16 + 20. Vision, Auditory and Vestibular Systems 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 different classifications of the brightness of light.
What are the two main parts of the eye involved in focusing light to form an image on the retina?
- Cornea
- Lens
Draw a simple diagram showing how an image is formed on the retina in this situation.
What is the equation for the power of the eye?
P = µ/f
Where:
- P = Power (Dioptres)
- µ = Refractive index of the media of the eye
- f = Focal length of the eye (0.022m)
What is the unit for optic power of the eye?
Dioptres
(Where 1 dioptre is the power of a lens to focus parallel rays at a focal point of 1m)
Calculate the optic power of the eye.
- P = µ/f
- f is focal length of the eye (22mm or 0.022m)
- µ is the refractive index of the media of the eye (approximately that of water, which is 1.333)
- Therefore, for the eyeball, P = 60 Dioptres
What are the 3 main refractive errors of the eye that you need to know about?
- Myopia
- Hypermetropia
- Astigmatism
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 another name for hypermetropia?
Hyperopia
What is emmetropia?
When there are no visual defects (i.e. normal vision).
What is emmetropisation?
- Babies tend to be be hyperopic
- Over time, they tend to grow emmetropic (normal vision), so that they have normal vision by the time they are adults
- This is due to changes in the shape of the eyeball
What is the driving force of emmetropisation?
It is driven by defocus of the retinal image (not the effort of accommodation).
In other words, the hyperopia presents as a blurry image, which drives the changes in shape of the eyeball. This leads to emmetropia.
Give some experimental evidence for how emmetropisation happens.
Experiments in animal models have shown that:
- Fitting negative power spectacles causes faster axial eye growth -> This eventually leads to myopia because the eye changes in shape to compensate.
- Fitting positive power spectacles causes slower axial eye growth -> This eventually leads to hyperopia because the eye changes in shape to compensate.
An example: (Whatham, 2001)
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.
Describe the consequences of astigmatism.
The top left is normal vision. The other two are examples of astigmatism.
How is astigmatism corrected?
With spectacles that have a cylindrical component in their surface curvatures.
What is accommodation?
Increase in power of lens caused by contraction of annular ciliary muscle.
How does accommodation occur?
In order to increase power of a lens:
- Parasympathetic innervation drives contraction of annular ciliary muscle
- This reduces tension in radial zonular fibres, allowing lens to relax to a more convex state
How does contraction of ciliary muscles affect the power of the lens?
Contraction leads to increase in power of lens (since it becomes more rounded).
Note: This is sort of counter-intuitive.
What is presbyopia?
- A failure of accommodation of the lens with age
- Usually considered to be caused by lens material becoming stiffer and less elastic with age
Give an example of an experiment demonstrating presbyopia.
This shows how the maximum accommodation of the eye changes with age (due to stiffening of the lens).
What is a cataract?
[IMPORTANT]
- Condition in which the lens becomes cloudy -> This is due to the fibres in the lens becoming disordered (so they scatter light).
- In wealthier countries largely a condition of old age.
What can cataracts lead to?
Blindness
How can cataracts be treated?
Straightforward to treat by removing the lens surgically and replacing it with an artificial lens. BUT cataracts still cause millions to be blind.
What are some risk factors for cataracts?
- Tends to occur in old age
- Higher incidence near the equator -> May be due to severe dehydration
What is aqueous humour?
It is the watery fluid that fills the anterior and posterior chamber of the eye. It is not to be confused with the vitreous fluid behind the lens.
Describe the circulation of the aqueous humour.
- Produced by the ciliary body
- Flow through the posterior then anterior chambers
- Flows out through the trabecular meshwork in the corner of the anterior chamber
What is normal intraocular pressure?
Around 16.5mmHg
What can cause increase intraocular pressure?
[IMPORTANT]
Usually it is due to decreased drainage of aqueous humour (e.g. due to blockage of the trabecula meshwork).
What is glaucoma and what causes it?
[IMPORTANT]
Damage to the optic nerve caused by raised intraocular pressure (>20mmHg). It can lead to blindness.
What device can be used to look into the eye?
Ophthalmoscope
Label this ophthalmoscope image.
On an ophthalmoscope image, where do all the blood vessels originate from?
Near the optic disc.
(It is also worth noting that almost no vessels go to the macula).
What is the problem with studying the inside of someone’s eye and how is this overcome in ophthalmoscopy?
- When viewing the retina, the doctor’s eye is in the way of the source (figure a)
- Therefore, an ophthalmoscope uses a beam splitter to ensure that the doctor is aligned with the source practically but not literally
What are the two types of light receptor in the eye?
- Rods
- Cones
Describe the function and distribution of rods and cones in the eye.
[IMPORTANT]
Cones:
- Colour vision
- Centre of the eye (in fovea)
Rods:
- Night vision
- Periphery of eye
(Just remember: CCC)
Draw a graph showing the distribution of cones and rods on the retina.
How many rods and cones are there?
- 100 million rods
- 7 million cones
How can the function of rod photoreceptors be tested experimentally?
How large is the fovea and what visual angle is that equal to?
- 1.5mm
- Equal to 5* of visual angle
What are some functional adaptations of the fovea of the eye?
Retinal nerve cell bodies are shifted aside from the central fovea, so light has a more direct path to photoreceptors.
Is the fovea along the midline of the eye?
No, it is slightly off to the side.
(Check if this is due to the angling of the lens)
Draw the histological structure of the retina.
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 are the two main types of modulator cells in the retina? Describe the position of each.
[IMPORTANT]
- 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 different layers of the retina?
What type of receptive field do ganglion cells and bipolar cells in the retina have? What does it mean?
[IMPORTANT]
- 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
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.
What neurotransmitter do photoreceptors release?
[IMPORTANT]
Glutamate
What are the two types of bipolar cells? How do they work?
[IMPORTANT]
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?)
For this centre-surround receptive field of an ON bipolar/ganglion cell, draw the action potentials that are fired.
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
Do ganglion cells receive input from both ON and OFF bipolar cells?
No, they receive input from only one type of bipolar cell, which is why we also sometimes talk about ON and OFF ganglion cells (since they are corresponding).
Summarise signal processing in the retina.
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)
What is the difference between different opsins?
Different opsins have different spectral sensitivities.
Describe the structure of photopigments.
Consist of two parts:
- Opsin (a type of protein) -> Determines the type of photopigment
- Chromophore (in the human retina, the type is called ‘retinal’)
Give some clinical relevance of photopigments.
[EXTRA]
Retinal (the chromophore in animal photopigments) is an aldehyde of vitamin A. Therefore, vitamin A deficiencies can lead to visual defects.
How does light trigger photopigments?
It leads to isomerisation of retinal (the chromophore), which triggers the transducin pathway that hyperpolarises the cell (phototransduction).
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 is Purkinje shift?
[EXTRA]
Sensitivity to colour shifts towards red on moving from dim (‘scotopic’) light, where only rods are active, to bright (‘photopic’) light, where cones are used. This is due to difference in the shapes of their spectral curves.
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 is visual acuity?
- Commonly refers to the clarity of vision, but technically rates an examinee’s ability to recognize small details with precision
- It is a measure of spatial resolution
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
With normal 20/20 (a.k.a. 6/6) vision, what is the smallest fraction of the visual field that can be resolved?
