Vision Flashcards
How does the optic nerve travel through the eye?
Travels Nasally
What is the structure of the eye?
Has an outer fibrous layer containing
- Sclera: Quite flexible
- Cornea: The transparent bit of the eye
How is the sclera flexible?
- To do this, it need an element which holds it rigid
- This is done by providing a certain amount of intraocular pressure
- Pressure generated through the production of aqueous humor
How is the flexibility of the sclera maintained?
As long as the production and drainage in balance properly, it will produce just enough pressure in your eye to keep it rigid
What are the optics of the eye?
- Cornea
- Lens
- Both bend light
What is one feature about the Cornea and the lens?
- The Cornea bends the light rays inwards to the eye
- The lens can change shape in order to change the focus of the eye
What supports the lens to help keep it in place?
The lens is held in place by a ring of suspenseful ligaments from the ciliary body
What can the ciliary body do to the lens?
Can change the shape of the lens as it’s a ring of muscle, when it contracts, it’s diameter becomes smaller causing the lens to become fatter.
When relaxed, it becomes wider, diameter becomes larger flattening the lens for distance vision
What is the iris part of the eye?
The ring of muscle which Creates the colour part of the eye
- Function: Produces an Aperture in the middle known as the pupil
- Maintains the smallest aperture it can for the illumination conditions
- The smaller the aperture, the better the focus conditions
What happens with the retina indifferent lights
Adjusts the eye based on brightness
- Becomes larger in a dim lighted area
- Becomes narrower in bright sunlight
Describe what happens when light touches the eye?
- Some photons strike the cornea and pass through
- Some are stopped by the iris
- Those which pass through the pupil will be bought to a single point for focus by the cornea and the lens
What are the key elements of the eye when under the sunlight?
- The cornea is the most powerful refractive surface of our eye
- The lens provides additional variable fine focus
- The iris controls how much light enters the eye via the pupil
Where is the retina found?
Found at the back of the eye
What are the two parts of the retina?
- Neural retina
- Retinal pigment epithelium: Provides a lot of support for photoreceptors. Also holds the retina in place
Where was the retinal pigment epithelium developed
- Developed embryonically from the neural tube
- This means because they both came from the neural tube, they’re both CNS structures
What is special about the retina?
- Holds basically the whole massive surface of the eye
- Technically apart of the brain
- Has photoreceptors
- Also has Afferens which are retinal ganglion cells
What do the retinal ganglion cells have?
- Have axons which run across the surface of the retina which form the optic nerve
Describe the Primary visual pathway (PART 1)
- The ganglion cell axons project down the optic nerve to the optic chiasm
- One on the nasal part of the retina swap sides however those from the temporal sides stay on the same side
Describe the Primary visual pathway (PART 2)
They then project back to the lateral geniculate nucleus which serves the visual system
- Cells in this region then send their axons through a region of white matter known as optic radiation
- Back to the opsiticsl cortex where you find the primary visual area
What are the two forms which photoreceptors come in?
- Rod photoreceptors: Night vision
- Cone Photoreceptors: Day vision
What is the similarities & difference between rods and cones?
- They use the same neural circuitry
- But at the retina, you’ve either got rods and their circuitry or cones and their circuitry
How do the rods become react to the level of light overtime?
- They become super sensitive since they’ve had a lot of time to react
- This allows you to see a lot more clearly in the dark
Describe the structure of the cones photoreceptors
Has an outer and an inner segment
Contains
- A membrane disc
- Nucleus
- Axon
- Synaptic terminal
What does the inner segment contain?
Contains the nucleus as well as the replication machinery
What does the outer segment contain?
Contains the transduction mechanism
- A bag of tightly packed layers containing phospholipid layers
What is the resting membrane potential in cone photoreceptors?
-45 mV
What does this mean about the membrane potentials?
- Meaning they’re depolarised even at rest
- This is due to in the outer segment there are sodium channels which are open by default
What is the cones photoreceptors response to darker light?
- When the light gets darker
- More Na+ channels open depolarising the cell
- Causing it to release more glutamate
What is the cones photoreceptors response to brighter light?
