20.3 Vision Flashcards
What is some evidence that the eye is the dominant sense?
About one-third of the human cerebral cortex is devoted to visual analysis.
Describe the function and distribution of rods and cones in the eye.
Cones:
- Colour vision
- Centre of the eye (in fovea)
Rods:
- Night vision
- Periphery of eye
(Just remember: CCC)
Draw the histological structure of the retina.
What are the two main types of modulator cells in the retina? Describe the position of each.
- Horizontal cells -> Connect laterally between pedicles of the photoreceptor cells
- Amacrine cells -> Connect across ganglion cells
Describe the different classes of interneuron in the retina.
- Bipolar cells -> Connect between photoreceptors and ganglion cells
- Horizontal cells -> Connect laterally between pedicles of the photoreceptor cells
- Amacrine cells -> Connect across ganglion cells
What are the output neurons in the retina called (i.e. those going out from the retina as the optic nerve)?
Ganglion cells
Aside from neurons, what are some other cell types in the retina?
- Astrocytes
- Muller glial cells [EXTRA]
- Pigment epithelial cells
What is the function of horizontal cells in the retina?
- Increase contrast via lateral inhibition -> Leading to centre-surround receptive fields.
- Adapting both to bright and dim light conditions.
Horizontal cells provide inhibitory feedback to rod and cone photoreceptors.
What is the function of amacrine cells in the retina?
- Intercept retinal ganglion cells and/or bipolar cells
- Create functional subunits within the receptive fields of many ganglion cells
- Vertical communication within the retinal layers
- Paracrine functions -> e.g. Release of dopamine and acetylcholine
- Through their connections with other retinal cells at synapses and release of neurotransmitters, contribute to the detection of directional motion, modulate light adaption and circadian rhythm, and control high sensitivity in scotopic vision through connections with rod and cone bipolar cells
What is the function of Muller glial cells?
Maintain the structural and functional stability of retinal cells:
- Uptake of neurotransmitters
- Removal of debris
- Regulation of K+ levels
- Storage of glycogen
- Electrical insulation of receptors and other neurons
- Mechanical support of the neural retina
Summarise the basic circuitry of the retina.
- There is a straight-through pathway from receptor to bipolar cell to ganglion cell
- There are also two lateral pathways:
- Horizontal cells communicate between receptors (sending signals between them)
- Amacrine cells serve a similar function between ganglion cells
What are the two types of bipolar cells? How do they work?
ON bipolar cells:
- When the photoreceptors are activated and hyperpolarise, ON bipolar cells depolarise
- They synapse in sublamina B of the inner plexiform layer
OFF bipolar cells:
- When the photoreceptors are activated and hyperpolarise, ON bipolar cells hyperpolarise
- They synapse in sublamina A of the inner plexiform layer
It is worth noting that the photoreceptors all release glutamate, it is just the way the bipolar cells respond to it that differs (different receptors?)
What are the two types of ganglion cells (in terms of morphology)?
- Magnocellular
- Parvocellular
Note that each of these can be either ON or OFF-centre cells.
Compare magnocellular and parvocellular ganglion cells in the retina.
Magnocellular:
- Transient responses
- High temporal resolution
- Low spatial resolution
- Monochrome
Parvocellular:
- Sustained responses
- Low temporal resolution
- High spatial resolution
- Colour
In other words, magnocellular cells are useful for detecting movement, while parvocellular are useful for detecting colour, texture and depth.
Give some examples of retinal disease.
What is diabetic retinopathy?
- High blood glucose damaged blood vessels
- This causes proliferation of blood vessels, which become swollen and damage the retina
- There is loss of vision
What is macular degeneration?
[IMPORTANT]
- Damage to the macula (part of the retina) that occurs with age.
- The pathophysiology is not known, but theories have been suggested, relating to oxidative stress, mitochondrial dysfunction, and inflammatory processes.
- This results in blurred or no vision in the centre of the visual field.
What vessel supplies blood to the retina?
- Central artery of the retina
- Branches off the ophthalmic artery, running inferior to the optic nerve within its dural sheath on the way to the eyeball
- Supplies the inner layers of the retina
What cells are involved in the generation of centre-surround receptive fields in the retina? How?
