PBL Topic 3 Case 3 Flashcards
What occurs when a light ray travelling in a beam strikes an interface that is perpendicular to the beam?
- Same course
- Decrease in velocity
- Shorter wavelength
What occurs when a light ray travelling in a beam strikes an angulated surface?
- Refraction
- Light rays bend
What happens when parallel light rays enter a convex lens?
- Convergence of rays at the focal point
What happens when parallel light rays enter a concave lens?
- Divergence of rays
Identify the four refractive interfaces in the lens system of the eye.
- Air and anterior surface of cornea
- Posterior surface of cornea and aqueous humor
- Aqueous humor and anterior surface of lens
- Posterior surface and vitreous humor
What is meant by the term diopter?
- A unit of refractive power
- Which is reciprocal to the focal length of a given lens
What is the total refractive power of the eye?
- 59 diopters
What is the accommodation reflex?
- Voluntary increase in refractive power
- To focus on a nearby object,
- The eyes converge, the lens becomes more convex and the pupils constrict
Outline the mechanism of accommodation
- Efferent impulses pass in the oculomotor nerve to the orbit
- There they synapse in the ciliary ganglion which give rise to small ciliary nerves
- These nerves stimulate contraction of ciliary muscle, which relaxes ligaments of the lens
- Which enables the lens to assume a more convex shape.
What is presbyopia?
- With age the lens grows larger and thicker and less elastic
- The ability of the lens to change shape decreases, and the accommodation reflex decreases
What is am emmetropic eye?
- Parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed
What is hyperopia?
- Farsightedness due to a short eyeball
- Parallel light rays are not bent sufficiently by the relaxed lens system to come to focus
- So ciliary must contract to increase the strength of the lens
What is myopia?
- Shortsightedness due to a long eyeball
- Parallel light rays from distant objects are not focused in front of the retina when the ciliary muscle is completely replaced
What is astigmatism?
- Refractive error in which curvature of the cornea in one plane of the eye is too great
- Such that light rays do not come to a focal point.
Why can the accommodation power of the eye not compensate for astigmatism?
- The curvature of the lens changes approximately equally in both planes
- Each of the two planes requires a different degree of accommodation which is not possible
What is visual acuity?
- The ability of the eye to discriminate between two points of light
What is the purpose of intraocular fluid?
- To maintain sufficient pressure in the eyeball to keep is distended
Where is intraocular fluid located?
- Aqueous humor, located in front of the eyes
- Vitreous humor, located between posterior surface of the lens and retina.
Outline how aqueous humor is formed and reabsorbed
- It is secreted by the epithelia of the ciliary processes
- Diffuses of sodium and chloride ions into the spaces between epithelial cells
- Which causes osmosis of water into the same area
- Flows through the pupil into the anterior chamber of the eye, lens, corona and iris
Outline how aqueous humor is reabsorbed?
- Passes through trabeculae before emptying into the canal of Schlemm
- Which empties into the extra ocular veins
What is the average intraocular pressure?
- 15 mmHg
How can intraocular pressure be measured?
- Tonometry
- Small force is applied to a plunger
- Causing cornea to be displaced inward
- Degree of displacement is calibrated in terms of intraocular pressure
Outline the pathology of glaucoma?
- Intraocular pressure rises above 60 mmHg
- Which puts pressure on the axons of the optic nerve
- Resulting in lack of nutrition of the fibres, resulting in death of the fibres
Identify three treatments of glaucoma?
- Drugs that reduce secretion of intraocular fluid
- Drugs that increase absorption of intraocular fluid
- Surgical procedure include increasing spaces of trabeculae
What is the retina?
- Light-sensitive portion of the eye
- Which contains photoreceptors (e.g. rods and cones)
Identify the eight layers of the retina
- Pigmented layer (vitamin A and melanin)
- Layer of rods and cones
- Outer nuclear layer (cell bodies of rods and cones)
- Outer plexiform layer (horizontal cells)
- Inner nuclear layer (bipolar cells)
- Inner plexiform layer (amacrine cells)
- Ganglionic layer
- Layer of optic nerve fibres
- Inner limiting membrane
What is the role of melanin?
- Melanin, which prevents light reflection throughout the globe of the eyeball
What is the fovea?
