Case 3 Flashcards

Question 1

Q

anatomy of the eye:

pupil
iris
cornea
sclera
extraocular muscles
orbit
conjunctiva
optic nerve

Answer

A

Pupil – this is the opening that allows light to enter the eye and reach the retina; it appears dark because of the light-absorbing pigments in the retina.
Iris – the pupil is surrounded by the iris, whose pigmentation provides the ‘eye colour’. The iris contains two muscles that can vary size of the pupil; one makes it smaller when it contracts, the other makes it larger.
Cornea – this is the glassy transparent external surface of the eye that covers the pupil and the iris.
Sclera – the cornea is continuous with the sclera, the ‘white of the eye’, which forms the tough wall of the eyeball.
Extraocular muscles – these are three pairs of muscles that insert into the sclera. The move the eyeball in the orbit. These muscles normally are not visible because they lie behind the conjunctiva.
Orbit – this is the bony eye socket in the skull, in which the eyeball sits.
Conjunctiva – this is a membrane that folds back from the inside of the eyelids and attachs to the sclera. The extraocular muscles lie behind this.
Optic nerve – this is the nerve responsible for vision. It carries axons from the retina, exits the back of the eye, passes through the orbit (optic foramen), and reaches the base of the brain near the pituitary gland where it decussates (optic chiasma) before reaching the primary visual cortex in the occipital lobe.

Question 2

Q

where do retinal blood vessels originate from?

Answer

A

the optic disc

Question 3

Q

where can the sensation of light not occur?

Answer

A

Sensation of light can’t occur at the optic disk because there are no photoreceptors here, nor can it occur where the large blood vessels exit because the vessels cast shadows on the retina.

Question 4

Q

macula

what is it
what’s it for
how’s it distinguished
what improves quality of central vision

Answer

A

yellow tissue at the centre of the retina, surrounding the fovea
it is for central vision
besides its colour, it’s distinguished by the relative absence of large blood vessels
the retinal blood vessels arc from the optic disc to the macula; this is also the trajectory of the optic nerve fibres from the macula en route to the optic disc
the absence of blood vessels improves the quality of central vision

Question 5

Q

what is the fovea?

Answer

A

dark spot about 2mm in diameter
at centre of retina
thinnest part of retina

Question 6

Q

what is defined as the nasal retina and temporal retina?

Answer

A

from the fovea, anything medial is the nasal retina and anything lateral is temporal retina
- optic disc is in the nasal retina

Question 7

Q

what colour does the retina appear and why?

Answer

A

orange

- due to the choroidal circulation under the photoreceptor layer

Question 8

Q

what brings about refraction of light? what is most responsible for it?

Answer

A

cornea and lens

- cornea is responsible for most of the refraction of light

Question 9

Q

how are light rays refracted by the cornea?

Answer

A

The light rays entering the eye are parallel rays.
The light rays that strike the curved surface of the cornea bend (as they enter the aqueous humor) so that they converge on the back of the eye; those that enter the centre of the eye pass straight to the retina.

Question 10

Q

what is the focal distance?

Answer

A

the distance from the refractive surface to the point where the parallel light rays converge

Question 11

Q

the more curved the cornea, the x the focal distance?

Question 12

Q

what is a dioptre?

Answer

A

the unit of the power of refraction

Question 13

Q

refractive power (dioptres) =

Answer

A

1/focal distance (m)

Question 14

Q

what does the cornea have a refractive power of? what does this mean in terms of focal distance?

Answer

A

42 dioptres

this means that the light rays striking it will be focused about 2.4cm (0.024m) behind it, about the distance from the cornea to the retina

Question 15

Q

what is the refractive power of the lens compared to the cornea?

Answer

A

it’s less

- about 12 dioptres

Question 16

Q

what is accomodation?

Answer

A

the refraction supplied by the lens is adjustable, allowing objects at various distances from the observer to be brought into sharp focus
the additional focusing power is provided by changing the shape of the lens = accommodation

Question 17

Q

describe how accommodation comes about

Answer

A

The ciliary muscle forms a ring around the lens.
For near vision, the ciliary muscle contracts and swells in size, thereby making the area inside the muscle smaller and decreasing the tension in the suspensory ligaments.
The lens becomes rounder and thicker because of its natural elasticity.
This increases the curvature of the lens, and thus its refractive power increases.
For viewing distant objects, relaxation of the ciliary muscle increases the tension in the suspensory ligaments, and the lens is stretched into a flatter shape, and its refractive power is reduced.

Question 18

Q

what does the pupillary light reflex do?

Answer

A

it aids the clarity of images formed on the retina

Question 19

Q

how does narrowing the pupil lead to sharper images? however what happens under conditions of dim illumination?

Answer

A

it reduces both spherical and chromatic aberration
(aberration = failure of rays to converge at one focus due to a defect in lens)
- spherical aberration = occurs due to increased refraction of light rays when they strike a lens
- chromatic aberration = the effect produced by the refraction of different wavelengths of light through slightly different angles, resulting in a failure to focus

reducing size of pupil also increases the depth of field - the distance within which objects are seen without blurring
however, a narrowed pupil also limits the amount of light that reaches the retina, and, under conditions of dim illumination, visual acuity becomes limited by the number of available photons rather than by optical aberrations.

Question 20

Q

what is the visual field?