- Gaps of about 1 min arc (1/60 degree) can just be read.
- But under ideal conditions, gaps of 0.5 min can be resolved.
What is the limiting factor of visual acuity?
The spacing of cones in the fovea -> The width of individual foveal cone outer segments is 2 microns.
Draw a diagram to show how there are limits to visual acuity and there may be times when you cannot tell two lines apart if they are close together.
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.
[IMPORTANT]
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 experimental evidence for the functions of the two types of ganglion cells in the retina.
- The diagrams in the middle show the way in which banded lines of line are moved back and forth between two positions
- The graphs on each side show the firing of magnocellular and parvocellular cells in response to each diagram
- This shows that the magnocellular cells have better temporal resolution and can therefore detect movement better -> This is shown by the fact that there are peaks every time the light changes
- It also shows that the parvocellular cells have better spatial resolution and essentially produce an average of the receptive field -> This is shown by the fact that there are peaks when there is either light or dark, but not both
Give some examples of retinal disease.
[IMPORTANT]
What is diabetic retinopathy?
[IMPORTANT]
- 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.
Where does vision information from the retina travel to first?
Lateral geniculate nucleus (LGN)
What is the lateral geniculate nucleus and where is it found?
- It is a relay center in the thalamus for the visual pathway.
- It receives a major sensory input from the retina.
What word can be used to describe the structure of the lateral geniculate nucleus?
Layered
Describe the laminar organisation of the lateral geniculate nucleus.
- Layers 1-2 receive input from magnocellular ganglion cells
- Layers 3-6 receive input from parvocellular ganglion cells
- The layers alternate inputs from the two eyes (so 3 layers per eye)
- There are also extensive inputs from the brainstem and from V1
It is also worth noting that there are koniocellular cells in the interlaminar spaces.
What is the function of the LGN?
- It was first thought that it was nothing more than a relay of visual information to the visual cortex, but it is now thought to play a far more complex role as a gate for controlling the passage of signals.
- For example, it can cause fewer signals to pass through during sleep.
Where does the LGN relay information and via what?
To the primary visual cortex (V1) via the optic radiations.
How does areas of the retina correspond to areas in the LGN?
- The retinal (hereafter called “retinotopic”) map is preserved. Axons from the retina preserve their order.
- There is an entire map of a visual hemi-field in each layer of the LGN.
Compare the vision deficits caused by lesions prior to and after the LGN.
- Prior to the LGN -> Left or right eye defects
- After the LGN -> Left or right field defects
Draw the structure of the visual pathway.
[IMPORTANT]
Optic nerve fibres from which part of the retina decussate at the optic chiasm?
Nasal retina
What sort of lesions of the visual pathway do you need to know about?
- Optic nerve
- Optic chiasm
- Optic tract
- Visual cortex
So no optic radiations or LGN.
Summarise the different field defects that occur along the visual pathway.
Give a summary of colour perception disorders of the retina.
[EXTRA]
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 the LGN/optic tract?
[EXTRA]
- Dependent on what part of the optic radiations are lesioned -> This is because the radiations do not all run together
- e.g. Meyer’s loop -> Loss of vision in the upper quadrant on the opposite side of the visual field to the injury. Also known as pie in the sky disorder.
- Lesion of upper optic radiation -> Pie in the floor
- Complete lesion causes contralateral homonymous hemianopsia -> Loss of half of visual field (opposite side to lesion)
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 optic neuritis?
[EXTRA]
Inflammation of the optic nerve
The lateral geniculate nucleus is not the only target that fibres of the optic nerves go to. What are some other sub-cortical visual centres and what are their functions?
[IMPORTANT]
- Superior colliculus -> Eye and head movements
- Suprachiasmatic nucleus -> Circadian rhythms
- Pre-tectal nucleus -> Pupillary light reflex
These should be covered more in other lectures -> If not, add more flashcards.
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.
Where is V1?
It is in the occipital lobe of the cerebral cortex.
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
In V1, there is a map of the visual field. How was this map first mapped?
[EXTRA]
- Studying the visual field losses caused by bullet & shrapnel wounds in the war between Russia and Japan early in the 20th century; and in WW1 (by Gordon Holmes)
- The parts of the brain that were damaged were compared with the parts of the visual field that were lost, allowing the map of the visual field in V1 to be detected
Do lesions of V1 always lead to blindness?
- They tend to lead to blindness, as would be expect
- However, if it is only V1 that is lesioned, sometimes there is a condition called blindsight
- This is where they can respond to visual stimuli that they do not consciously see.
- In studies, when a stimuli is shown in the blind parts of their visual field, they have a higher than chance likelihood of guessing when the stimulus is.
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
Describe the concept of V1 neuron orientation selectivity and who discovered it.
(Hubel and Wiesel, 1968):
- Found that each V1 neuron responds to a bar of light differently depending on the bar’s orientation
- Each neuron has an orientation at which it fires most rapidly, and the it fires less at other orientations
Describe Hubel and Wiesel’s model for how orientation selectivity in V1 works.
Each V1 neuron receives inputs from multiple receptive fields arranged in a line, meaning that the neuron only achieves peak activation at a certain orientation, when all of the receptive fields are fired.
What are the different types of cell in V1?
[EXTRA?]
- Simple cells -> These are the orientation-selective neurons that respond to a line at a given orientation
- Complex cells -> Like simple cells, but they have a high degree of spatial invariance, such that there is a large receptive field, and the line can be responded to regardless of the exact position
- Hypercomplex cells -> Like complex cells, but they are excited specifically by short bars or corners
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.
Give some experimental evidence for the existence of orientation columns in V1.
[EXTRA]
In cats:
Give some experimental evidence for the existence of ocular dominance columns in V1.
[EXTRA]
Do neurons in V1 receive information from only one eye?
No, they receive information from both eyes, but they preferentailly respond to input from one, resulting in ocular dominance columns. The fact that they can receive information from both eyes is because of partial decussation at the optic chiasm.
Name an experimental technique that can be used to study orientation columns in V1.
[EXTRA]
- Two-photon calcium imaging
- This is where a mouse is modified so that neurons in V1 show colour upon calcium entry (when the neuron is excited)
- This allows the researchers to see how excited individual cells get when exposed to given orientations of lines
What is the concept of developmental plasticity of V1?
[IMPORTANT]
- The ocular dominance columns can change sizes in an adaptive manner.
- If one eye is deprived of light, then its ocular dominance columns shrink and the other eye takes over parts of them.
Describe an experiment to prove developmental plasticity of V1.
[EXTRA]
- One eye in a monkey can be covered from birth til 8 weeks
- The ocular dominance columns in that monkey will be much larger for the dominant eye than for the covered eye, especially when compared to a control monkey
What is amblyopia?
[IMPORTANT]
- It is a disorder when the visual cortex fails to process input from one eye as strongly as input from the other eye (a.k.a. lazy eye)
- Causes include: Poor alignment of the eyes, an eye being irregularly shaped such that focusing is difficult, one eye being more nearsighted or farsighted than the other, or clouding of the lens.
- Symptoms may not be noticeable, but can include poor depth perception, poor pattern recognition, poor visual acuity, and low sensitivity to contrast and motion.