- The small area in the centre of the retina
- Composed of long thin cones
Describe the structure of the photoreceptors
- Outer segment is composed of photochemicals (either rhodopsin or colour pigment)
- Discs, which are unfolded shelves of cell membrane
- Inner segment containing cytoplasm and cytoplasmic organelles e.g. mitochondria
- Synaptic body that connects to horizontal and bipolar cells
Identify the two components of rhodpsin
- Scotopsin
- 11-cis retinal
Identify the pathway in which rhodopsin is activated
- Light energy is absorbed by rhodopsin
- 11-cis retinal is converted to 11-trans retinal
- 11-trans retinal is unable to bind with 11-cis retinal
- So rhodopsin decomposes into a number of intermediate products e.g. bathorhodopsin, lumirhodopsin, metarhodopsin I
- And finally to metarhodopsin II (activated rhodopsin)
Identify two ways in which rhodopsin is reformed
[A] From 11-trans retinal
[B] From vitamin A
- [A] 11-trans retinal is converted to 11-cis retinal by retinal isomerase
- [B] 11-trans retinal is converted to Vitamin A, which is then converted to 11-cis retinol, which is converted to 11-cis retinal by retinal isomerase
Why does night blindness occur in people with vitamin A deficiency?
- Less 11-cis retinal formed from Vitamin A
- So less rhodopsin is formed (light-sensitive chemical)
- So light-sensitivity in the dark is reduced
Which enzyme inactivates metarhodopsin II?
- Rhodopsin kinase
Identify the two components of the cone photochemicals?
- Photopsin
- Retinal
Name the three colour pigments present in each cone.
- Blue
- Green
- Red
How is the nervous system able to interpret different colours other than blue red and green?
- Red, green and blue cones are stimulated in different ratios depending on the colour
What is meant by the term protanope?
- A person with loss of red cones
What is meant by the term deuteranope?
- A person with loss of green cones
What is light adaptation?
- When light energy is absorbed, the light-sensitive chemical rhodopsin is reduced to scotopsin and 11-trans retinal
- So sensitivity to light is reduced
What is dark adaptation?
- When there is less light energy acting on rods, 11-cis retinol is able to bind with scotopsin to form rhodopsin, a light-sensitive chemical
- So sensitivity to light is increased
Identify two other mechanisms for light and dark adaptation that do not involve rods or cones
- Change in pupillary size, reducing the amount of light through the pupillary opening
- Neural adaptation in which signals transmitted by bipolar, horizontal, amacrine and ganglion cells are intense when light intensity increases
Horizontal cells:
- In which layer are they located?
- What do they transmit?
- What type of signals do they transmit?
- Located in outer plexiform layer
- Which transmit signals from photoreceptors to bipolar cells
- Inhibitory signals
Bipolar cells:
- In which layer are they located?
- What do they transmit?
- What type of signals do they transmit?
- Between outer plexiform and inner plexiform layer
- Which transmit signals from rods, cones and horizontal cells to ganglion and amacrine cells
- Both excitatory and inhibitory signals
Amacrine cells:
- In which layer are they located?
- What do they transmit?
- Inner plexiform layer
- Which transmit signals from bipolar cells to ganglion cells
Ganglion cells:
- In which layer are they located?
- What do they transmit?
- Ganglion cell layer
- Which transmit output signals from retina through the optic nerve to the brain
Identify the three type of ganglion cells
- W cells which transmit rod vision
- X cells which transmit visual image and colour
- Y cells which transmit instantaneous changes in the visual image
Identify the neurotransmitters involved at each synapse in the retina
- Photoreceptors release glutamate
- Amacrine cells release GABa, glycine, ACh and dopamine
- Bipolar and horizontal cells, unclear, but inhibitory neurotransmitters
In which retinal cell type are signals transmitted by means of action potential?
- Ganglionic cells
What is the importance of electrical conduction in all other cell types?
- Allows for graded conduction
- Meaning the strength of hyperpolarisation output signals is directly related to intensity of illumination
- It is NOT an all or one principle like an action potential
Why is electrical conduction not used in the ganglion cells?
- Due to long distance
How are colour signals transmitted by ganglion cells?
- One colour type while excite a specific ganglion cell by the direct excitatory route through a depolarising bipolar cell
- While another will inhibit it by the indirect inhibitory route through a hyperpolarising bipolar cell
Outline the pathway taken by optic nerve fibres at the optic chiasm?