Answer

A

the total amount of space that can be viewed by the retina when the eye is fixated straight ahead

Question 21

Q

what happens to the image of an object in the visual field?

Answer

A

it’s inverted on the retina

Question 22

Q

what is binocular and uniocular vision?

Answer

A

Binocular visual field is the visual field when both eyes are open.
With only one eye open the field is uniocular and is restricted inwards by the nose.

Question 23

Q

what is the most common visual field loss due to?

Question 24

Q

what is visual acuity? what does it depend on? what can be used to talk about visual acuity?

Answer

A

the ability of the eye to distinguish two nearby points

acuity depends on several factors - mainly on the spacing of photoreceptors in the retina and the precision of the eye’s refraction.

distance across the retina can be described in terms of degrees of visual angle. We can speak of the eye’s ability to resolve points that are separated by a certain number of degrees of visual angle.

Question 25

Q

what test is used to test visual acuity? what is normal?

Answer

A

the Snellen eye chart - tests our ability to discriminate letters and numbers at a viewing distance of 6 metres (20 feet)

normal vision = 6/6

Question 26

Q

describe the development of the retina

Answer

A

During development, the retina forms as an outpocketing of the diencephalon, called the optic vesicle, which undergoes invagination to form the optic cup.

- The inner wall of the optic cup forms the retina.
 - The outer wall gives rise to the retinal pigment epithelium (RPE).

Question 27

Q

what is retinal pigment epithelium (RPE)?

Answer

A

this epithelium is a thin melanin-containing structure that reduces backscattering of light that enters the eye
it also plays a critical role in the maintenance of photoreceptors, renewing photopigments and phagocytosing the photoreceptor discs, whose turnover at a high rate is essential to vision

Question 28

Q

what cells is the retina composed of?

Answer

A

photoreceptor cells (rods and cones)
bipolar cells
ganglion cells
horizontal cells
amacrine cells

Question 29

Q

what are the different layers called (outward to inward)

Answer

A

pigment epithelium
photoreceptor outer segments
outer nuclear layer
outer plexiform layer
inner nuclear layer
inner plexiform layer
ganglion cell layer
nerve fibre layer

Question 30

Q

where are the cell bodies of the photorecpetor cells located?

Answer

A

in the outer nuclear layer

Question 31

Q

where are the cell bodies of the bipolar cells, the horizontal cells and the amacrine cells located?

Answer

A

in the inner nuclear layer

Question 32

Q

where are the cell bodies of the ganglion cells located?

Answer

A

in the ganglion cell layer

Question 33

Q

what’s located in the outer plexiform layer?

Answer

A

the synaptic contacts between the photoreceptor cells, the bipolar cells and the horizontal cells

Question 34

Q

what’s located in the inner plexiform layer?

Answer

A

the synaptic contacts between the bipolar cells, the ganglion cells and the amacrine cells

Question 35

Q

what do both rods and cones have?

Answer

A

an outer segment composed of membranous disks that contain light-sensitive photopigment (photopigments absorb light, thereby triggering changes in the photoreceptor membrane potential)
an inner segment that contains the cell nucleus and gives rise to synaptic terminals that contact bipolar or horizontal cells

Question 36

Q

what is the life span of the membranous disks in the outer segment? what’s continuously being formed? what happens during life span?

Answer

A

12 days

new outer segment disks are continuously being formed near the base of the outer segment
during their life span, disks move progressively from the base of the outer segment to the tip, where the pigment epithelium plays an essential role in removing the expended receptor disks
shedding involves ‘pinching off’ a clump of receptor disks by the outer segment membrane of the photoreceptor
this enclosed clump of disks is then phagocytosed by the pigment epithelium

Question 37

Q

what blood vessels are photoreceptors supplied by?

Answer

A

choroidal blood vessels

Question 38

Q

what do the processes of horizontal cells enable?

Answer

A

lateral interactions between photoreceptors and bipolar cells that maintain the visual system’s sensitivity to luminance

they help integrate and regulate the input from multiple photoreceptor cells

Question 39

Q

what are the processes of amacrine cells?

Answer

A

they are postsynaptic to bipolar cell terminals and presynaptic to the dendrites of ganglion cells

Question 40

Q

were it retinal pigment epithelium found? what does it do?

Answer

A

it surrounds the tips of the outer segments of each photoreceptor
as well as being involved with the phagocytosis of expended membranous disks, it also regenerates photopigment molecules after they have been exposed to light

Question 41

Q

which cells in the retina are light-sensitive? how are cells influenced by light?

Answer

A

only the photoreceptors

all other cells are influenced by light only via the direct and indirect synaptic interactions with the photoreceptors:

direct synaptic interactions = photoreceptors > bipolar cells > ganglion cells > brain
indirect synaptic interactions = photoreceptor cells > bipolar cells (+ horizontal cells) > ganglion cells (+ amacrine cells) > brain

Question 42

Q

what are the only source of output from the retina?

Answer

A

ganglion cells - no other retinal cell type projects an axon through the optic nerve

Question 43

Q

describe rod photoreceptors

Answer

A

these have a long, cylindrical outer segment, containing many disks
this makes them extremely sensitive to light
rods have a low spatial resolution
it is therefore specialised for sensitivity at the expense of resolution

Question 44

Q

describe cone photoreceptors

Answer

A

these have a shorter, tapering outer segment with fewer membranous disks
this makes them relatively insensitive to light
cones have a high spatial resolution - it is therefore specialised for acuity at the expense of sensitivity
the properties of cones allow humans to see colour

Question 45

Q

what happens as light intensity increases? what about at low levels of light? what is the names of the different types of vision involving different receptors?