- Treatment can be done by wearing an eye patch over the stronger eye to allow the weaker eye to take over more of V1.
What is strabismus?
[IMPORTANT]
- Strabismus is a condition in which the eyes do not properly align with each other when looking at an object.
- It can lead to amblyopia.
How do some cells in V1 allow for depth perception?
- Many cells in the V1 are disparity-selective.
- The receptive fields are in slightly different horizontal positions on the two retinae.
- Hence they respond best to single objects at particular distances.
What are the two types of colour detecting cells in V1 and where are they found?
- Double-opponent cells -> Found in blobs (in layers 2 and 3)
- Single-opponent cells -> Found in the rest of V1 and as retinal ganglion/LGN cells
What are double opponent cells? What is their function?
[IMPORTANT]
- Cells in blobs of V1 that are, for example, excited by green and inhibited by red in the receptive field centre, and excited by red and inhibited by green in its receptive field surround.
- They are important in detecting colour contrast, such as in patterns, textures and boundaries.
What are single opponent cells? What is their function?
[IMPORTANT]
- Cells in V1 that are, for example, excited by green in its receptive field centre, and inhibited by red in its receptive field surround.
- They are important in responding to large areas of colours, such as in large colour scenes and atmospheres.
Compare how double and single opponent cells respond to monochromatic and black-white contrast illumination.
- Double opponent cells do not respond to monochromatic and black-white contrast illumination since the signals from the centre and surround would cancel each other out each time.
- Single opponent cells respond to monochromatic colour and also to a white spot in its centre against a black surround.
Can cells in V1 be both colour and orientation selective?
Yes
How can we tell apart different areas of the cerebral cortex?
They can be differentiated by their:
- Cytoarchitecture (different cell types and distributions)
- Myeloarchitecture (different myelination)
- Connectivity
Is V1 the only part of the cortex that receives visual inputs?
No, there are multiple cortical areas for vision.
What Brodmann area is V1?
17
What are the different areas of the visual cortex?
V1-V5
Where in the brain are the different parts of the visual cortex?
[EXTRA?]
- They are in the occipital lobe
- V1 is intact, but the other parts are split on either side of it
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
As a signal is passed from V1 down visual pathways, what happens to the receptive field?
Receptive fields increase in size since each neuron at each level integrates information from multiple neurons, so that there is progressive integration.
What is visual agnosia?
[IMPORTANT]
- Visual agnosia is where a patient can see objects as normal, and in general their vision is unimpaired, but they are unable to recognise what each object is
- This is usually due to damage to the “what” visual pathway (such as in the lateral occipital cortex)
Draw the appearance of a sound wave.
What is the period of a wave?
- The wave period is the time it takes to complete one cycle.
- The standard unit of a wave period is in seconds.
Describe the sound produced by speech.
- The opening and closing of the vocal cord produces sound waves
- They are not sinusoidal and there is a range of frequencies
- The pitch can be changed by changing the tension in the muscles
What are the main sections of the ear?
- Outer ear
- Middle ear
- Inner ear
What is the external ear?
It is everything outside of the ear drum (i.e. ear canal and auricle).
Label the external ear.
What are the main parts of the external ear that you need to know about?
- Pinna/Auricle -> This is the whole part that is visible from the outside
- Tragus -> The small prominence on the anterior side of the pinna, which slightly covers the ear canal
- External auditory meatus (ear canal)
- Tympanic membrane (ear drum)
Label this diagram of the pinna.
What is the function of the external ear?
- Collection of sound -> The pinna catches sounds and deflects them into the external auditory meatus.
- Acoustic gain -> The auricle and start of the ear canal have resonant properties that increase the amplitude of sounds of certain frequencies
- Sound location -> Identical sounds from different directions will elicit different frequency spectra when passing through the pinna.
Draw a graph to show how the ear canal and pinna cause acoustic gain of certain frequencies.
Draw a diagram to show how the external ear is involve in sound location.
Sounds from different locations produce different frequency spectra.
What is secreted into the external auditory meatus?
Secretions of the ceruminous gland (ear wax).
What is the external auditory meatus made of?
Outer third is cartilaginous, continuous with the pinna, whilst the inner two thirds are formed by the temporal bone.
What nerves innervate the external auditory meatus and tympanic membrane?
[IMPORTANT]
- Trigeminal (V)
- Vagus (X)
- Facial (VII) -> Contributes to tympanic membrane
What is the middle ear?
It is an air-filled cavity between the ear drum and cochlea. It contains the auditory ossicles.
What are the parts of the middle ear that you need to know about?
- Auditory ossicles (malleus, incus and stapes)
- Stapedius muscle + Tensor tympani muscle
- Pharyngotympanic tube connection
- Oval and round windows
What tube is connected to the middle ear and what is its function?
Pharyngotympanic tube (Eustachian tube) -> Connects the ear to the nasopharynx for pressure equalisation
Describe the auditory ossicles.
- Malleus -> Largest of the ossicles and is attached to the tympanic membrane at its handle.
- Incus -> Shaped rather like a pre-molar tooth and has a long limb terminating at the stapes.
- Stapes -> Has two limbs and a base which connects to the oval window (i.e. the inner ear).
What is impedance matching?
- The middle ear must match low-impedance sounds from the air to the higher impedance of the fluid in the inner ear.
- Impedance describes a medium’s resistance to movement.
- Normally, when sound travels from a low-impedance medium like air to a much higher-impedance medium like water, almost all of the acoustical energy is reflected.
How is impendance matching achieved in the middle ear?
- Difference in membrane area -> The tympanic membrane (eardrum) is much larger than the footplate of the stapes, so that the pressure transferred to the inner ear is increased
- The ossicles act as levers, multiplying the force
- The tympanic membrane buckles as it moves, increasing the force transferred to the inner ear
What are the two openings between the middle and inner ear? What is the function of each?
- Oval window -> Membrane made to vibrate by the stapes
- Round window -> Membrane vibrates with opposite phase to the oval window, so that the cochlear fluid can move
What muscles are in the middle ear, what do they do and what are they innervated by?
[IMPORTANT]
Stapedius muscle:
- Connects to the stapes
- Innervated by the facial nerve (VII)
Tensor tympani muscle:
- Connects to the malleus
- Innervated by the trigeminal nerve (V)
When the muscles contract, they increase the tension in the auditory ossicles, so that there is less transmission of sound.
Label this diagram of the middle ear.
What is the middle ear reflex?
- The contraction of the stapedius muscle by loud, low-frequency sounds.
- The contraction causes stiffening of the auditory ossicles in the middle ear, so that low frequency sounds are not as easily transmitted through the middle ear.
- Because of latency, this is not that useful in protecting against loud external noises, but it is triggered in advance of your own speech to protect against that
What is the inner ear?
Everything inwards of the cochlea.
What parts of the inner ear do you need to know about?
- Internal acoustic meatus
- Cochlea
- Cochlear nerve
- Spiral ganglion
- Vestibular apparatus
Within which bone is the inner ear?
[IMPORTANT]
Petrous part of the temporal bone
Outflow from the inner ear happens through what?