- Nerve fibres from the nasal halves cross to the opposite side where they join temporal fibres of the opposite retina
- To form the optic tract
Outline the nerve pathway from the optic tract to the primary visual cortex
- Synapse in dorsal lateral geniculate nucleus of the thalamus
- Geniculocalcarine fibres pass through geniculocalcarine tract to the primary visual cortex
What is the role of those nerve fibres that pass to the suprachiasmatic nucleus of the hypothalamus?
- Control circadian rhythms that synchronise changes of the body with night and day
What is the role of those nerve fibres that pass to the pretectal nuclei of the midbrain?
- Pupillary light reflex
What is the role of those nerve fibres that pass to the superior colliculus of the midbrain?
- Control rapid directional movements of the eyes
What is the role of those nerve fibres that pass to ventral lateral geniculate nucleus of the thalamus?
- Control some of the body’s behavioural functions
Identify the different layers of the dorsal lateral geniculate nucleus and their respective roles.
- Layers 2,3 and 5 receive signals from the lateral half of the ipsilateral retina
- Layers 1,4 and 6 receive signals from the medial half of the opposite eye.
Outline the ‘gate’ function of the dorsal lateral geniculate nucleus
- DLGN receives corticofugal fibres returning from primary visual cortex and from reticular areas of mesencephalon
- These are inhibitory signals that can turn off transmission through the DLGN
What are the magnocellular layers of the DLGN?
- Layers 1 and 2
- Receiving input from Y cells (instantaneous changes in visual image)
What are the parvocellular layers of the DLGN?
- Layers 3, 4, 5 and 6
- Receiving input from X cells (visual image and colour)
Where is the primary visual cortex located?
- Calcarine fissure
- Extending from occipital pole on the medial aspect of each occipital cortex
Where are the secondary visual areas located and what is their role?
- Anterior, lateral, superior and inferior to the primary visual cortex
- Responsible for analysis of visual meanings
How many layers are there in the primary visual cortex?
- 6
In which layer do geniculocalcarine fibres terminate?
- 4
In which layer do rapidly conducted signals from Y-cells terminate?
- 4c alpha
In which layer do signals from X- cells terminate?
- 4a
- 4c beta
Identify the two fields of vision
- Nasal field seen to the nasal side
- Temporal field seen to the temporal side
How is visual field assessed?
- Perimetry
- Patient closes one eye while looking at a central spot with the other eye
- Central spot moves and the patient indicates when they can / cannot see the spot
Where is the blind spot?
- 15 degrees lateral to the central point of vision
What causes the blind spot?
- Lack of rods and cones over the optic disc
What is scotoma?
- Blind spots in other portions of the visual field other than over the optic disc
- Caused by damage to the optic nerve e.g. gluacoma
Identify the three pairs of muscles responsible for eye movements
- Medial and lateral recti, which move the eyes from side to side
- Superior and inferior recti, which move the eyes up and down
- Superior and inferior obliques which contract to rotate the eyeballs to keep the visual fields in the upright position
What is meant by reciprocal innervation?
- Muscles of a pair are innervated such that one muscle contracts when the other relaxes
What is the voluntary fixation mechanism?
Where in the brain is this mechanism controlled from?
- Allows person to moves eyes voluntarily to find the object to fixate on
- Premotor cortical areas of the frontal lobes
What is the involuntary fixation mechanism?
Where in the brain is this mechanism controlled from?
- Holds eyes firmly on the object once it has been found
- Secondary visual areas in the occipital lobe
What is meant by opticokinetic movement?
- Persons eyes jump from one point to the next (saccades)
What is meant by pursuit movement?
- Persons eyes fixate on a moving object
What neuronal mechanisms are involved in the turning of the head towards a sudden visual disturbance?
- Superior colliculus sends oculomotor signals to brain stem nuclei of oculomotor nerves
What is stereopsis?
- The perception of depth produced by the reception in the brain of visual stimuli from both eyes in combination
What is strabismus?
- Lack of fusion of the eyes in one or more visual coordinates
Identify three types of stabismus
- Horizontal
- Torsional
- Vertical strabismus?
What is the cause of strabismus?
- Abnormal set of the fusion mechanism of the visual system
- One eye is able to fixate successfully
- The other eye is unsuccessful and the eye becomes abnormally set in another neuronal control pathway
What is a cataract?