Answer

A

• As light intensity increases, cones become more and more dominant in determining what is seen, and they are the major determinant of perception under relatively bright conditions.
• The response of an individual cone does not saturate at high levels of steady illumination.
• The contributions of rods to vision drops out nearly entirely in so called ‘photopic’ vision because their response to light saturates—that is, the membrane potential of individual rods no longer varies as a function of illumination because all of the membrane channels are closed.
 At the lowest levels of light, only the rods are activated – ‘scotopic’ vision.
 ‘Mesopic’ vision occurs in levels of light at which both rods and cones contribute.

Question 46

Q

people who have lost cone function are what?

Answer

A

legally blind

Question 47

Q

people who have lost rod function experience what?

Answer

A

night blindness

Question 48

Q

describe convergence of rod and cone cells and the ganglion cells - what does this mean in terms of spatial resolution and acuity?

Answer

A

• In most parts of the retina, rod and cone signals converge on the same ganglion cells depending on the level of illumination.
• The early stages of the pathways that link rods and cones to ganglion cells, however, are largely independent:
 Rod bipolar cells:
- These do not contact retinal ganglion cells.
- Instead, rod bipolar cells synapse with the dendritic processes of a specific class of amacrine cell that makes gap junctions and chemical synapses with the terminals of cone bipolar cells; these processes, in turn, make synaptic contacts on the dendrites of ganglion cells.
- Each rod bipolar cell is contacted by a number of rods, and many rod bipolar cells contact a given amacrine cell.
 Cone bipolar cells:
- Each retinal ganglion cell that dominates central vision (midget ganglion cells) receives input from only one cone bipolar cell, which, in turn, is contacted by a single cone.

Convergence makes the rod system a better detector of light, because small signals from many rods are pooled to generate a large response in the bipolar cell.
However, convergence reduces the spatial resolution of the rod system, since the source of a signal in a rod bipolar cell or retinal ganglion cell could have come from anywhere within a relatively large area of the retinal surface.
The one-to-one relationship of cones to bipolar and ganglion cells maximizes acuity.

Question 49

Q

are there more rod or cones in the human retina?

Answer

A

In the human retina, there are more rods (~90 million) than there are cones (~4.5 million). This means that the density of rods in the human retina is far greater than the cone density.

Question 50

Q

what happens in the fovea? how is this achieved?

Answer

A

In the fovea, the cone density increases dramatically reaching, at its centre, the highest receptor packing density anywhere in the retina.
 This high density is achieved by decreasing the diameter of the cone outer segments such that foveal cones resemble rods in their appearance.
 The increased density of cones in the central 300 μm of the fovea, called the foveola, is due to the absence of rods.

Question 51

Q

what allows the cone system to mediate high visual acuity?

Answer

A

The high cone density in the fovea, coupled with the one-to-one relationship with the bipolar cells and retinal ganglion cells, allows the cone system to mediate high visual acuity.

Question 52

Q

what does the exclusion of rods from the fovea and their presence in high density away from the fovea explain?

Answer

A

why the threshold for detecting a light stimulus is lower outside the region of central vision

Question 53

Q

what contributes to superior visual acuity in the fovea?

Answer

A

 The layers of cell bodies and processes that overlie the photoreceptors in other areas of the retina are displaced around the fovea, and especially the foveola.
 The retinal blood vessels are diverted away from the foveola. This central region of the fovea is therefore dependent on the underlying choroid and pigment epithelium for oxygenation and metabolic sustenance.

Question 54

Q

what is phototransduction?

Answer

A

when the photoreceptors convert, or transduce, light energy into changes in membrane potential

Question 55

Q

what happens to the membrane in a photoreceptor when there is light stimulation of the photopigment?

Answer

A

light stimulation of the photopigemtn leads to membrane hyperpolarisation rather than depolarisation

Question 56

Q

in the dark, what is the membrane potential of the rod outer segment? why is it like this? how maintained?

Answer

A

In the dark, the membrane potential of the rod outer segment is about -30mV.
 This depolarisation is caused by the steady influx of Na+ through special channels in the outer segment membrane.
 At the same time, K+ ions leave the inner segment, so as to balance the electrochemical gradient across the photoreceptor cell.
 The movement of positive charge (Na+ and K+ - diagram) across the membrane, which occurs in the dark, is called dark current.
 These gated sodium channels are simulated to open by an intracellular second messenger called cyclic guanosine monophosphate (cGMP).
 cGMP is produced continually in the photoreceptor by the enzyme guanylyl cyclase, keeping the sodium channels open.

Question 57

Q

how does light stimulation of the photopigment cause hyperpolarisation?

Answer

A

 Light stimulation of the photopigment activates G-proteins, which in turn activate an effector enzyme that reduces cGMP.
 This causes the cGMP-gated-Na+ channels to close, thus hyperpolarising the cell.

Question 58

Q

what is the relationship between luminance changes and the rate of transmitter release?