[IMPORTANT]
Internal acoustic meatus (this is a canal in the temporal bone)
Is the coiling of the cochlea of any particular function?
Not really, it is just to make it compact.
Describe the structure of the cochlea.
- It is coiled about 2.5 times
- If we now consider any given spiral:
- On the very outside, there is the bony labyrinth, containing perilymph
- In the perilymph, there is the membranous labyrinth, containing the endolymph
Draw a cross-section of the cochlea.
What are the chambers of the cochlea called?
- Scala vestibuli (perilymph)
- Scala media (endolymph)
- Scala tympani (perilymph)
Describe the structure of the membranous labyrinth.
- The membranous labyrinth is the structure that surrounds the scala media (contain endolymph) in the cochlea.
- It is made up of the basilar membrane and Reissner’s membrane
Compare and explain the composition of endolymph and perilymph in the cochlea.
- Perilymph is high in sodium (like normal extracellular fluid)
- Endolymph is high in potassium, which is pumped in through the stria vascularis (which contains ATPases)
What is the endocochlear potential?
- It is the potential difference of 80mV between the (more positive) endolymph and the perilymph, across the basilar membrane.
- The high potassium in the endolymph enables transduction of the mechanical signal into an electrical one.
Where does transduction happen during hearing?
In the hair cells of the organ of Corti, on top of the basilar membrane in the scala media.
Draw the structure of the organ of Corti.
It contains hair cells and support cells, and is covered by the tectorial membrane.
Draw the structure of hair cells in the organ of Corti in the cochlea.
- Note how there is no axon (just like with photoreceptors), since the hair cells do not fire action potentials. Instead there is an afferent fibre that innervates the cell.
- There is also efferent innervation.
- There are stereocilia on the surface that are arranged by height.
What are the different types of hair cell in the cochlea and how are they arranged?
- Inner hair cells -> Only one row, stereocilia not embedded in tectorial membrane
- Outer hair cells -> 3 rows, stereocilia embedded in tectorial membrane
How do hair cells detect sound?
- When the stapes vibrates, it moves the membrane in the oval window (connected to the scala vestibula)
- Since the membrane in the round window at the other end (connected to the scala tympani) can also move, pressure waves are sent through the perilymph
- The basilar membrane separates the scala tympani (containing perilymph) and the scala media, so it is deflected as the pressure waves pass
- The hair cells, fixed to the basilar membrane, have stereocilia that are deflected as they brush against the tectorial membrane above them
How do the stereocilia on hair cells in the cochlea enable transduction?
- The stereocilia are arranged from shortest to longest, and the tip of each cilia is attached to its longer neighbour
- When the sterocilia are brushed in the direction of the longest stereocilia (as the basilar membrane moves up), the tip-links are stretched
- Stretching of the tip-links leads to opening of transduction channels, TMC1 and TMC2 (transmembrane channel-like 1 and 2)
- This leads to entry of potassium into the hair cell (due to electrical gradients) and therefore depolarises the hair cell
- Voltage-gated calcium channels open and there is an influx of calcium, which triggers neurotransmitter release onto the afferent nerve fibre
- On the other hand, when the sterocilia are brushed in the direction of the shortest stereocilia (as the basilar membrane moves down), the tip-links are relaxed. This leads to hyperpolarisation.
Label this cross-section of the cochlea.
How is the cochlea able to detect different frequencies of sound?
- The stiffness of the basilar membrane decreases as you go along the cochlea
- This means that as the wave of displacement travels along it, the point at which it peaks will be different for every frequency
- This means that specific hair cells detect specific frequencies, based on their position along the cochlea
What is a frequency tuning graph?
- It is a graph of the minimum amplitude of a sound at a range of frequencies that is required to activate hair cells
- It is done for a single point along the basilar membrane in the cochlea
What are the two mechanisms that contribute to the shape of a frequency tuning graph like this?
- Stiffness of the basilar membrane changes along its length -> So that peak deflection occurs at a specific point along the cochlea (and therefore at corresponding hair cells)
- Amplification of the sound (at the right frequencies) by outer hair cells
Note: The importance of amplification is shown by the kanomycin, which is an ototoxic drug.
How is amplification of sounds done in the cochlea?
- Outer hair cells become longer when hyperpolarised and become shorter when depolarised
- This means that when they are stimulated in the cochlea, they appear to bounce
- SInce their stereocilia are embedded in the tectorial membrane, this causes it to move
- Therefore all of the hair cells are deflected more and therefore activated more
Note how this is selectively amplifiying the sound at the right frequencies since it only happens at points where the outer hair cells are already being stimulated.
How do outer hair cells change size when stimulated?
Prestin is the protein in the basolateral membrane responsible for the changes in size.
Compare the innervation of inner and outer hair cells in the cochlea.
Describe the afferent innervation of hair cells in the cochlea.
- Almost all of the afferent innervation is of the inner hair cells
- Many of the inner hair cells are innervated by multiple afferent fibres
What is the characteristic frequency?
- The frequency of sound that AUDITORY NERVE FIBRES (that innervate hair cells in the cochlea) are most sensitive to
- Since auditory nerve firbes only innervate one hair cell in the cochlea, the graph shown should correspond to the frequency tuning curve of the hair cell (which is in turn dependent on its position along the basilar membrane)
Does cochlear amplification occur up to any amplitude of sound?
- No, the vibration of the tectorial membrane eventually becomes saturated (CHECK THIS), so that at a certain amplitude the response plateaus.
- This is shown on the graph, where the line flattening out shows that the ear cannot detect difference in amplitude of the sound beyond that amplitude.
What is two-tone suppression?
- When a sound in the grey area is played simultaneously with the one at the characteristic frequency, the response of the auditory nerve to the characteristic frequency sound is decreased.
- This is not mediated by inhibitory synapses, but is due to the non-linear response of the basilar membrane (i.e. the amplification mechanism is saturated).
What is phase locking?
- It is the way in which the frequency of the sound corresponds to how frequency the firing rate of an auditory nerve fibre alternates between high and low.
- i.e. An auditory nerve fibre will only fire at moments when the basilar membrane deflects upwards, which occurs at the peaks of the waves -> This happens at constant intervals
What ganglion are the cell bodies of neurons innervating the hair cells of the cochlea found in?
Spiral ganglion
Where in the brain does the auditory nerve (VIII) go?
(Cochlear nucleus of the) Medulla.
Draw the central auditory pathway.
[IMPORTANT]
- Cochlea
- Spiral ganglion
In brainstem:
- Cochlear nucleus
- Superior olivary complex (including the medial nucleus of trapezoid body)
In midbrain:
- Inferior colliculus
In cerebrum:
- Medial geniculate nucleus
- Auditory cortex (in the superior temporal gyrus)
How does tonotopicity work in the brain?
- Each hair cell can receive afferent innervation from about 10 auditory nerve fibres
- Each of these bundles corresponds to a different characteristic frequency, and these areas are topologically organised in the brain (e.g. in the cochlear nucleus)
- Therefore, there is essentially a map of different frequencies on the brain
What are some different cochlear nucleus cell types? What makes them different?
- The cells all differ in their morphology and pattern of firing in response to a tone burst
- Their pattern of firing is partly dependent on the physical properties of the membrane channels they have, and partly dependent on local inhibitory neurons that affect the appearance of their frequency response areas
Does the cochlear nucleus receive innervation from both cochleas?