[IMPORTANT]
- Horizontal cells
- When exposed to light, a photoreceptor releases less glutamate onto the horizontal cell
- This causes hyperpolarisation of the horizontal cell
- The horizontal cell is connected to other adjacent photoreceptors and leads to their depolarisation (remember that activation of photoreceptors leads to hyperpolarisation)
- Thus, horizontal cells provide negative feedback to nearby photoreceptors, meaning that a spot of light will have a ring of inhibition around it.
*this establishes an antagonistic receptive field surround.
What neurotransmitter do photoreceptors release?
[IMPORTANT]
phototransduction leads to modulation of glutamate release from terminal
What is phototransduction?
The process through which photons are converted into electrical signals in photoreceptors.
How does phototransduction in photoreceptors work?
[IMPORTANT]
Light causes HYPERPOLARISATION of the photoreceptors:
- Light causes isomerisation of photopigment
- This leads to transducin (G-protein) activation
- Transducin activates a phosphodiesterase (PDE)
- PDE reduces levels of cGMP
- Reduced cGMP causes sodium channels to close
- Therefore, the membrane is hyperpolarised
This enables large and adjustable amplification.
What is dark current?
[IMPORTANT]
- It is the depolarising inwards sodium current in photoreceptors while it is dark.
- Upon sensing light, the sodium channels are closed and the current stops.
What are the main photopigments found in rods and cones of the retina?
[IMPORTANT]
- Rods -> Rhodopsin
- Cones -> Other opsins (3 different types -> Red, green and blue)
Describe adaptation of vision to the dark and how it happens.
[IMPORTANT?]
- It takes about 20 minutes for sensitivity to reach maximal values after lights turned off.
- This corresponds to the time needed for rhodopsin in rods to regenerate (rhodopsin isomerises when activated, so it must regenerate for full dark vision)
- The sensitivity to light also depends on the colours shown, since rods are less sensitive to red light
What are most common examples of colour defects?
[IMPORTANT]
- Deuteranomaly -> Abnormal green-absorbing pigment (5% -> Most common)
- Protanomaly -> Abnormal red-absorbing pigment
- Protanopia -> Absence of red-absorbing pigment
- Deuteranopia -> Absence of the green-absorbing pigment
What happens to retinal when it absorbs a photon?
Retinal → absorbs photon
11-cis retinal –photon→ all-trans retinal (more stable)
Causes conformational change in opsin
Results in activation of opsin
Transducin stimulates phosphodiesterase → ↓ cGMP → light sensitive cation channel closes (cGMP gated) → dark current reduced → hyperpolarisation → transmitter release falls
Dark - guanylyl cyclase keeps cGMP levels ↑
Describe the transmission of information in the retina.
[IMPORTANT]
- Photoreceptors (cones and rods) at the back of the retina detect light
- This information is passed to bipolar neurons
- This information is then passed to ganglion cells -> These form the fibres that flow out as the optic nerve
Photoreceptors -> Bipolar neurons -> Ganglion cells
(Note: The front of the retina is at the bottom)
What is found in the disks in the outer segment of rods and cones which is involved in phototransduction? How are they similar?
Rhodopsin in rods
Photopsins in cones
Both contain retinal
Why can we not rely on rods for daytime vision?
In bright light cGMP levels fall to the point where the response becomes SATURATED and increasing light causes no additional hyperpolarisation
Cones needed which pigments that require more bleaching to produce a response
What are the two factors contributing to dark adaptation?
Pupil size (increased diameter from 2-8mm)
Regeneration of unbleached rhodopsin
What happens during light adaptation?
Cones initially hyperpolarised as much as possible
GRADUAL DEPOLARISATION BACK TO -35mV
What is the mechanism of light adaptation?
- cGMP gated Na channels also admit Calcium ions
- In the dark Ca has an inhibitory effect on guanylyl cylase which synthesises cGMP
- In light: cGMP breakdown causes cGMP-gated ion channels to close, Ca influx is stopped
- leads to less inhibition on guanylyl cyclase and increased cGMP over time
- Allows more cGMP-gated channels to open again
- Gradually depolarises membrane
(channels also open gradually even when light level does not change)
What are the two types of bipolar cells?
ON and OFF bipolar cells
What determines the difference between ON and OFF bipolar cells?