Answer

A

• There is a consistent relationship between luminance changes and the rate of transmitter release from the photoreceptor terminals.
• Transmitter release from the synaptic terminals of the photoreceptor is dependent on voltage-sensitive Ca2+/Na+ channels in the terminal membrane.
 In the dark, when photoreceptors are relatively depolarized, the number of open Ca2+/Na+ channels in the synaptic terminal is high, and the rate of transmitter release is correspondingly great.
 In the light, when receptors are hyperpolarized, the number of open Ca2+/Na+ channels is reduced, and the rate of transmitter release is also reduced.

Question 59

Q

what can the photopigment be thought of as? what’s it called?

Answer

A

thought of as a receptor protein with a pre-bound light absorbing ‘chromophore’
rhodopsin

Question 60

Q

what is the light absorbing ‘chromophore’ of rhodopsin called? what is it?

what is the receptor protein called?

how are the both of them linked?

Answer

A

retinal (11-cis retinal) - a substance derived from vitamin A
(all-trans retinol -> 11-cis retinal)

transducin (opsin)

opsin and 11-cis retinal are covalently linked

Question 61

Q

what happens to rhodopsin when light is absorbed? what is this called?

Answer

A

Absorption of light causes a change in the conformation of 11-cis retinal to all-trans retinal.
This activates opsin, causing it to be released from 11-cis retinal.
Opsin protein is constitutively active
Ligand = 11-cis retinal – acts as an ‘inverse agonist’ – keeps opsin in inactive state
Photoisomerization to all-trans (‘agonist’) and retinal dissociation triggers signalling

 The activation of opsin causes the activation of transducin, resulting in the conversion of GTP to GDP (by GTPase).
 GDP, along with phosphodiesterase (PDE) hydrolyses cGMP, thus reducing it’s concentration.
 This leads to the closure of the cGMP-gates-Na+ channels.
• Following isomerisation and release form the opsin protein, all-trans retinal is reduced to all-trans retinol.
• This process is called bleaching.

Question 62

Q

what type of protein in transducin? how does termination of its activity occur?

Answer

A

a G-protein

via GTPase activity

Question 63

Q

the restoration of retinal to a form capable of signalling photons is a complex process known as what? describe it

Answer

A

retinoid cycle:
 The all-trans retinol is transported out of the outer segment and into the retinal pigment epithelium.
 Enzymes convert it to 11-cis retinal.
 After it is transported back into the outer segment, the 11-cis retinal recombines with opsin in the receptor disks to form rhodopsin.

Question 64

Q

why is the recycling of rhodopsin needed?

Answer

A

for maintaining the light sensitivity of photoreceptors

Answer 63

A

One important feature of this biochemical cascade of transduction is signal amplification.
Many G-proteins are activated by each photopigment molecule, and each phosphodiesterase enzyme breaks down more than one cGMP molecule.
This amplification gives our visual system the ability to detect as little as a single photon, the elementary unit of light energy.

Answer 64

A

sensitivity to light decreases, preventing the receptors from saturating and thereby greatly extending the range of light intensities over which they operate.

Answer 65

A

red, green and blue

Answer 66

A

cones only contain a single photopigment
however, each type of cone differs in the photopigment it contains
each of these photopigments have a different sensitvity to light of different wavelengths
- blue or ‘short’ wavelength (only 5-10% of cones, virtually absent at the fovea centralis)
- green or ‘medium’ wavelength
- red or ‘long’ wavelength

Answer 67

A

Under normal conditions, most people can match any colour by adjusting the intensity of three superimposed light sources generating long, medium, and short wavelengths.
 Colour vision is therefore trichromatic.

Answer 68

A

When all types of cones are equally active, as in broad-spectrum light, we perceive “white”.

Answer 69

A

 The inherited failure to make one or more of the cone pigments.
 An alteration in the absorption spectra of cone pigments.

Answer 70

A

 This is because only two bandwidths of light are needed to match all the colours that these individuals can perceive. This is called dichromacy.
 The third colour category is simply not seen.

dichromats can match any colour they see with a mixture of no more than two pure spectral lights.

Answer 71

A

Dichromacy is inherited as a recessive, sex-linked characteristic and exists in two forms:

Protanopia – blindness to red light so blue and green cones are used.
Deuteranopia – blindness to green light so blue and red cones are used.

Answer 72

A

 This is because the red and green pigments show a high degree of sequence homology and lie adjacent to each other on the X chromosome, thus explaining the prevalence of colour blindness in males.
 In contrast, the blue-sensitive pigment gene is found on chromosome 7.

Answer 73

A

The transmission from all-cone daytime vision to all-rod nighttime vision is not instantaneous; it takes about 20-25 minutes.

Answer 74

A

 Pupil dilation – this allows more light to enter the eye.
 Regeneration of unbleached rhodopsin.
 An adjustment of the functional circuitry of the retina so that information from more rods is available to each ganglion cell.

Answer 75

A

Sensitivity to light increases greatly during this period.
 Because of this tremendous increase in sensitivity, when the dark-adapted eye goes back into bright light, it is temporarily saturated.

Answer 76

A

• After this temporary saturation, light adaption begins.
• It involves reversing the changes in the retina that accompanied dark adaption.
• Role of calcium:
 cGMP-gated sodium channels also admit Ca2+ ions.
 In the dark, Ca2+ enters the cones and has an inhibitory effect on the enzyme (guanylyl cyclase) that synthesizes cGMP, thus closing the cGMP-gated channels.
 When the cGMP-gated channels close, the flow of Ca2+ into the photoreceptor is reduced; as a result, more cGMP is synthesized (because the synthetic enzyme is less inhibited), thereby allowing the cGMP-gated channels to open again.