No, each cochlear nucleus receives input from the cochlea on the same side of the body.
What does the superior olivary complex receive innervation from?
Both of the cochlear nuclei.
What are the main parts of the superior olivary complex and what is the input to each?
- Medial superior olive
- Receives excitatory input from the cochlear nuclei on contralateral AND ipsilateral side
- EE (excitatory-excitatory)
- Lateral superior olive
- Receives excitatory input from the cochlear nucleus on the ipsilateral side and inhibitory input from the cochlear nucleus on the contralateral side (via the medial nucleus of the trapezoid body)
- EI (excitatory-inhibitory)
- Medial nucleus of the trapezoid body
- This is where some fibres from one cochlear nucleus synpase and inhibitory neurons continue to innervate the contralateral superior olive
What is the trapezoid body and what is its function?
- It is the site in the pons where fibres from the cochlear nuclei decussate to the other side
- The medial nucleus of trapezoid body is where some of these fibres synapse and continue as inhibitory neurons to the contralateral lateral superior olive
After the superior olivary complex, where does auditory information go?
To the inferior colliculus in the midbrain, via the lateral lemniscus.
After the inferior colliculus, where does auditory information go?
Medial geniculate body (a.k.a. nucleus)
After the medial geniculate body, where does auditory information go?
Auditory cortex
In tonotopy maintained throughout the entire auditory system?
Yes, all the way to the auditory cortex.
Describe tonotopy in the primary auditory cortex.
- There are isofrequency bars.
- On top of these are mapped areas of EE (excitatory input from both ears), EI (excitatory from one ear, inhibitory from the other) and frequency modulated (changing frequencies).
Note that this is not a very well understood mapping.
Describe the outflow from the primary auditory cortex (A1).
- Posterior parietal cortex (parietal lobe) -> “Where” a stimulus is
- Inferotemporal cortex (temporal lobe) -> “What” a stimulus is
What determines the dynamic range of hearing?
- Each hair cell in the cochlea has its own dynamic range
- The minimum deflection of sterocilia required for depolarisation determines the minimum amplitude detected, while the maximal opening of channels determines the maximum amplitude detected
- Thus, the dynamic range of hearing is the sum of all of these dynamic ranges
What is an audiogram?
[IMPORTANT]
- It is a graph of the minimum amplitude of sound that an individual can hear at a range of frequencies.
- It is useful in diagnosing hearing conditions.
What determines the human frequency hearing range and the frequencies at which the ear is most sensitive?
- The cochlea’s properties (length and stiffness) determine the range of frequencies the ear can detect
- The external ear amplifies all sound entering the ear, but it is done in a frequency-dependent manner based on the physical properties of the external ear, meaning the ear is most sensitive around 2-5kHz
Is phase locking temporal coding or rate coding of a sound?
Temporal coding, since the amplitude of the wave may affect that rate at which action potentials are fired. Phase locking simply means that the action potentials will be fired in bursts at set intervals depending on the frequency.
What are the two main sound localisation cues?
- Interaural time differences (ITDs) -> The difference in arrival time of the sound at each ear
- Interaural level differences (ILDs) -> The difference in amplitude of the sound at each ear
How can you study the accuracy of sound localisation?
Just like it the sound practicals we did.
Which medial superior olive processes sounds that are located on the on the right side of the body?
- The left MSO
- This is because the sound arrives at the right ear first, so it must be delayed slightly (via the longer route) in order for the action potentials to arrive at the MSO at a similar time
- Thus, each MSO processes sounds located on the contralateral side of the body
How does the MSO detect ITDs?
- The MSO receives signals from both the contralateral and ipsilateral ear
- Each MSO uses a counterflow delay system so that signals arrive at each MSO neuron at different times
- The more a sound is on the contralateral side (and therefore the greater the ITD), the higher the rate of firing of neurons is at the MSO, since (in net) the signals converge at the MSO at closer intervals and therefore there is more firing
- Thus, ITD is estimated by comparing the activation of the two MSO channels
How does the LSO detect ILDs?
- Each LSO receives stimulatory inputs from the ipsilateral side and inhibitory inputs from the contralateral side
- This means that if a sound is on the ipsilateral side to a LSO, then the excitatory response dominates, while if it is on the contralaterl side, then the inhibitory response dominates
- Thus, the LSO neurons fire more when the sound is on the ipsilateral side
How can a sound directly in front of us be differentiated from one directly behind us (since the ITDs and ILDs are 0)?
- The external ear provides cues about the direction of the sound. They are also useful for the vertical direction of the sound.
- These cues are processed largely in the dorsal cochlear nucleus, which contains type 4 cells.
Aside from sending information up to the medial geniculate nucleus (and then auditory cortex), where else can the inferior colliculus send information to? Why?
- Superior colliculus
- This is involved in producing a sensory map of the directions of various modalities of stimuli (sound, vision, somatosensory)
What is the function of the superior colliculus?
- It receives sensory input from the auditory, visual and somatosensory systems
- It has maps of all of these, superimposed on each other
- Thus, it integrates the positions of all of these stimuli
- It also has a corresponding motor map, so that fast responses (such as eye movements towards a stimulus) can be coordinated
Note that the superior colliculus is not essential for sound localisation however.
What are the two main types of hearing loss?
[IMPORTANT]
- Conductive -> Problems with sound conduction in the external or middle ear
- Neural -> Problems with sound transduction and processing (e.g. hair cells, etc.) in the inner ear or further downstream
Describe the appearance of a clinical audiogram.
- It accounts for the normal U-shaped curve of human hearing.
- Therefore, it just shows the deviation from normal hearing at each frequency and gives ranges for different types of hearing loss.
How does conductive hearing loss present on a clinical audiogram?
- The hearing loss is around constant at every frequency
- Therefore, the audiogram is almost a flat line
What is a common example of neural hearing loss?
Presbycusis -> This is the progressive hearing loss at high frequencies as you age.
How does presbycusis appear on a clinical audiogram?
The highest frequencies of hearing are most affected.
How do some antibiotics and loud sounds cause hearing loss?
[IMPORTANT?]
- They induce metabolic changes that cause loss of outer hair cells -> These are responsible for cochlear amplification.
- They also cause swelling of the auditory nerve fibres.
How do cochlear implants work and when are they used?
- They use a microphone outside of the head to detect sounds
- These sounds are then transduced using an electrode array that is inserted into the scala tympani.
- This is done in situations where cochlear hair cells are lost and therefore transduction is the problem, so that no amount of amplification using a hearing aid will help.
Label diagram.
What is the canal of Schlemm?
- This is also known as the scleral venous sinus
- Circular canal found in the posterior part of the corneoscleral junction
- Allows the collection of aqueous humour from the anterior chamber of the eyeball and for it to be delivered to the veins of the of the eyeball
What is the lamina cribrosa?
- A mesh-like structure that occupies the hole in the sclera through which optic nerve fibres and the central retinal artery and vein pass
- Formed by a multilayered network of collagen fibres that insert into the scleral canal wall
- Thought to support the optic nerve as it leaves the eyeball
What is the ciliary body?
