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
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
3
Q
What fundamentally limits visual acuity?
A
- neural factors
2. optical factors
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
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
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
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
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
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
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
11
Q
What is the neurotransmitter in rods?
A
glutamate
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)
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)
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
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