The glutamate receptor
What does the release of glutamate onto OFF bipolar cells do?
Glutamate released in the dark to OFF bipolar cells
Cation channels = increased in cation influx into bipolar cells
DEPOLARISES OFF BIPOLAR CELLS
What does the release of glutamate do to ON bipolar cells?
Glutamate released in the dark to ON bipolar cells
Coupled to G protein which CLOSES cation channels (APB receptor)
DARKNESS causes HYPERPOLARISATION of ON bipolar cell and BLOCKS ON pathway
Which glutamate receptors are found on OFF bipolar cells?
IONOTROPIC receptors
AMPA cationic ionotropic channels (iGluR)
Which glutamate receptors are found on ON bipolar cells?
Metabotropic glutamate receptors
What does the release of glutamate onto OFF bipolar cells do?
Glutamate released in the dark to OFF bipolar cells
Cation channels = increased in cation influx into bipolar cells
DEPOLARISES OFF BIPOLAR CELLS
What does the release of glutamate do to ON bipolar cells?
Glutamate released in the dark to ON bipolar cells
Coupled to G protein which CLOSES cation channels (APB receptor)
DARKNESS causes HYPERPOLARISATION of ON bipolar cell and BLOCKS ON pathway
What do ON bipolar cells excite?
ON ganglion cells
What is the overall effect of dim light on each type of bipolar cell?
Dim light = increased cGMP = opening of cGMP dependent Na channels
Depolarisation causes glutamate release
ON bipolar cells have mGluR which closes cation channels causing HYPERPOLARISATION
OFF bipolar cells have iGluR which causes cation influx and DEPOLARISATION
Define receptive field
delimited region of a sensory space (e.g., the retina) within which a stimulus influences the electrical activity of a sensory cell (e.g., a ganglion cell).
How do horizontal cells influence the receptive field?
Synpase with photoreceptors and have gap junctionc connecting to other horizontal cells
They MEASURE THE AVERAGE LEVEL OF EXCITATION OF A PHOTORECEPTOR REGION and sends inhibitory signals back to the photoreceptor
Photoreceptor under TWO opposing influences: light striking it and causing hyperpolarisation and light falling on the surround and depolarising it via horizontal cells
What type of receptive field do ganglion cells and bipolar cells in the retina have? What does it mean?
- Centre-surround receptive field
- This is where the receptive field consists of a centre (on) and surround (off) region
- If photoreceptors in the centre of the receptive field are stimulated, then there is excitation of the ganglion cell
- If photoreceptors in the edge of the receptive field are stimulated, then there is inhibition of the ganglion cell
- If both are stimulated, then the excitatory response is weak
Do ON and OFF bipolar cells have the same receptive fields?
- ON bipolar cells have an on-centre, off-surround field
- OFF bipolar cells have an off-centre, on-surround field
What is the visual pathway?
Optic nerves -> partial decussations at optic chiasm -> optic tracts -> lateral geniculate nucleus -> optic radiations -> primary visual cortex
Where is the optic chiasm found? Why is this important?
Superior to the pituitary gland - important in relation to pituitary tumours which can compress the lateral tracts as they dessucate
Summarise the different field defects that occur along the visual pathway.
What visual field defect is seen with damage to the optic nerve?
[IMPORTANT]
Loss of vision in one eye
What visual field defect is seen with damage to the optic chiasm?
[IMPORTANT]
Bitemporal hemianopia (vision loss in the outer half of both the left and right visual fields)
What visual field defect is seen with damage to the LGN/optic tract?
[IMPORTANT]
Contralateral homonymous hemianopsia -> Loss of one half of visual field. If right side affected, sight is lost of left side of visual field (and vice versa).
What visual field defect is seen with damage to V1?
[IMPORTANT]
- Complete unilateral damage -> Contralateral homonymous hemianopsia (Loss of one half of visual field on the opposite side to lesion)
- Macular sparing (preservation of the centre of the field of view) can occur in some cases, such as posterior cerebral artery stroke -> Proposed to be due to double blood supply to the macular region from the posterior and middle cerebral arteries.
- Damage to just the lower bank of calcarine fissure -> Pie in the sky
- Damage to just the upper bank of calcarine fissure -> Pie on the floor
What is another name for the primary visual cortex (V1)?