- This is the part of the eye that connects the iris to the choroid
- Secretes aqueous humour
- Made up of the ciliary muscle, the ciliary ring and the ciliary processes
- Ciliary muscle adjusts the curvature of the lens (contraction causes lens to become more spherical and increases focussing power)
- Ciliary ring joins the ciliary body to the choroid
- Ciliary processes are radial structures from which the lens is suspended by ligaments
What is the conjunctiva?
- This is the tissue that lines the insides of the eyelids and covers the sclera
- Composed of unkeratinised, stratified squamous epithelium with goblet cells, and stratified columnar epithelium
- Provides and lubrication of the eye through the production of mucus and tears
- Also provides an anti-microbial barrier and aids in immune surveillance
What is the iris?
- Pigmented region of the eye which regulates the amount of light that is allowed to enter the eye
- Made up of circular and radial muscles (is a functional sphincter)
- When circular muscles contract, the aperture radius decreases
- When radial muscles contract, the aperture radius increases
What is the pupil?
- The aperture in the eye through which light can enter
- Size is governed by the muscles in the iris
- Aperture is smaller in bright light, wider in dim light
- Responds during the pupillary light reflex
What is the retina?
- Thin layer of tissue that makes up the innermost layer on the back of the eye
- Contains rod and cone cells, which are able to transduce light into neural signals, which can then be transmitted to the brain via optic nerve fibres
- The retina does not cover the entirety of the eye, the innermost layer on the iris and more anterior structures of the eye is the choroid
- Junction between the two layers is the ora serrana
What is the optic disc?
- Also known as the optic nerve head
- This is the point of exit for ganglion nerve cells as they leave the eye
- These cells form the optic nerve once they have left the eye
- There are no overlying rods or cones, so this region is a blind spot on the retina
What is the fovea?
- This is a tiny pit located in the macula of the retina
- The macula is the functional centre of the retina
- Provides the clearest vision possible in the eye, as the other layers of the retina are not present at this region, allowing light to fall directly onto the cones
- These cells have the highest acuity, and so a clear image is formed
What is the lens?
- Transparent biconvex structure that helps to refract light to be focussed onto the retina
- Can change shape depending on the optical power needed/distance from the object
- This is achieved through the action of surrounding structures (ciliary bodies and suspensory ligaments)
What are the suspensory ligaments?
- Also known as the zonule
- Series of fibres that connects the ciliary body to the lens, holding it in place
- When the ciliary muscle contracts, the ligaments are allowed to loosen and the lens bunches back into its natural shape (thicker and more spherical)
- When the ciliary muscle relaxes, the ligaments become stretched/become tight, and the lens is also stretched into a thinner and flatter shape
What are the anterior and posterior chambers?
- The anterior chamber is between the cornea and the iris
- Chamber is filled with aqueous humour, which is produced by the ciliary bodies, but first passes into the posterior chamber before entering the anterior chamber via the pupil
- The posterior chamber is between the suspensory ligaments and the iris
- Chamber is also filled with aqueous humour
What is the choroid?
[EXTRA]
- One of the layers of the eye, containing some blood vessels that then augment the main blood vessels found in the retina
- Also mostly made up of connective tissues
- Is the middle layer of the eye
- Is pigmented to limit reflection of light within the eye that would otherwise cause the image to be degraded
- Fused with the ciliary body
- The junction between the retina and the choroid is the ora serrata
What is the vitreous body?
- This space is filled with vitreous humour (transparent, ‘glass-like’)
- Region between the lens and the retina
- Has protective functions and also gives the eye its spherical shape
What is the iridocorneal angle?
[EXTRA]
- This is the angle between the iris and the cornea (red region on diagram)
- Lies external to the lens and is responsible for the outflow of aqueous humour from the anterior chamber (into the canal of Schlemm)
What are the 5 layers of the cornea?
- External stratified epithelial layer
- Thin anterior limiting lamina (aka Bowman’s membrane, free of fibroblasts)
- Stroma of the cornea (thick layer of parallel running collagen fibrils, interspersed with fibroblasts)
- Posterior limiting lamina (Descemet’s membrane)
- Layer of endothelial cells
Describe the different components of the retina.
- Photoreceptor layer is lined by a thin sheet, which forms the pigmented epithelium
- Beneath this layer is the choroid, which contains many blood vessels
- Outer and inner segments of the photoreceptive rods and cones lie interally to this layer, and their nuclei lie in the outer nuclear layer
- Cell-free layer lying internally is the outer plexiform layer, which contains the processes of the horizontal and bipolar cells, and synaptic processes of photoreceptors
- Horizontal and bipolar cells both synapse with rods and cones, but horizontal cells also synapse with bipolar cells (thought to intergrate information laterally)
- Bipolar cells contact ganglion cells directly to receive from cones, others form contacts with amacrine cells to receive from rods
- Bipolar cells form contacts with amacrine and ganglion cells in the inner plexiform layer
- Amacrine cells are also associated with lateral integration, and their cell bodies are either in the inner nuclear layer or the ganglion cell layer
-
Ganglion cell layer is largely made up of ganglion cells, which send their axons to the visual layers
- Between the ganglion cell layer and vitreous humour, there is a thick fibre layer of ganglion cell axons which converge on the optic disc to form the optic nerve
- These fibres are unmyelinated, but become myelinated as they leave the eye
-
Fovea has the thinnest inner cell layers, allowing this region to be the most sensitive to light, but the macula is also rod-free/there are only cones present in this location
- Fovea lies in the temporal retina, which is lateral to the optic disc
- Other regions of the retina are comparatively thick
- Muller cells are vertically orientated glial cells with end feet forming a layer adjacent to the inner limiting membrane of the retina
- These cells are equivalent to astrocytes, and are important in maintaining the ionic composition of extracellular fluid
What are the different components shown on this diagram of the retina?
- Red square: pigmented epithelium
- Red bracket: choroid layer
- Yellow highlight: rods
- Black outline: cones
- Blue bracket: outer nuclear layer
- Blue square: outer plexiform layer
- Black bracket: inner nuclear layer, containing horizontal, amacrine and bipolar cells
- Black square: Inner plexiform layer
- Red triangle: ganglion cell layer
- Red circle: ganglion cells
What structure does this image show?
The fovea
What are the cells stained in red on this image?
Muller cells
Label this diagram of rod and cone cells.
How many muscles move the eye within the orbit, and what are they innervated by?
- There are 6 oculomotor muscles
- The are innervated by 3 muscles:
- Oculomotor - the largest of the three, supplies all of the eye muscles except for the lateral rectus and superior oblique
- Trochlear - innervates the superior oblique muscle
- Abducens - innervates the lateral rectus muscle
Label this diagram
Label the diagram
What is the ciliary ganglion?
- Innervated by the parasympathetic supply of the oculomotor nerve
- This ganglion controls pupil constriction and the smooth muscle of the ciliary body
- Contraction of the ciliary muscle reduced the diameter of the ring of muscle formed by the ciliary body
- This reduced tension in the lens, causing it to relax and become spherical
What are the different muscles responsible for eye movement?
