lecture 6 Flashcards

Vision 1: the retina - photoreceptors and phototransduction - retinal circuitry - ganglion cell function - light is converted to a neural signal by photoreceptors: phototransduction - information is carried in parallel down via bipolar cells to ganglion cells - ganglion cells are tuned to encode edges -- central part of the receptive field acts differently to the peripheral part

1
Q

Describe Joan’s case.

A
  • 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
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2
Q

What is the anatomy of the eyeball?

A
  • 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
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3
Q

What fundamentally limits visual acuity?

A
  1. neural factors

2. optical factors

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4
Q

What are optical factors affecting visual acuity?

A
  • pupil size
  • clarity of optical media: cataracts, corneal opacities etc
  • refractive errors –> blur: myopia, hypermetropia, astigmatism, presbyopia
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5
Q

Describe the neural retina

A
  • 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
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6
Q

What are the photoreceptor cells?

A

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
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7
Q

How do we optimise our ability to see?

A
  • 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
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8
Q

How do photoreceptors function?

A
  • 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
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9
Q

What is the structure of a Rod?

A
  • outer segments = contains the proteins that are sensitive to light
  • cell body
  • axon and synaptic terminals
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10
Q

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

A
  • 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
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11
Q

What is the neurotransmitter in rods?

A

glutamate

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12
Q

How is the retina wired up?

A

“Through” pathway:

  • Photoreceptors
  • Bipolar cells
  • Ganglion cells

Lateral interactions:

  • horizontal cells (outer retina - modify signal)
  • amacrine cells (inner retina - modify signal)
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13
Q

What are second order neurons?

A
  • 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)
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14
Q

What are ganglion cells?

A
  • 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
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15
Q

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

A
  • 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
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16
Q

How do ganglion cells convey parallel information?

A
  • 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
17
Q

What is lateral inhibition?

A

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)
18
Q

So what’s wrong with Joan?

A

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
-