and retinal
• photoreceptors contain photopigments that are activated by light
• rods contain Rhodopsin
• cones contain one of three different coneopsins
• opsins bind to vitamin A (all-trans Retinal)
• in the dark retinal is kinked and does not activate rhodopsin, thus allowing a continuous influx of sodium ions through a cGMP gated sodium channel: this depolarises the cell
• Retinal picks up the up the light, changes and then changes the rhodopsin protein (activates it)
• initiates a cascade of events that ultimately leads to the closure of cGMP gated sodium channels and prevents the flow of sodium ions
• Rh –> transducin –> PDE (phosphodiesterase) –> breaks down cGMP
• closure of sodium channels –> hyperpolarisation
• respond to light with graded changes in membrane potential (not action potentials)
• continuous release of neurotransmitter that goes down when hyperpolarised, or up if the cell is slightly depolarised

Question 9

What is the structure of a Rod?

Answer 9

• outer segments = contains the proteins that are sensitive to light
• cell body
• axon and synaptic terminals

Question 10

How does the structure of retinal change when hit by light?

Answer 10

• retinal usually in 11-cis retinal form - kinked
• when light hits it, it becomes unkinked and straight forming All-trans retinal form
• this gets the whole process going

Question 11

What is the neurotransmitter in rods?

Answer 11

glutamate

Question 12

How is the retina wired up?

Answer 12

“Through” pathway:

• Photoreceptors
• Bipolar cells
• Ganglion cells

Lateral interactions:

• horizontal cells (outer retina - modify signal)
• amacrine cells (inner retina - modify signal)

Question 13

What are second order neurons?

Answer 13

• bipolar cells
• important in “through”
• 10 different types:
• • 1x rod bipolar cell
• • 9x cone-bipolar cells
• important for spatial vision, and colour vision
• found in the inner nuclear layer (second layer)

Question 14

What are ganglion cells?

Answer 14

• output neurons of the retina
• many different types: On, Off, M (motion) and P (important for how we see colour) (maybe 22 different types)
• release glutamate
• fire action potentials

Question 15

What are the receptive field properties of ganglia? How does this allow ganglion cells to integrate information over time?

Answer 15

• ganglion cells respond to light by either increasing or decreasing their action potential firing rate
• receptive field of a ganglion cell: is the area of retina that when stimulated with light changes the cell’s membrane potential
• allows us to pick up edges
• response of a ganglion cell can vary over time:
• • transient: sudden burst of APs at the onset of stimulus (i.e. transient)
• • sustained: continuous APs during stimulation
• GCs are especially tuned for edges

Question 16

How do ganglion cells convey parallel information?

Answer 16

• 20 different types of GCs
• ganglion cell responses:
• • increase or decrease in firing
• • transient or sustained response
• visual information is passed to higher cortical centres in parallel
• ganglion cells deconstruct what we see and the brain puts it all back together

Question 17

What is lateral inhibition?

Answer 17

horizontal cells

• input from photoreceptors
• provide output onto photoreceptors
• use inhibitory neurotransmitter GABA
• respond to light by hyperpolarising
• this is what develops the receptor field property in ganglion cells

amacrine cells

• many different types
• axonless cells
• important for lateral inhibition
• for the most part ACs are considered inhibitory cells (release inhibitory NTs: glycine, GABA)

Question 18

So what’s wrong with Joan?

Answer 18

Retinitis pigmentosa
- 1:5000
- genetic defect in rhodopsin, or proteins involved in phototransduction (around 150 different mutations in rhodopsin)
- tunnel vision
- has a field of the order that’s about 5º
- eventually causes blindness
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