Flashcards in lecture 6 Deck (18)
Describe Joan's case.
- 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
What is the anatomy of the eyeball?
- 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
What fundamentally limits visual acuity?
1. neural factors
2. optical factors
What are optical factors affecting visual acuity?
- pupil size
- clarity of optical media: cataracts, corneal opacities etc
- refractive errors --> blur: myopia, hypermetropia, astigmatism, presbyopia
Describe the neural retina
- 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
What are the photoreceptor cells?
- night vision
- very sensitive
- one type only
- no colour vision
- 100 million
- absent from fovea
- day vision
- less sensitive
- three types
- allow colour vision
- 5 million
- densest in fovea
How do we optimise our ability to see?
- 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
How do photoreceptors function?
- 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
What is the structure of a Rod?
- outer segments = contains the proteins that are sensitive to light
- cell body
- axon and synaptic terminals
How does the structure of retinal change when hit by light?
- 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
What is the neurotransmitter in rods?
How is the retina wired up?
- Bipolar cells
- Ganglion cells
- horizontal cells (outer retina - modify signal)
- amacrine cells (inner retina - modify signal)
What are second order neurons?
- 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)
What are ganglion cells?
- 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
What are the receptive field properties of ganglia? How does this allow ganglion cells to integrate information over time?
- 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
How do ganglion cells convey parallel information?
- 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
What is lateral inhibition?
- 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
- many different types
- axonless cells
- important for lateral inhibition
- for the most part ACs are considered inhibitory cells (release inhibitory NTs: glycine, GABA)