7. VISION Flashcards
What are the two halves of the eye?
- Nasal
- Temporal
- The fibres from the nasal half of the retina cross over into the other optic tract, whilst the temporal fibres stay on the same path
What is the fibrous layer of the eye & what structures does it contain?
- The fibrous layer is the outermost layer which consists of the:
1. Sclera
2. Cornea - The sclera & cornea are continuous
What is the sclera?
- The sclera is the white part of the eye
- It is the non-stretchy layer that provides attachment for or anchors the extraocular eye muscles involved in movement
- Sclera is held rigid by the intraocular pressure
What is the cornea?
- The cornea is transparent & is positioned centrally at the eye
- Light is refracted off the cornea and focused
What is the vascular layer of the eye & what structures does it contain?
- The vascular layer is located beneath the fibrous layer
- It consists of:
1. Choroid
2. Ciliary body
3. Iris
What does the ciliary body do?
- Ciliary body controls the shape of the lens & contributes to the formation of aqueous humour
- The (ring of) suspensory ligaments of the ciliary body control the shape of the lens
What is the iris & what does it do?
- The iris is a circular structure with an aperture in the middle (pupil)
- The smooth muscles of the iris contract to change the size of the pupil
- Iris is located in between the cornea & the lens
What is the pupil?
- The pupil is the aperture within the iris
- The pupil controls how much light enters the eye
- The pupil maintains the smallest aperture it can, as there’s better focus
- As the diameter of the pupil gets smaller, the lens gets fatter
What is the lens & what does it do?
- The lens is located between the pupil & the vitreous humour
- Lens is a transparent structure involved in focusing light
- Shape of the lens can be changed by the suspensory ligaments of the ciliary body
What two structures of the eye bend light rays?
- CORNEA - Focuses light rays
2. LENS - Provides additional, variable fine focus
What is Vitreous humour?
- Vitreous humour is the transparent, gel-like layer found behind the lens. It holds all the layers of the retina in place
What is the aqueous humour?
- Aqueous humour is produced by the ciliary body, It’s a clear, plasma like fluid that protects the eye
- It’s constantly drained & produced, generating intraocular pressure
- The intraocular pressure helps keep the sclera rigid
What’s the primary visual pathway involving the lateral geniculate?
- RGC -> OPTIC NERVE -> OPTIC CHIASM -> LGN -> OPTIC RADIATION -> OCCIPITAL CORTEX -> PRIMARY VISUAL CORTEX
- Axons of retinal ganglion cells project down the optic nerve into the optic chiasm back to the lateral geniculate nucleus in thalamus
- Cell of the LGN project their axons through a region of white matter known as optic radiation, back to the primary visual cortex in the occipital cortex
What are the two types of photoreceptors?
- Cones - involved in colour & day vision
2. Rods - involved in dim light & night vision
What are the three segments of a cone cell?
- OUTER SEGMENT
- INNER SEGMENT
- SYNAPTIC TERMINAL
What does the outer segment of a cone cell contain?
- The outer segment contains the photopigment/chromophore which is the light sensitive area
- Na+ channels are located in the outer segment
What does the inner segment of a cone cell contain?
- The inner segment contains the K+ channels
What does the synaptic terminal of the cone cell do?
- Doesn’t fire axon potentials
- Synaptic terminal releases glutamate which is excitatory
Describe the membrane potential of photoreceptors at rest
- The photoreceptors are already slightly DEPOLARISED at rest, they have a resting membrane potential of around - 45V
- This is because the inner segment of cone cells contain K+ channels, leaking K+ . The outer segment also contains Na+ channels so Na+ leaks
Describe what happens to the membrane potential of photoreceptor cells in response to light?
- When the outer segments are illuminated/exposed to bright light the cone cells HYPERPOLARISE
- Hyperpolarisation means that less glutamate is released & there’s an inhibitory effect
- When the brightness of light goes down, depolarisation occurs where Na+ channels open & glutamate is produced
- More light = HPERPOLARISATION
- Less light = DEPOLARISATION
How do photoreceptors adapt to bright light?
