Principles of Neuroscience Lecture 6, Vision, the Retina Flashcards
What are the two types of photo transducer cells?
Rods and Cones
Rods : numerous, night vision, few in fovea, black and white vision, sensitive
Cones : few in number, day vision, concentrated in fovea, colour vision, less sensitive
Describe the basic structures of the eye
Cornea :
Sclera : tough material protecting eye
Lens : through which light passes through and is refracted
Vitreous humour: fluid filled cavity inside eye
Retina : contains photo transducer cells, along the back of the eye wall
Fovea : central vision, has many cone cells
Compare the sensitivity of rods and cones
Rods more sensitive, cones less
Compare colour vision in rods and cones
Rods : black and white
Cones : colour vision
Compare the number of rods and cones
Rods : numerous
Cones : few
Compare the localisation of rods and cones in the fovea
Rods : very few
Cones : concentrated to the fovea
What does the differential distribution of rods and cones in the fovea mean?
Colour vision is localised to the fovea –> colour vision is most clear in central vision
Colours are hard to distinguish in peripheral vision
Night vision is clearest off centre
Describe photo transduction in photoreceptor cells
Light hits the rods and cones.
The rods and cones have a protein in their cell membranes called rhodopsin and one of three coneopsins
This protein has Retinal (vitamin A) bound in the cis conformation.
When light hits the retinal, it changes shape to become in the trans conformation.
This initiates a transduction pathway.
A G-protein is activated (ATP - ADP)
2. cGMP –> GMP
3. cGMP no longer a ligand for a sodium channel
4. Hyperpolarisation occurs
5. Hyper polarisation of the cell
6. Hyper polarisation causes neurotransmitter release to stop
Which is the Neurotransmitter released by photo transducer cells?
When is released, and when is it not released?
The neurotransmitter is Glutamate.
I is released in the dark, when the cell is depolarised.
When light hits the photoreceptor cells, the sodium channels close and the cell becomes hyperpolarised, and neurotransmitter release stops.
Describe the structure of the retina
Pigmented epithelium
The innermost layer of cells in the retina is photoreceptors : rods and cones.
Bipolar cells : 2nd order neurons
Ganglion cells: Magnocllular and Parvocellular : 3rd order neurons, which become the optic nerve
Lateral communication :
Horizontal cells : between photoreceptors
Amacrine cells : fine tuning of signals
Which neurons fire action potentials, and which don’t ?
Don't fire action potentials: Photoreceptors Bipolar cells Horizontal cells Amacrine cells
Do fire action potentials:
Ganglion cells
Describe the properties of the receptive field in the retina
When light is shone directly on a cell or group of cells, they respond by increasing the rate of action potential firing.
In the surrounding areas, action potentials are inhibited.
What does the different response in the periphery mean for vision?
Edges are encoded
What does the phrase : “the eye is a comparator, not a detector” mean?
That the eye does not give us absolute information about the intensity of light, for example, but compares the things we are seeing at once, and distinguishes between them.
Describe the different responses the different ganglion cells can have
Transient: incident light causes a burst of action potentials firing which stop after a while
Sustained : the action potentials continue firing for the duration of the stimulus being present.
How many different types of ganglion cell are there, and how do they differ
There are 20
Different ganglion cells detect different aspect of the light stimulus.
This information is sent to the brain in parallel, where it is the reconstructed to form an image
What are the two types of lateral inhibition?
In the horizontal and Amacrine cells
Describe lateral inhibition in horizontal cells
Horizontal cells receive input from photoreceptors, and output to other photoreceptors.
Horizontal cells release GABA onto neighbouring photoreceptors. This inhibits the other photoreceptors.
Describe lateral inhibition in Amacrine cells
Fine tuning of information occurs when Amacrine cells perform lateral inhibition
Amacrine cells release Glycine and GABA onto neighbouring bipolar cells
What optical factors affect visual acuity?
- Pupil size
- Clarity of visual media
- Refractive errors
How many types of cone cell are there?
3
Differentiate between the terms scotopic and photopic. How do they relate to photoreceptor cells?
Scotopic: vision in the dark : rods
Photopic: vision in the light : cones
How many different types of bipolar cells are there?
10
1 rod bipolar cell
9 cone bipolar cells
What is the receptive field of ganglion cells?
These are the cells that change their firing rate when stimulated by light
Ganglion cells convey … information
Parallel
How do horizontal cells respond to light?
By hyper polarising and releasing inhibitory neurotransmitters onto bipolar cells
What is retinitis pigmentosa?
This is a genetic defect in rhodopsin
Gradual death of photoreceptors
Tunnel vision
What are the two broad categories of factors affecting visual acuity?
- Optical factors
- Neural factors
Rod and cone cells don’t undergo action potentials. Hw do they signal?
Graded changes in membrane potential
What is dark current?
This is the constant release of neurotransmitters when we are in the dark.
When we transduce light, we stop releasing glutamate onto bipolar cells
What does the through pathway refer to?
This is the pathway from photoreceptor to bipolar cell and on to the ganglion cells
Which neurotransmitter do Ganglion cells release?
Glutamate
Describe how Ganglion cells respond to stimulation
They either increase or decrease their firing rate
This response can be either sustained or transient, depending on the cells type
Ganglion cells are especially tuned for …
Edges