- Lateral rectus muscle -> Abduction
- Medial rectus muscle -> Adduction
- Superior rectus muscle -> Elevation + Adduction
- Inferior rectus muscle -> Depression + Adduction
- Superior oblique muscle -> Depression (counter-intuitive) + Abduction + Inward rotation
- Inferior oblique muscle -> Elevation (counter-intuitive) + Abduction + External rotation
What are the effects of an abducens nerve lesion?
- The abducens nerve usually causes contraction of the lateral rectus muscle, causing the eye to be abducted/turn outwards
- With a lesion, the eye deviates towards the midline
How can you detect glaucoma?
- The optic nerve disc/head can be viewed using an ophthalmoscope
- Ophthalmascope is a tool that introduces light into the back of the retina to allow visualisation by eye (large light aperture requires the pupil to be dilated using mydriatic drops)
- Compression of the optic nerve onto the lamina cribrosa causes cupping of the optic nerve head
- Nerves fibres in the optic nerve begin to die, causing the cup around the disc to become unstable and appear larger than the disc, since support has been lost
- The optic disc/nerve head appears pale as the small capillaries of the nerve are compressed
Describe the path of the optic nerve and fill in the diagram
- The optic nerves pass first to the ventral diencephalon, where they form the optic chiasm
- Retinal ganglion cells in the temporal half of each retina (which, due to the reversing lens, views the nasal part of each visual field) send their information to the same part of the brain (ipsilateral)
- Retinal ganglion cells on the nasal retina cross to the contralateral side
- This partial decussation results in each side of the brain receiving information from the opposite side of our visual fields
- After the optic chiasm, the nerve fibres become the optic tracts
- The optical tract runs to the lateral geniculate body (LGN)
- This is the main visual relay in the thalamus
- From the LGN, fibres sweep rostrally and laterally to the retrolentiform part of the internal capsule, passing around the concave surface of the ventricle where the body runs into the inferior horn
- Fibres spread into a broad sheet, superior and lateral to the lateral ventricle, and the inferior fibres sweep far rostrally on the superior aspect of the inferior horn of the lateral ventricle
- This causes upper visual field neurons to project onto the lower banks of the calcarine fissure, and the lower visual field to project onto the upper banks
- The macula has a relatively large representation on the cortex and projects to the caudal pole of the occipital cortex
Describe the structure of the lateral geniculate nucleus (LGN)
- Laminated structure
- Outer two cell layers are made up of large cells
- Magnocellular
- Inner four cell layers are made up of smaller cells
- Parvocellular
What is papilloedema?
[IMPORTANT]
- Swelling of the optic nerve disc due to raised intracranial pressure
- The dural anatomy of the optic nerve shows that it is continuous with the subarachnoid space, therefore subject to changes in intracranial pressure (ICP)
- If the ICP increases, it is transmitted to the optic disc, causing it to bulge outwards
Image shows unilateral papilloedema (healthy retina on the left)
What is the vestibular system and what is its function?
- It is part of the inner ear, continuous with the cochlea.
- It is responsible for detecting motion, head position and spatial orientation.
Describe the position of the vestibular system.
It is within the temporal bone and it lies adjacent to the cochlea.
What are the different parts of the vestibular system? What is the function of each?
[IMPORTANT]
- 3 semi-circular canals -> Each involved in detecting the angular acceleration of the head in a different plane (rotation)
- Utricle -> Detects acceleration in the horizontal plane + head position
- Saccule -> Detects acceleration in the vertical plane + head position
Draw the structure of the vestibular system with correct orientation with respect to the head.
What do the semi-circular canals detect and how?
- They detect head acceleration in all 3 planes of rotation
- In the ampula (swelling) of each canal, there are hair cells which function like in the cochlea
- Stereocilia in the hair cells project into the cupula (somewhat analogous to the tectorial membrane in the cochlea)
- When the head accelerates in one direction, the sterocilia are deflected, activating the hair cells
What do the utricle and saccule detect and how?
- Utricle -> Detects acceleration in the horizontal plane + head position
- Saccule -> Detects accleration in the vertical plane + head position
- Movement causes the otolith (calcium carbonate) layer to move, displacing the fluid below it
- This deflects the sterocilia, activating the hair cells
Where are the cell bodies of neurons in the vestibular nerve found?
In the vestibular ganglion.
Where are the vestibular nuclei found and what are they called?
They are at the junction between the pons and medulla:
- Medial and lateral
- Superior
- Inferior (descending)
What are the two hair cell types in the vestibular system?
[EXTRA?]
- Type I hair cells have a large afferent calyx surrounding their base
- Type II hair cells do not
What is the name for the structure that is deflected during angular movement of the head? How does it lead to detection of movement?
- The cupula
- It is a structure analogous to the tectorial membrane, but in the ampulla of the semi-circular canals
- Hair cells are embedded within the cupula
- When the head rotates, the endolymph lags behind and drags the cupula within it, which deflects the hair cells that are embedded in the base
Describe the concept of the time constant of the vestibular response.
- The hair cells in the semicircular canals are only activated upon acceleration, not stationary movement or constant speed
- Thus, the graph shows that there is a spike in firing rate when the head accelerates
- There is a time constant for the decay of this spike once the head reaches a constant velocity (about 6 seconds)
- Note how the spikes are in opposite directions on each side of the head
Draw and name the 3 axes of head rotation that the semicircular canals detect.
Describe the mapping of afferent inputs from the vestibular system onto the vestibular nuclei.
- Semi-circular canal afferents terminate mainly in the superior and medial nuclei
- Afferents from the utricle and saccule terminate mainly in the inferior and medial nucleus
- The lateral nucleus receives afferents from both
Draw the anatomy of the vestibulo-ocular reflex.
Note that this is for horizontal head rotation. Other axes of rotation will involve different nuclei and nerves.
Describe how inhibitory connections contribute to the vestibulo-ocular reflex.
What are the main vestibular system output pathways?
The vestibular system outputs to the vestibular nuclei, which then output to:
- Vestibulo-ocular reflex pathways
- Vestibulocerbellum (also directly)
- Thalamus, then to cortex
- Spinal cord
How can you remember which way round the utricle and saccule are?
- The saccule sounds like “sack”, which hangs vertically.
- Therefore it detects acceleration in the vertical plane, while the utricle detects acceleration in the horizontal plane.
What are the hair cells in the utricle and saccule embedded in?
The otolithic membrane that surrounds the otoliths (a.k.a. otoconia).
Compare the sensory apparatus in the semicircular canals and utricle/saccule.
Aside from detecting linear acceleration, what else do the otolith organs (utricle and saccule) detect and how?
[IMPORTANT]
- They can also detect head position (e.g. tilt of the head)
- This is because when the head is tilted, the otolith moves, disrupting the stereocilia on hair cells
Describe the distribution and arrangement of hair cells in the utricle and saccule.
[EXTRA?]
- Both the utricle and saccule are zoned with central specialisations “strioli” which have type I hair cells whereas the extra striolar region has type II hair cells.
- The hair cells are arranged in complementary sets on each side of the stiola, with each set arranged along a certain direction
- The cells are mirrored across the striola, such that a movement will excite cells on one side and inhibit those on the other side
- This means that the cells can respond to changes in motion in any given direction
How do the utricle and saccule detect head position (e.g. head tilt)?