- Error in refraction
- Cloudy opaque area in the lens caused by denaturation of proteins within the lens that coagulate
- Most common cause of preventable blindness in the world
Identify three metabolic causes of cataracts
- Diabetes
- Hypocalcaemia
- Wilson’s disease
Identify an inflammatory cause of cataracts
- Uveitis
Identify the treatment of a cataract
- Small incision extracapsular or phacoemulsificaiton extraction
- With insertion of intraocular lens
Identify three risk factors of primary open angle glaucoma
- Age
- Race (Black African)
- Positive family history
- Myopia
Why is timolol used in the treatment of primary open angle glaucoma?
- Reduces aqueous production
Why is latanoprost used in the treatment of primary open angle glaucoma?
- Increases aqueous drainage
Why are carbonic anhydrase inhibitors such as acetazolamide used in the treatment of primary open angle glaucoma?
- Slows the formation of bicarbonate
- Reduces sodium and fluid transport
How do the clinical features of primary open angle glaucoma differ from those of acute angle-closure glaucoma?
- POAG: Gradual, painless loss of peripheral visual field
- AACG: Red painful eye, blurred vision, nausea, vomiting, severe ocular pain
Why is pilocarpine used in the treatment of acute angle-closure glaucoma?
- Constricts pupil to improve aqueous outflow
- Prevents iris adhesion to trabecular meshwork
What is trabeculectomy?
- Making a hole in the periphery of the iris increasing spaces of trabeculae
Outline the pathology of retinal detachment
- Tear in retina
- Fluid collects in potential space between sensory retina and pigment epithelium
What are the clinical features of retinal detachment?
- Visual field loss
- Floaters
- Flashes of light (photopsia)
What is the treatment of retinal detachment?
- Laser surgery to scar edge of retinal tear
- Thereby attaching the retina back to the eye
Outline the pathology of retinal pigmentosa
- Common chronic inherited degenerative disease. Several genes are implicated.
- Constriction of the peripheral vision leading to tunnel vision and progressive loss of night vision.
- Spicule deposits and attenuated retinal vessels.
.
What is the treatment of retinal pigmentosa?
- Vitamin A supplementation may slow progression
- Gene therapy is being investigated
Identify risk factors in age related macular degeneration
- Smoking
- Hypertension
- Hypercholesterolaemia
- UV exposure
Identify two types of AMD
- Non-exudative (dry) with lipofuscin deposits
- Exudative (wet) AMD with subfoveal choroidal neovascularisation
What is the treatment of AMD?
- Vitamins C, E, beta carotene, zinc copper
- Intravitreal injections of anti-vascular endothelial growth factor such as ranibzumab
How are visual fields assessed at bedside?
- Confrontation
- Comparing the examiner’s eye and patient’s fields, one eye at a time, quadrant by quadrant
Identify two detailed tests of visual fields?
- Goldmann (manual) perimetry testing
- Humphrey (automated) perimetry testing
What is meant by hemianopia?
- When half of visual field is affected
What is meant by quandrantanopia?
- When a quadrant of a visual field is affected
Lesions of which region produce homonymous field defects?
- Lesions posterior to the optic chiasm
- Indicating involvement of the same part of the visual field in both eyes
- As information from the two visual fields is separated beyond this point
Lesions damaging which fibres cause bitemporal defects?
- Lesions damaging the decussating nasal fibres at the optic chiasm
Mononuclear field loss is caused by a nerve lesion located at [A] and may be due to [B]
- [A] Optic nerve
- [B] Optic neuritis
BItemporal hemianopia is caused by a lesion located at [A] and may be due to [B]
- [A] Optic chiasm
- [B] Pituitary tumours
Incongruous contralateral homonymous hemianopia is caused by a lesion located at [A] and may be due to [B]
- [A] Optic tract
- [B] Tumour, stroke
or inflammatory disease
What type of visual field loss occurs in damage to
[A] Temporal lobe
[B] Parietal lobe
- [A] Contralateral homonymous upper quadrantanopia
- [B] Contralateral homonymous lower quadrantanopia
What is meant by macular sparing and why can damage to the occipital lobe be macular sparing?
- Signals from the macula terminate at the occipital pole,
- Therefore a lesion located anterior to the optic pole can produce macula sparing
- Which means that the centre of vision is spared
Outline process of phototransduction
- Breakdown of rhodopsin to activated metarhodopsin
- Which activated transducin
- Which activates cGMP-phosphodiesterase
- Which breaks down cGMP
- Closure of cationic channels
- Hyperpolarisation which excites cell
What is the effect of phenylephrine on pupillary diameter?