The striate cortex -> It is given this name because it has this pale stripe along it, which is formed by small granule cells. The boundary between V1 and V2 is shown by the end of the pale stripe, as indicated by the red arrows.
Describe the layers of V1. What does each receive input from?
- Like all parts of the cerebral cortex, it has 6 layers (layer 1 not shown here)
- In layers 2 and 3, there are cytochrome oxidase ‘blobs’, which receive input from koniocellular neurons from the LGN (these are the interlaminar neurons that are not usually considered as the 6 layers of the LGN)
- In layer 4, the axons of parvocellular LGN neurons terminate deeper than those of magnocellular LGN neurons
What are blobs in V1? How can they be detected?
[IMPORTANT]
- They are regions of the primary visual cortex (V1), which are most clearly seen in layers 2 and 3
- The neurons in the blobs are sensitive to COLOUR
- The blobs can be detected histologically by staining with cytochrome oxidase stain
Describe how the visual field is mapped on V1.
- Central field -> Posterior pole of the occipital lobe
- Upper field -> Lower bank of the calcarine fissure
- Lower field -> Upper bank of the calcarine fissure
- Peripheral field -> Deep in the calcarine fissure
What are the two types of column in V1? How are they arranged?
[IMPORTANT]
- Ocular dominance columns -> These are alternating stripes of neurons that respond preferentially to input from one eye than the other
- Orientation columns -> These are columns of neurons that are excited by line stimuli at a specific orientation
These are arranged at right angles to the layers of cortex.
What are the two main outflow pathways from the visual cortex? What is the function of each?
[IMPORTANT]
“What” pathway:
- Via V4 to inferotemporal cortex (in temporal lobe)
- Involved in colour and pattern recognition, as well as memory (such as recognising known faces and objects)
“Where” pathway:
- Via V5 to the posterior parietal cortex
- Involved in location, motion and space sense
How are the orientation columns arranged in the visual cortex?
Arranged in a pinwheel structure
What are orientation columns?
cells with similar orientations are grouped into columns
Across the cortex there is a clockwise and counter clockwise cycling of orientation preference (full 180 degrees repeated every 750 micrometres)
What are the fibres in the optic radiation?
outer = Meyers loop
middle = central bundle
inner = dorsal bundle
What is myopia, what causes it and how can it be treated?
[IMPORTANT]
- Short-sightedness -> Objects in the distance appear blurry
- Caused by the eye being too long, so that the light waves focus in front of the retina
- Corrected using negative power spectacles or contact lenses (concave)
What is hypermetropia, what causes it and how can it be treated?
[IMPORTANT]
- Long-sightedness -> Objects in proximity appear blurry
- Caused by the eye being too short, so that the light waves focus behind the retina
- Corrected using positive power spectacles or contact lenses (convex)
What is astigmatism?
- An eye defect where there is different focus in different planes.
- This leads to two different foci -> One for the horizontal and one for the vertical planes.
How is testing of visual acuity done?
[IMPORTANT]
Using a Snellen chart:
- The patient is presented with a set of letters of different sizes
- They stand at a set distance, usually 6m or 20ft
- They read to the smallest text they can comfortably read
- Each lines is labelled with a number
- The visual acuity can be quoted as a fraction -> Distance from object / Number of line
- 6/6 or 20/20 vision is normal vision
What are the inputs in a blue/yellow receptive field?
Blue - receives inputs from S cones
Yellow - receives inputs from both M and L cones
Why would the response seen in diffuse red light (image) on a R+G- receptive field differ to that of diffuse blue light on a B+Y- receptive field?
BASED ON ABSORPTION CURVES
Red light is absorbed by inhibitor green cones (overlap in curves)
In the B+Y- neuron, very little blue light is absorbed by red and green cones that make up the surround, so the inhibitory response is less and diffuse blue light acts as a stronger stimulus than diffuse red light
What type of eye movements are coordinated by the frontal eye field?
Saccadic eye movements (saccades)
Rapid, simultaenous movement of both eyes jumping from one point to another
Generally occur in response to rapid movement - reactive nature
How do the regions of the brain controlling eye movements bring about the effects?
Signal to oculomotor, trochlear and abducens nuclei
