U14W2: the eye

Question 1

How does the eye transduce visual information to the brain?

Answer 1

Optic circuitry - happy.

Question 2

How do photorecepotrs turn light changes into potentials?

Answer 2

happy

Question 3

How does photoreceptors activity affect the activity of bipolar cells?

Answer 3

Increased PR activity in high light = hyperpolarisations = less glutamate release
1. Glutamate has an inhibitory affect at MGlur6 receptors on ‘on pathway’ bipolar cell - reduced inhibition increases activity down the on bipolar cell
2. Glutamate has an excitatory affect at AMPA receptors on ‘off pathway’ bipolar cell. Reduced activation decreases activity down the off pathway bipolar cell.

Question 4

How does the activity of photoreceptors affect the activity of horizontal cells?

Answer 4

light = hyperpolarised PR = decreased glutamate
Glutmater norm activates horizontal cells - reduced activation of horizontal cells - horizontal cells have decreased activity - reduced inhibition from horizontal cells on neighbouring PR - neighbouring PR depolarises - perceived dark - inc contrast in the image.

Question 5

What is the function of CN2 the optic nerve?

Answer 5

Is a specialised visceral afferent
Sense of vision - visual acuity, colour vision and afferent limb of pupillary reflexes.

Question 6

What is the function of CN3 - the oculomotor nerve?

Answer 6

General somatic efferent - extraocular muscles
Efferent limb of the pupillary constriction reflex
General visceral efferent - Efferent limb of the lens accommodation reflex via contraction of the ciliary muscles.

Question 7

What is the function of CNIV trochlear?

Answer 7

General somatic efferent - SO4 - motor innervation to superior oblique (an extraocular eye muscle)

Question 8

What is the function of CN6 abducens?

Answer 8

General somatic efferent - for contraction of lateral rectus (one of the extraocular muscles of the eye) fo eye abduction.

Question 9

How does RAPD (relevant afferent pupillar defect) test work?

Answer 9

Identifies damage to the afferent pathway of the pupillary reflex e,g the optic nerve.
On the side of the lesion will only show consensual contraction( as efferent pathway working) but will dilate/unconstrict when light is shone directly into the damaged eye (as the afferent pathway is not working).
Therefore the lack of light causing dilation in the non-affected eye, is stronger than the inactive constrictor reflex to light in the affected eye.

Question 10

What does delayed evoked potential mean?

Answer 10

Evoked potential tests measure the electrical activity of area in your brain and spinal cord in response to stimulation. Often tested as response to visual auditory and electrical stimuli with elctodes position on the brain
Delayed = Slowed transmission of nerve signals, indicates damage to the nerve pathway, sometimes identifiable before patients start reporting symptoms.

Question 11

What do the lumbar puncture results indicate in MS?

Answer 11

Analyses a sampe of CSF.
MS CSF will have - elevated wbc (autoimmune underpinning), neurofilaments and myelin basic protein (from myelin degradation) and oligoclonal bands (immunoglobulins) found in a protein gel.

Question 12

What does an MRI show in a MS patient?**

Answer 12

Lesions (inflammation and odema) are best visible on a T2 weighted flair - appear as white
Can differentiate between old and new lesions using a T1 weighted MRI with gadolinium contrast, where older (non-enhancing) lesions will appear more black and newer (enhancing) lesions as a greyer colour as highlights inflammation

Question 13

What is the use of beta interferon in the treatment of MS?**

Answer 13

Clinical: is a disease-modifying therapy for relapsing MS and in some cases used in secondary progressive MS treatment. Adminstered as an injection.
Is am immunomodulatory agent
Chem: recombinant human interferon
Pharm: Agonist at type 1 interferon receptors IFNAR1/2c, are tyrosine kinases, activation immunomodulatory nad antiviral responses via direct effect on gene expression.
Physio: suppresses inflammatory responses, including T cell activation and secretion of pro-inflammatory cytokines (shifts to anti-inflammatory cytokine phenotype).
Also encourages differentiation of neural stem cells to oligodendrocytes to repair damaged nerve cells.

Question 14

What is the mechanism of action of methylprednisolone in MS?

Answer 14

Class - corticostreroid
Chem - methyl - enables injectable form and more potent than prednisolone
Pharm - glucocorticoid receptors Nuclear receptor agonist, hetero, sheet shock, transolcation, GRE, transcription factors, transactivation and transrepression.
Phyio - Inc Annexin A1 - inhibits PLA2, reduced prostaglandins, reduce TNFalpha, reduce COX, reduce pro-inflammoatry signals, increase INFy and IL-10 anti-inflammatory signals, Change to anti-inflammatory phenotype
Clinical - helps to reduce inflammation and speed up recovery during a relapse or exacerbation of MS symptoms. Can be given orally or IV.

Question 15

What is a certificate of visual impairement?

Answer 15

A formal document completes by an consultant ophthalmologist to certify someone as either slightly impaired (partially sighted) or severely sight impaired (blind).
Based on personal visual acuity and field of vision.
This enables the individual to access support services, financial benefits and education resources including SEN in children.
Can now register with local social services, legal standing to access rehabilitation, mobility training and adaptation funding.

Question 16

Why does beta interferon lose efficacy over time?

Answer 16

Sometimes whilst on treatment patients have a felt ineffectiveness of the drug when they experience worsening symptoms.
Thought to be due to production of interferon- neutralisng antibodies during therapy. Found in up to 40% of patients.

Question 17

How does a Snellen Chart work?

Answer 17

Rows of letters getting gradually smaller, sizing of letters and the distance you are standing from the chart (norm use 6m chart and mirror) relates to your visual acuity
Compared to the average person can see at the same distance = 6/6 vision.
Identify deficits in vision is only able to see shorter distance sizing what other people can see at six meters for example 6/16.