- Mydriasis (dilation)
- Since it is an alpha-1 agonist
What is the effect of tropicamide on pupillary diameter?
- Mydriasis (dilation)
- Since it is an M3 antagonist
What is the effect of pilocarpine on pupillary diameter?
- Miosis (constriction)
- Since it is an M3 agonist
What is miosis?
- Pupillary constriction
Outline the mechanism behind miosis
- Light stimulus excites photoreceptors
- Signal sent via the optic nerve and optic tract to the pretectal nucleus
- Signal is relayed to the Edinger Westphal nucleus
- Efferent impulses pass along parasympathetic fibres of oculomotor nerve
- Synapse in ciliary ganglion
- Short ciliary nerves pass to sphincter pupillae
- Which constricts the pupil
What is mydriasis?
- Pupillary dilation
Outline the mechanism behind mydriasis
- Fibres arise from T1 and T2
- Impulses pass via sympathetic trunk to superior cervical ganglion
- Long ciliary nerves pass to dilator pupillae
- Which dilates the pupil
What is the cause of Horner’s syndrome?
- Interruption to sympathetic nerves in the cervical sympathetic chain
- E.g. tumour
Outline 3 clinical features of Horner’s syndrome
- Ptosis
- Miosis
- Anhydrosis
How is a myopic eye corrected?
- Concave lens
- Which diverges light rays
- Enhanced focus of the image on the retina
How is a hyperopic eye corrected?
- Convex lens
- Which converges light rays
- Enhanced focus of the image on the retina
Identify the two components of the pupillary light reflex
- Direct component mediates constriction of the ipsilateral pupil
- Consensual component mediates constriction of the contralateral pupil
What is amblyopia?
- Reduction in visual acuity
- Due to disruption of the visual image of one eye but not the other in early development
- Causing failure of development of the visual system
At what age does amblyopia develop and before which age must it be detected and treated to prevent blindness?
- Age 2
- Age 8
Why can strabismus cause amblyopia?
- Misalignment of each eye results in different retinal images being sent to the visual cortex
Why does amblyopia not occur in adults? What occurs in adults when a visual image is suppressed?
- The visual cortex pathway are developed
- Diplopia
Which of Brodmann’s area are the secondary visual areas located?
- 18 and 19
Where is the ‘What?’ Pathway located and what is it concerned with?
- Medially in association area
- Colour medially
- Faces in the middle region
- Form laterally
Where is the ‘Where?’ Pathway located and what is it concerned with?
- Laterally in association area
- Movement in contralateral visual hemifield
What is felt stigma?
- A change in self identity that is manifested as shame, guilt and withdrawal
What is enacted stigma?
- Societal reactions that cause actual discriminatory experiences either:
- Directly, treated less favourable
- Indirectly, particularly groups are likely to suffer effects of rules, systems, procedures
Outline the negative feedback circle of stigma
- Disability causes restriction of activities and social roles
- Which causes negative labelling
- Which causes diminished self-esteem and ‘felt stigma’
- Resulting in isolation and withdrawal from social life
- Resulting in lack of confidence and skills
- Which causes further restriction of activities and social roles
What is a virtuous circle in relation to stigma
- Positive challenges to stigma can change attitudes that lead to enacted stigma
- Which in turn reduce felt stigma
- Which itself reduces enacted stigma by changing behaviours such as stigmatisation and withdrawal
Identify five strategies to tackle stigma
- Legislation
- Education
- Language
- Public acknowledgement
- Treatment
Outline the Medical Model of Disability
- Disability caused by health condition or impairment
- By fixing the body, disabled people can participate in society like everyone else
Outline the Social Model of Disability
- Distinguishes between impairment (loss of functioning part of body) and disability (meaning society attaches to impairment)
- Three barriers include environment, attitudes and organisations
What is Activities of Daily Living and how is it measured?
- Assessment of disability
- How an impairment affects everyday activities
- Measured as elf-report and observation
What is sensation?
- The detection of simple properties of stimuli
- Such as brightness, colour and sweetness
What is perception?
- Detection of objects, their locations, movements and backgrounds
What is bottom’s up processing?
- Analysis of stimuli into a set of features
- Which are compared to existing sets in the brain
- Recognition occurs if a match occurs
What is top-down processing?
- Contextual and based on expectancy
- People see what they expect or want to see
Define attention
- Directing and focusing of perception
- May be selective, divided or focused and can be negatively affected by stress or fatigue