- Photireceotirs are very sensitive, so they produce a large response to small changes
- When light is first detected, there’s a rapid response due to the phototransduction cascade
- Once the light becomes constant, the photoreceptor cells return to resting membrane potential ready to respond to changes
- The photoreceptors change their sensitivity so that they only respond to further changes in light
What are the two components of the photopigment?
- OPSIN
2. RETINALDEHYDE
What happens to the structure of retinaldehyde in response to light?
- Retinaldehyde has 11C, all the carbons are in the trans configuration except for the 11th carbon which is in the cis configuration
- When light hits the photopigment, the unstable cis bond of the 11th carbon breaks and reform in the trans configuration
- This forms ALL-TRANS RETINAL
What are the steps of the phototransduction cascade?
- Once all-trans retinal has been formed it will act as an agonist & bind to G-protein coupled receptor
- The G-protein coupled receptor is linked to an enzyme which causes cGMP levels to fall
- cGMP is associated with Na+ channels & is needed to keep them open. Falling levels of cGMP causes Na+ channels to close so Na+ can’t enter
- Hyperpolaristaion occurs, glutamate isn’t produced & the response is inhibitory
Describe how the phototransduction cascade is terminated?
- The opsin is enzymatically capped so that it can’t activate any enzymes
- All trans retinal is transported to the retinal pigment epithelium where the trans bond of the 11th carbon is broken. A new molecule of 11-cis retinaldehyde can bind to the opsin
- Levels of cGMP are restored by an enzyme which allows Na+ channels to open. Glutamate is produced
- The cascade may be terminated in response to low light -> depolarisation
Give two examples of diseases that can lead to loss of peripheral vision?
- GLAUCOMA
- RETINITIS PIGMENTOSA
- Loss of peripheral vision, but fovea is still present so able to read & recognise faces
Give an example of a disease that causes the loss of central vision
- Age-related macular degeneration - loss of central vision due to loss of fovea
- Unable to read or recognise faces
Describe the structure of the peripheral retina
- Photoreceptors covered by a layer of retinal pigment epithelium
- Interneurone layer between RGC & photoreceptors, feed into the optic nerve
- Ganglion cells communicate with photoreceptors via bipolar cells
How does the arrangement of cells in the central retina allow for central vision?
- FOVEAL PIT allows for central vision
- No image blur as there’s no overlying layer (retinal pigment epithelium) over the cone receptors
- Only has red & green cone receptors, which are packed closely together to provide good focus as there’s no convergence
How does the arrangement of the cells of the peripheral retina allow for peripheral vision?
- Peripheral retina has large cone receptors which are separated by large groups of rods
- Visual image is blurred
- Signals from many cones converge onto one ganglion cell so the focus isn’t very good
**Why is the image that we see inverted?
- Bipolar cells are depolarised by inverting the synapse that excites ganglion cells
What are the two classes of retinal ganglion cells?
- RGC can be divided into:
1. Parvocellular
2. Magnocellular - These RGC are also selective to wavelength
What are the properties of parvocellular RGC?
- Small field with string surround
- Fine resolution
- Able to accurately follow changes in light
- Needs a stable image as it’s not sensitive due to small input
- Selective input from “red & green” cones, comparison of these cones to encode either RED vs GREEN
What are the properties of magnocellular RGC?
- Large field with weak surround
- Coarse resolution
- Able to respond well to fast movement/moving stimuli
- Transient responses to change
- Selective inputs from “blue” or “red + green” cones for comparison to encode either BLUE vs YELLOW
What’s the key difference between magnocellular & parvocellular RGC?
- Magnocellular RGC have a greater convergence than parvocellular RGC
What does the cortical area do?
- Cortical area processes colour
What does the inferotemporal visual area do?
- Inferotemporal visual area encodes visual information about object identity
What does the parietal visual area do?
- Encodes information about location & movement