- In head tilt, gravity causes displacement of the otolithic membrane
- This leads to firing of the hair cells
What is a common cause of vertigo and how is it treated?
- Benign paroxysmal positional vertigo (BPPV)
- Pieces of otolithic membrane break off and fall into semicircular canal displacing fluid. Common in elderly.
- Benign paroxysmal positional vertigo is commonly treated by allowing gravity to move the dislodged otoconia from the canals
What is Meniere’s disease?
[EXTRA?]
What two systems are integrated in order to maintain balance and posture?
Vestibular and proprioceptive -> These input to the vestibular nuclei, which control muscles via the vestibulo-spinal tracts.
Summarise the roles of the medial and lateral vestibulospinal tracts.
[IMPORTANT]
- Lateral vestibulospinal tract
- Descends to lumbar regions to influence limb extensors involved in balance.
- Medial vestibulospinal tract
- Projects bilaterally to the cervical spinal cord to mediate the vestibulocollic reflex (a neural reflex that activates neck muscles when head motion is sensed by the vestibular organs in the inner ear).
How is proprioception involved in the control of posture?
- Neck muscle proprioceptors send signals to vestibular nuclei.
- Here the information integrates with the information from the vestibular systems.
- The vestibulospinal tracts then project downwards and control posture via the muscles.
Describe an experiment that demonstrates adaptive learning in posture.
[EXTRA]
- The subject is made to stand on a platform that moves forwards or backwards, causing the person to lean over
- They must compensate for this or they fall over
- Over multiple trials, the compensatory response becomes faster and stronger, demonstrating adaptive learning
Give an example of postural reflexes due to pressure on the feet.
[EXTRA?]
When there are shifts in the centre of gravity, such that there is more pressure on one of the sets of limbs, those limbs detect the extra pressure and the response is to extend those limbs.
Give an example of vestibulospinal reflexes.
[IMPORTANT]
- When the head is tilted downwards/upwards, it is detected by the vestibular system
- If the head is tilted downwards, there is reflex extension of the forelimbs and retraction of the hind limbs
- This is mediated by the vestibular nuclei outputting to the vestibulospinal tracts that affect muscles
Compare when the tonic neck reflex and vestibulospinal reflex are used.
Give some experimental evidence for the influence of the visual system on the vestibular system.
[EXTRA]
When a person is sat in a fixed position in a room that is rotating, they may feel as if they are spinning even though they are not.
How do these sound waves appear on a spectogram?
How does this complex waveform appear on a spectogram?
Describe how speech is produced.
[IMPORTANT]
- Speech is produced by exhaling air from the lungs through the trachea into the larynx
- The larynx consists of the glottis (an opening) and the vocal folds (two flaps of muscle)
- During voiced (nonwhispered) speech, the vocal folds are stretched over the glottis. Air pressure from the lungs causes them to vibrate rapidly, producing a sound.
- As we make our vocal folds more tense, they vibrate faster, producing a higher perceived pitch.
- Above the larynx there are several chambers: the throat, the nasal /oral cavity, and the lips. Each chamber allows some frequencies to flow through while filtering others.
- Speech production involves altering the shape and size of these chambers (e.g. by moving the tongue and lips), to alter the resonant frequencies (formants) produced, in order to produce different speech sounds (phonemes).
What is the fundamental frequency and what are harmonics? What determines each?
- The fundamental frequency is the lowest frequency heard in the complex sound that is speech
- It determines the pitch at which we perceive the sound
- The harmonics are frequencies that are multiples of the fundamental frequency
- When the vocal cords are made more tense, they vibrate faster, producing a higher fundamental frequency and thus a higher perceived pitch. The harmonics are still multiples of the fundamental frequency.
Describe how speech sounds change from the lungs to the mouth.
What is a vowel and how is it detected?
- A vowel is a relatively long-lasting, unchanging speech sound
- The oral tract is kept mostly open from the glottis to the lips and there are segments of time during which the articulators do not move
- Each vowel can be detected by peaks of energy called “formant” frequencies (F1, F2, F3, etc).
- The first 2 formant frequencies are usually sufficient to characterize each vowel. The rest are useful for identifying a speaker’s voice.
How do formant frequencies of vowels appear on a spectogram?
What are consonants and what differentiates different consonants?
- Consonants are formed by blocking passage of air, either partially or completely.
- The constonants are in pairs, with each pair distinguished by the place of articulation and manner of articulation.
- In each pair, one of the phonemes is unvoiced and one is voiced
How can different consonants be told apart by the listener?
For stop consonants (b-p, d-t, and g-k):
- Place of articulation -> This produces different formant frequencies (similar to vowel perception - see flashcard), which must be learned.
- Voice Onset Time (VOT) -> The duration of the period of silence between the stop consonant and the proceeding vowel (e.g. the gap between the d and a in “da” is shorter than the gap between t and a in “ta”). This allows two sounds within a phoneme pair to be differentiated.
Give some experimental evidence for the learning of consonant formants.
[EXTRA]
(Kuhl, 2006):
- The sounds r and l in English can be distinguished by their formant frequencies resulting from their place of articulation
- Over time, the ability of English infants to discriminate the two sounds increases, while the ability of Japanese infants to do so decreases
- This shows the learning of consonant formant frequencies
Where is Broca’s area?
Left inferior frontal gyrus (Brodmann areas 44 and 45) of left cerebral hemisphere
Where is Wernicke’s area?
Posterior part of superior temporal gyrus (Brodmann area 22) of left cerebral hemisphere
Draw the position of Wernicke and Broca’s areas and what the consequence of their lesion is.
Describe a model of the of the language systems in the brain (derived from observations of patients with aphasia).
Geschwind model (1960s) explains that when speaking a heard word:
- Auditory cortex passes auditory information to Wernicke’s cortex
- Wernicke’s cortex accesses the meaning of the word
- Wernicke’s area communicates with Broca’s area via the arcuate fasciculus
- Broca’s area stores motor information associated with that word and outputs to the motor cortex
- This leads to the speech of that word
What is conduction aphasia?
- It is an aphasia due to lesions in the arcuate fasciculus (connecting Wernicke’s and Broca’s areas).
- More recently this idea has been challenged and it is thought that it could also be due to lesions of other areas such as the left superior temporal gyrus/left supramarginal gyrus.
- The superior temporal gyrus/left supramarginal gyrus are involved in phonological working memory (processes that underlie the short-term maintenance of language sounds for processes like memory during a sentence), while the arcuate fasciculus is involved in integrating the auditory and motor systems.
- These lesions result in an inability to repeat things exactly (e.g. “the football player celebrated” becomes “the sportsman cheered”)
- Also show impaired naming, especially phonemic paraphasis (e.g. they may say “wife” instead of “knife”)
- Speech is fluent and grammatical, and auditory comprehension normal -> Since Wernicke’s and Broca’s areas are mostly undamaged
Give some experimental evidence for the lesion that causes conduction aphasia.
[EXTRA]
(Buchsbaum, 2011):
- Mapped the areas of lesion in patients with conduction aphasia and found the most common areas of overlap
- Mapped the active areas in healthy patients performing a phonological working memory task
- The area that was most overlapping between the two groups was determined to be Area Spt (which is involved in phonological working memory)