 Essentially, when the channels close, a process is initiated that gradually reopens them even if the light level does not change.

Answer 77

A

the area of retina that, when stimulated with light, changes the cell membrane’s potential

Two parts:

a circular centre area of retina providing direct photoreceptor input, called the receptive field centre
a surrounding area of the retina providing indirect photoreceptor input via horizontal cells, called the receptive field surround

• The response of a bipolar cell’s membrane potential to light in the receptive field centre is opposite to that of light in the surround! (on centre/off suround, or off centre/on surround)
 If illumination of the centre causes depolarization of the bipolar cell (an ON response - as light hyperpolarises, and on bipolar cells reverse the sign of the cone), then illumination of the surround will cause an antagonistic hyperpolarization of the bipolar cell. (as the surrounding cones becomes hyperpolarised too, so the horizontal cells are less excited so there is less of an inhibitory response to the centre cone, so more neurotransmitter released from centre cone, so on bipolar cell is less excited so hyperpolarisation?

Answer 78

A

• Each ganglion cell responds to stimulation of a small circular patch of the retina, which defines the cell’s receptive field (RF).
 Turning on a spot of light in the RF centre of an ON-centre ganglion cell produces a burst of action potentials.
 Turning on a spot of light in the in the RP centre of an OFF-centre ganglion cell reduces the rate of discharge.

• Thus:
 ON-centre cells increase their discharge rate to luminance increments in the receptive field centre.
 OFF-centre cells increase their discharge rate to luminance decrements in the receptive field centre.

• However:
 In uniform illumination, the centre and surround cancel to yield some low level of response.

Answer 79

A

ON center/OFF surround cell: Flashing small bright spot in the center subregion increases the cell’s response. Flashing a bright annulus in the surround subregion inhibits the cell’s response. There is little or no response to a large (full field) spot of light that covers both the center and the surround because excitation in the center cancels the inhibition from the surround, called lateral inhibition.

An OFF-center/ON-surround ganglion cell has the opposite arrangement. It gets inhibition from a small spot of light in the center, and excitation from an annulus in the surround.

both types are present in roughly equal numbers

http://www.cns.nyu.edu/~david/courses/
perception/lecturenotes/ganglion/ganglion.html

Answer 80

A

differences between the level of illumination that falls on the receptive field centre and the level of illumination that falls on the surround = luminance contrast

Answer 81

A

so that every point on the retina is analysed by severeal ON-centre and several OFF-centre ganglion cells

Answer 82

A

carried separately to the brain by the axons of these two different types of retinal ganglion cells
having separate luminance ‘channels’ means that changes in light intensity, whether increases or decreases, are always conveyed to the brain by an increased number of action potentials - this provides unambiguous information about both luminance increments and decrements

Answer 83

A

the terminals of ON and OFF- centre bipolar cells in the inner plexiform layer
depolarisation of the central bipolar cells causes the release of glutamate at their terminal synapse with the centre ganglion cells
glutamate binds to AMPA/kainite/NMDA receptors

Answer 84

A

ON- and OFF-centre bipolar cells express different types of glutamate receptors.

o OFF-centre bipolar cells have ionotropic receptors (AMPA and kainate) that cause the cells to depolarize in response to glutamate released from photoreceptor terminals.

o ON-centre bipolar cells express a G-protein-coupled metabotropic glutamate receptor (mGluR6). When bound to glutamate, these receptors activate an intracellular cascade that closes cGMP-gated Na+ channels, reducing inward current and hyperpolarizing the cell.
(ON-centre bipolar cells do the opposite, therefore the cone cell is depolarised as it’s dark, so the bipolar cell will be hyperpolarised)

• Normally, in a state of depolarisation (dark conditions), the photoreceptor terminals release glutamate, thus inhibiting the ON-bipolar cell but stimulating the OFF bipolar cell.
• Photoreceptors hyperpolarize in response to increased light, decreasing their release of neurotransmitter.
• Under these conditions:
 ON-centre bipolar cells contacted by the photoreceptors are freed from the hyperpolarizing influence of the photoreceptor’s transmitter, and they depolarize.
 In contrast, for OFF-centre cells, the reduction in glutamate represents the withdrawal of a depolarizing influence, and these cells hyperpolarize.

Answer 85

A

these are cells that are found in the outer plexiform layer (their terminals) and in the inner nuclear layer (their cell bodies)
horizontal cells release their neurotransmitter directly onto the photoreceptor terminals in the outer plexiform layer
subsequently, this regulates the amount of neurotransmitter that the photoreceptors release onto bipolar cell dendrites
- glutatmate release from photoreceptor terminals has a depolarising effect on horizontal cells
- GABA release from horizontal cells has a hyperpolarising effect on the photoreceptor terminals

Answer 86

A

A small spot of light centred on a photoreceptor supplying input to the centre of the ganglion cell’s receptive field produces a strong hyperpolarizing response in the photoreceptor.
Under these conditions, changes in the membrane potential of the horizontal cells that synapse with the photoreceptor terminal are relatively small, and the response of the photoreceptor to light is largely determined by its phototransduction cascade.
With the addition of light to the surround, however, the impact of the horizontal network becomes significantly greater; the light-induced reduction in the release of glutamate from the photoreceptors in the surround leads to a strong hyperpolarization of the horizontal cells whose processes converge on the terminal of the photoreceptor in the receptive field centre.
The reduction in GABA release from the horizontal cells has a depolarizing effect on the membrane potential of the central photoreceptor.
This causes an increase in the release of glutamate from the photoreceptor terminals, thus inhibiting the ON-centre ganglion cell but stimulating the OFF-centre ganglion cell.