Question 18

How does being visually impaired affect a persons life?

Answer 18

Socially isolating
Stigma
Employment and education opportunities
Financial hardship - needs adaptations and support equipment.
Alters sense of identity.
Safety - alarms, cars, target for abuse.

Question 19

What is the prevalence of MS?

Answer 19

2.3 million people in the world are thought to be living with MS.
Affects around 100,000 adults in the UK.
7,000 newly diagnosed a year
This equates to 1 in 500 people

Question 20

What are some common refractive errors in the eye?

Answer 20

Short-sighted = myopic eye - refraction causes the focus point in front of the retina - blurred image on retina for long distance
Long sighted = hypermetropic eye - refraction error means focus point is behind the retina - blurred image on retina for short distance

Emmetropic eye - point focus on fovea - no need for corrective lenses.

Question 21

What are the different types of corrective lenses for refraction disorders of the eye?

Answer 21

Myopic - or divergent (concaving shape) contact lenses - correct short-sighted vision by raising focal length of light before enters eye
Hypermetropic - or convergent lenses (convex shape) - correct long sighted vision - bling light together before it enters the eye.

Question 22

What is often the problem underpinning diplopia?
What is diplopia?

Answer 22

Eyes are not in aligment causes object we are looking at to fall in the fovea in one eye but an extrafoveal location in the other eye.
Alters the degree of overlap between visual fields.
Results in two separates images being perceived.
Often neurological in origin (confirm by disappearance in monocular vision), causes defects in muscle movement.
Most common is ocular misalignment due to extraocular muscle dysfunction.

Question 23

Where are the different nuclei for oculomotor, trochlear and abducens nerve found?

Answer 23

Oculomotor - midbrain
Trochlear - caudal midbrain
Abducens - pontomedullar junction

Question 24

What is meant by the gaze centres of the brain?

Answer 24

The horizontal and vertical gaze centre are both part of the reticular formation.
They coo-ordinate activity of III,IV and VI to create smooth eye movements.

Question 25

What controls the gaze centres of the brain?

Answer 25

The superior colliculus (visual reactions/reflexes)
The frontal eye fields (conscious control over eye movement)

Question 26

What is the circuitry underpinning the vertical gaze centre?**

Answer 26

Recieves input from the FEF and the superior colliculi
Vertical gaze centre in located in the midbrain reticular formation.
Projects to the oculomotor nuclei and the the trochlear nuclei, bilateral communications link these nuclei via the medial longitudinal fasiculus.
Inhibit one and excite the other - performed unified vertical gaze movements.

Question 27

What is the ciruitry underpinning the horizontal gaze centre?

Answer 27

Question 28

What is meant by internuclear opthalmoplegia?

Answer 28

An eye movement disorders that impacts the ability to look to the side with both eyes at the same time.
Develops when cranial nerves fibres that work together to pivot both eyes horizontally become damaged, can affect one or both eyes.
Lack of coordination of eye movements can result in double vision.

Question 29

What are the different functions of the superior and inferior divisions of the oculomotor nerve?

Answer 29

Superior division - innervates levator palpebrae superioris and superior rectus muscle
Inferior division - medial rectus, inferior rectus and inferior oblique.

Question 30

Describe the normal appearance of the fundus during fundoscopy.

Answer 30

Optic disk - made from optic nerve fibres
With an inner optic cup - hollow centre where central retinal artery and vein pass through
Arteries - lighter in appearance as oxygenated
Macula - centre for light focus, fovea, darker depression within the macula.

Question 31

What is meant by demyelination and how does it affect the nerve?

Answer 31

Loss of myelin (protein and fattu subtances) with relative preservation of axons (not a failure to form myelin)
Loss of electrical insulation - slows nerve impulses down or even stop them.
Normally occurs when oligodendrocytes are damaged.
Can occur all over the PNS and the CNS.

Question 32

Where is the lesion normally located when there is a non-specific reduction of the visual field?

Answer 32

The optic nerve
A typical sign of pattern loss would indicate any lesion at or after the chiasm.

Question 33

What are the different neuroglia and what are their functions?

Answer 33

Support for neurons.
Astrocytes - trophic support for neurons (e.g regulate blood flow, neurotransmitter release)
Oligodendrocytes - myelination of axons (implicated in MS)
Microglia - immune cells of the brain (implicated in dementia)

Question 34

What is myelin basic protein?

Answer 34

Responsible for adhesion of cytosolic surfaces of multilater compact myeline, found in the intracellular domain.

Question 35

What is the most popular theory underpinning the pathology of MS?

Answer 35

Autoimmune disorders - molecular mimicry to EBV
Majority MS patients have had EBV, however EBV is relatively common only small proportion develop MS afterwards.
EBV antigens are very similar to antigens of glialCAM, a cell adhesion molecule expressed on oligodendrocytes.
When activated B cells undergo hypermutations can end up producing autoimmune antibodies.

Question 36

How can we measure for visual evoked potentials for visual information?
How do we interpret the results?

Answer 36

Use an EEG with electrodes focused on the occipital lobe.
Compare patient signals to normal average reference value
More accurate to also compare two hemispheres are individual variation occurs.
Graphs gives time along x axis, look for delay in activity between reference/control and result.

Question 37

Suggest why relating to the visual system a person may struggle to recognise faces (prosopagnosia)

Answer 37

Defect in the ventral stream projection from the occipital to the temporal lobe
Part of visual association areas.
Often accompanied by left sided visual field defects as right sided hemisphere show stronger activity in facial recognition tasks.

Question 38

Why might an MS patient have difficulty percieving their surroundings?

Answer 38

Difficult with visual association cortex
Struggle for top down processing due to degeneration in association areas, mainly the prefrontal cortex.