Does the opposite
- Releases less glutamate
What’s the difference between 11-cis retinaldehyde and the trans conformation?
The cis one is less stable
Where is 11 cis retinaldehyde found?
- In the membrane disc which is found in the outer segment
- There is ops in which contains a cis retinal
What happens when light touches the 11 cis retinaldehyde?
- Causes the unstable bond to rupture
- When it reforms, it reforms in the more stable trans configuration
- This forms a trans retinal containing an all trans retinaldehyde
- The opsin behaves in the same way a G protein linked receptor would behave when it’s activated
What occurs next when light touches this molecule?
- Activates a G protein which then activates many other G proteins
- Each G protein go onto activate an enzyme
- These enzymes destroy cGMP
- You get a fall in the level of cGMP intracellular
- Causes some cGMP to diffuse away from its channel and allows them to close
How long are these G proteins activated for?
- They’re only activated very briefly
How are the cGMP levels restored?
- When the G proteins are inactivated, a second enzyme comes along
- Allowing to rebuild the cGMP levels which reopens those channels
When do you lose your visual field?
- If you have a disease attacking the peripheral retina like glaucoma or retinitis pigmentosa
- You’ll lose some visual field
When do you lose the central vision?
- Something like age related macular generation
- This goes for the central retina and destroys it
- You won’t be able to see the centre of your vision but will be able to see the surrounding bits
- This person will then be registered as blind
What does the cross section of the through peripheral retina contain?
- Pigment epithelium
- Photoreceptors
- Interneurons
- Ganglion cells
What happens to the image as it passes through the retinal tissue?
The image blurs as light passes through retinal tissue
What is the central retina called?
Where is it found?
- Called the fovea centralis
- Found at the back of the eye/retina
- No blood vessels go over it
Describe the foveal centralis
- Has a fovea (1.5 mm)
- Has a foveal pit (0.35mm)
What is the foveal pit?
The region where the photoreceptors are uncovered
- There is no retina sitting in between them and the light path
- So there in no blur
- You’ll have very excellent sampling as there’s no rods but the cones are packed closely together
Describe the peripheral vision
Majority of the retina serves only coarse vision
- The visual image is optically blurred
- The cone photoreceptors are large and widely spaced (Separated by larger numbers of rods)
- The signals from many cones converge onto single ganglion cells
Describe central vision
The fovea is specialised for high resolution
- Good focus
- Only cone photoreceptors, primarily red and Green
- Which are narrow and closely packed
- The signals from the photoreceptors are kept separate throughout the primary visual pathway
Which part of the brain deals with vision?
The right hand side of the brain looks at the left hand side of the visual world through both eyes
How does the retinal circuitry work?
- It is designed to pull out changes in brightness from one place to the neighbouring place
- It does this by lateral inhibition
What does the retinal ganglion cells report?
- Changes in illumination from one location to another
What do retinal ganglion cells respond to?
Half of all retinal ganglion cells respond to increase in brightness
How do they respond to a decrease in brightness?
- Central photoreceptors depolarised (red) by decreased illumination
- Bipolar and ganglion cells depolarised by excitatory synapses
How do they respond to a increase in brightness?
- Central photoreceptor hyperpolarised (blue) by increased illumination
- Bipolar cell depolarised by inverting synapse, excites ganglion cell
Which two classes can retinal ganglion cells be divided into?
- Parvocellular
- Magnocellular
Describe the Parvocellular divison
- Small field with strong surround
- Fine resolution
- Accurately follows changes in light
- Needs stable image
Describe the magnocellular division
- Large field with weak surround
- Coarse resolution
- Transcient responses to changes
- Responds well to fast movement
Where do the Parvocellular cells get their inputs from?
- Selective inputs from the red or Green photoreceptors
- By comparing these responses they can encode wavelengths
- Red vs wavelength
Describe the bistratified input
- Selective inputs from blue or red+green photoreceptors
- By comparing these responses they can encode wavelength
- Blue vs yellow