Answer 87

A

• Most ganglion cells in the retina have a centre-surround receptive field with either ON or an OFF centre.
• They can be distinguished as;
 Large M-Type Ganglion cells – 5% of population. [M = ‘magno’ = large]
 Smaller P-Type ganglion cells – 90% of population. [P = ‘parvo = small]
 The remaining 5% is made up of a variety of nonM-nonP ganglion cell types. These are less well characterised.

• M-type ganglion cells :
 Have larger receptive fields than P-type ganglion cells.
 Their axons conduct action potentials more rapidly in the optic nerve.
 They are more sensitive to low contrast stimuli.
 Respond to stimulation of their receptive field centres with a transient burst of action potentials, while P-type ganglion cells respond with a sustained discharge as long as the stimulus is on.

• P-type ganglion cells can transmit information about colour, whereas M-type ganglion cells cannot.

As a result, P-type ganglion cells are sensitive to differences in the wavelengths of light striking their receptive field centre and surround.
Although M-type ganglion cells also receive inputs from cones, there is no difference in the type of cone (red, green or blue) input to the receptive field centre and surround; the centre and surround of each M-type cell receptive field is driven by all cone types.

Answer 88

A

Some P-type ganglion cells and nonM-nonP cells are sensitive to differences in the wavelength of the light.
These colour-sensitive neurons are called color-opponent cells, reflecting the fact that the response to one wavelength in the receptive filed centre is cancelled by showing another wavelength in the receptive field surround.

• Two types of opponency are found:

Red vs. Green
Blue vs. Yellow

Concepts
• Either colour in the combinations above can be the ON-centre or the OFF-surround.
• The neuron responds to its ON –centre by firing action potentials.
• If the centre coloured stimulus encompasses the surround as well, the firing of action potentials still occurs, but with a dampened effect.
• The neuron is only inhibited when its corresponding colour lands on the OFF-surround.

White light encompasses all visible wavelengths; therefore, both the centre and the surround would be equally activated, thereby cancelling the response of the cell.
The lack of colour opponency in M-type ganglion cells is accounted for by the fact that both the centre and surround of the receptive field receive input from more that one type of cone.

Example: Red – Green Opponency
• In this example, we will have a red ON-centre and a green OFF-surround.
• The centre of the receptive field is fed mainly by red cones; therefore, the cell responds to red light by firing action potentials.
• Note that even a red light that bathes the entire receptive field is an effective stimulus. However, the response is reduced.

Example: Blue – Yellow Opponency
• In this example, we will have a blue ON-centre and a yellow OFF-surround (B+Y-).
• Blue light drives blue cones that feed the receptive field centre.
• Yellow light activates both red and green cones that feed the surround.
• Diffuse blue light would be an effective stimulus for this cell with a dampened response.
• Yellow light on the surround would cancel the response.
• Diffuse white light too would cancel the response.

Answer 89

A

Light energy is first converted into membrane potential changes in the photoreceptors.
Photoreceptor membrane potential is converted into a chemical signal (the neurotransmitter glutamate), which is again converted into membrane potential changes in the postsynaptic bipolar and horizontal cells.
This process of electrical-to-chemical-to-electrical signaling repeats again and again, until the presence of light or dark or color is finally converted to a change in the action potential firing frequency of the ganglion cells. This is done through the complex processes outlined above (receptive fields, colour opponency etc).
The information from the 125 million photoreceptors is funneled into 1 million ganglion cells.
In the central retina, particularly the fovea, relatively few photoreceptors feed each ganglion cell, whereas in the peripheral retina, thousands of receptors do. They develop a one-to-one relationship at the fovea.
This specialization ensures high acuity in central vision but also requires that the eye move to bring the images of objects of interest onto the fovea.

Answer 90

A

they become myelinated - the fibre are unmyelinated in the retina to allow for better transmission of light onto the photoreceptors

Answer 91

A

ophthalmic artery and veins

Answer 92

A

‘intracranial’ pressure

The subarachnoid space surrounding the optic nerve is continuous with that of the brain; as a result, increases in intracranial pressure can be detected as papilloedema (swelling of the optic disc)

Answer 93

A

• Once past the chiasm, the ganglion cell axons on each side form the optic tract.
• The ganglion cell axons in the optic tract reach a number of structures in the diencephalon and midbrain.
• The major target in the diencephalon is the dorsal lateral geniculate nucleus of the thalamus.
• Neurons in the lateral geniculate nucleus send their axons to the cerebral cortex via the internal capsule.
• These axons pass through a portion of the internal capsule called the optic radiation and terminate in the primary visual cortex, (striate cortex, brodmann’s area 17, V1), which lies largely along and within the calcarine fissure in the occipital lobe.
 Ganglion cells that lie in the nasal division of each retina give rise to axons that cross in the chiasm.
 Ganglion cells that lie in the temporal retina give rise to axons that remain on the same side.

Answer 94

A

• Optic nerve fibres reach the optic chiasm.
• Projections are given to:
 Superior colliculus
o Coordinates head and eye movements to visual (and other) targets.
 Pretectum
o Coordinates the pupillary light reflex.
o Afferent fibres terminate in the pretectal nucleus and pass to the Edinger-Westphal nucleus, which then project to the ciliary ganglion of the oculomotor nerve, causing contraction of the constrictor pupillae muscles.
 Hypothalamus (suprachiasmatic nucleus)
o These fibres are involved in the circadian cycle.
o The fibres that project here express their own light-sensitive photopigment (melanopsin).

Answer 95

A

The fibres containing melanopsin (fibres to Pretectum and hypothalamus) are capable of modulating their response to changes in light levels in the absence of signals from rods and cones. Therefore, the circadian rhythms are maintained even after the degeneration of photoreceptors.

Answer 96

A

The crossover of retinal ganglion cell axons within the chiasm isn’t linear (as seen in the diagram).
The fibres from the nasal division of the retina decussate within the chiasm.
The fibres from the nasal division of the retina form a structure called the anterior/ posterior knee of Willbrand.
This is significant when considering lesions of the visual pathways.

Answer 97

A

The lateral geniculate nucleus receives input from both eyes via ganglionic cell nerve fibres > optic chiasm > optic tract.
Although it receives input from both eyes, the axons terminate in separate layers, so that individual geniculate neurons are monocular, driven by either the left or the right eye.
Inputs from the left and right eyes remain segregated beyond the geniculate because the axons of geniculate neurons terminate in alternating eye-specific columns within cortical layer IV—the so-called ocular dominance columns.
Bringing together the inputs from the two eyes at the level of the striate cortex provides a basis for stereopsis, the special sensation of depth that arises from viewing nearby objects with two eyes instead of one.
Anything that creates an imbalance in the activity of the two eyes—for example, the abnormal alignment of the eyes during infancy (strabismus)—can permanently reduce the effectiveness of one eye in driving cortical neurons, and thus impair the ability to use binocular information as a cue for depth.

Answer 98

A

part of the visual cortex that is involved in processing visual information
first cortical visual area that receives input from the lateral geniculate nucleus in the thalamus
named because it is ‘striated’ (layered)

Answer 99

A

Layers of the Lateral Geniculate Nucleus
• It consists of 6 layers.
 Right eye = layers 2,3,5 of the RLGN
 Left eye = layers 1,4,6 of the RLGN

• The LGN is organised into 6 distinct neuronal populations occupying separate layers:
 Magnocellular neurons (the 2 ventral layers):
 These are located in layers 1 and 2.
 These are composed of large neurons.
 These are called the magnocellular layers because they receive information from the M-type (‘magno’) ganglion cell fibres.
 These neurons are important for high temporal resolution – location, speed and direction of a rapidly moving object.
 These neurons convey motion signals [remember: Magnocellular = Motion].

 Parvocellular neurons (the 4 dorsal layers):
 These are located in layers 3, 4, 5 and 6.
 These are composed of small neurons.
 These are called parvovellular layers because they receive information from the P-type (‘parvo’) ganglion cell fibres.
 These neurons are important for high spatial resolution vision – detailed analysis of the shape, size and colour of objects.
 These neurons receive information from red and green cones (red-green opponency).

 Koinicellular/ Interlaminar neurons:
 These are located between the magno- and parvocellular layers.
 These neurons receive information from blue and yellow cones (blue-yellow opponency).
• The axons of relay cells in the magno- and parvocellular layers of the lateral geniculate nucleus terminate on distinct populations of neurons in layer IV of striate cortex.
 The magnocellular layers of the LGN, conveying information about movement and gross spatial features, project mainly to layer 4Cα.
 The parvocellular layers of the LGN, carrying fine spatial information, terminate mainly in layer 4Cβ.
• The koniocellular fibres (and some pavocellular fibres), carrying information about colour, project to the superficial layers (layers II and III) of striate cortex, called blobs.

Answer 100

A

Cortical neurons respond vigorously to light–dark bars or edges, but only if the bars are presented at a particular range of orientations within the cell’s receptive field.
The responses of cortical neurons are thus tuned to the orientation of edges; the peak in the tuning curve (the orientation to which a cell is most responsive) is referred to as the neuron’s preferred orientation - The vertical columns are called orientation columns.
All edge orientations are roughly equally represented in visual cortex. As a result, a given orientation in a visual scene appears to be “encoded” in the activity of a distinct population of orientation-selective neurons.

• In each orientation column, there are two types of cell.
1. Simple cells - these respond to stationary bars of a certain orientation from a single visual field.
 These cells are monocular.
2. Complex cells - these are not direction sensitive but respond preferentially to bars of light of the same orientation as the simple cells, but moving across the receptive field, parallel to the preferred orientation.
 These cells are binocular.

• Each class of orientation-selective neuron transmits only a fraction of the information in the scene – the part that matches its filter properties (i.e. the orientation it is encoded for) – but the information from these different filters contains all the spatial information necessary to generate a faithful representation of the original image.

Answer 101

A

• Afferent Pathway
 Light generates action potentials in optic nerve axons.
 Axons (some decussating at the chiasm) pass through each lateral geniculate body.
 Axons synapse at each pretectal nucleus.

• Efferent Pathway
 Action potentials pass to each Edinger-Westphal nucleus ‘bilaterally’ (thus effecting both eyes simultaneously) of the oculomotor nerve (CN III).
 This nucleus supplies preganglionic parasympathetic fibres to the ciliary ganglion.
 These cause the constriction of the concentric muscles (constrictor pupillae) in the iris, thus constricting the pupil.

Answer 102

A

constriction of both the stimulated eye (the direct response) and the unstimulated eye (the consensual response)

Answer 103

A

Sympathetic impulses via fibres in the nasociliary (long ciliary) nerve pass to dilator pupillae (radial muscle).

 Sympathetic preganglionic fibres to the eye (and face) originate in the hypothalamus, pass uncrossed through midbrain and lateral medulla, and emerge from the spinal cord at T1 (close to the lung apex) to form the superior cervical ganglion at C2.
 Postganglionic fibres form a plexus around the carotid bifurcation.
 Fibres pass to the dilator pupillae (in the iris) and cause it to constrict, thus dilating the pupils.

Answer 104

A

ipsilateral direct reflex = lost
contralateral direct reflex = intact
ipsilateral consensual reflex = lost
contralateral consensual reflex = intact

Answer 105

A

ipsilateral direct reflex = lost
contralateral direct reflex = intact
ipsilateral consensual reflex = intact
contralateral consensual reflex = lost

Answer 106

A

Move torch quickly from pupil to pupil. If there has been incomplete damage to the afferent pathway, the affected pupil will dilate (slightly) when light is moved from the normal eye to the abnormal eye. This is because there is reduced afferent input from the affected eye and so the consensual pupillary relaxation response from the normal eye predominates. This is Marcus gunn sign.

Looking for afferent pupillary defects
In dark room, you shine light in one eye and then swing over to the other eye, then swing back to the first eye and swing back to the second eye
If there’s a defect in on the afferent pathways, when you swing to that eye both eyes will dilate, because is bilateral innervation, but when you swing back to normal eye both pupils constrict again
No light
Normal response to light – both pupils constrict
Positive RAPD (relative afferent pupillary defect) of right eye – both pupils dilate

Answer 107

A

• Fixation on a near object requires convergence of the ocular axes and is accompanied by pupillary constriction.
• Afferent fibres in each optic nerve, passing through each lateral geniculate body, also relay to the convergence centre.
 This centre receives 1a spindle afferents from extraocular muscles.
• The efferent autonomic route is convergence centre to Edinger–Westphal nucleus to ciliary ganglion and pupils.

Answer 108

A

Information from the left half of the visual world, whether it originates from the left or right eye, is represented in the right half of the brain, and vice versa.

Answer 109

A

The surface of the retina is subdivided by vertical and horizontal lines that intersect at the centre of the fovea.
The vertical line divides the retina into nasal and temporal divisions and the horizontal line divides the retina into superior and inferior divisions.
Corresponding vertical and horizontal lines in visual space intersect at the point of fixation and define the quadrants of the visual field.
The crossing of light rays diverging from different points on an object at the pupil causes the images of objects in the visual field to be inverted and left-right reversed on the retinal surface.
As a result, objects in the temporal part of the visual field are seen by the nasal part of the retina, and objects in the superior part of the visual field are seen by the inferior part of the retina.
Objects in the monocular portions of the visual hemifields are seen only by the most peripheral nasal retina of each eye.

Answer 110

A

Superior Field = 60 degrees above the line of sight
Inferior Field = 75 degrees below the line of sight
Nasal Field = 60 degrees medial to the line of sight
Temporal Field = 100 degrees lateral to the line of sight

Answer 111

A

The superior field is limited by the frontal bone. It becomes restricted with ageing because the frontal bone depresses slightly and the eye ball is withdrawn into the orbit.

Answer 112

A

For the primary visual pathway, the map of the contralateral hemifield is established as such that information from the centre of vision (i.e. the fovea) is represented in the most posterior part of the striate cortex.
Information from the more peripheral regions of the retina is represented in progressively more anterior parts of the striate cortex.
Information from the upper visual field is mapped below the calacrine sulcus.
Information from the lower visual field is mapped above the calacrine sulcus.

Answer 113

A

Cortical magnification refers to the fact that the number of neurons in the visual cortex responsible for processing the visual stimulus of a given size varies as a function of the location of the stimulus in the visual field.
Stimuli occurring in the centre of the visual field that have been detected in the fovea of the retina are processed by a very large number of neurons in the primary visual cortex of the occipital lobe, though these neurons handle only a very small region of the central visual field.
Conversely, stimuli detected in the peripheral visual field tend to be processed by a much smaller number of neurons in the primary visual cortex.

Answer 114

A

• Damage to central visual structures is rarely complete.
• As a result, the deficits associated with damage to the chiasm, optic tract, optic radiation, or visual cortex are typically more limited than those shown.
 This is especially true for damage along the optic radiation, which fans out under the temporal and parietal lobes in its course from the lateral geniculate nucleus to the striate cortex. Some of the optic radiation axons run out into the temporal lobe on their route to the striate cortex, a branch called Meyer’s loop.

Answer 115

A

Injury to central visual structures can also cause ‘macular sparing’.
 Macular sparing is commonly found with damage to the cortex.
 Macula sparing is where the central 5-10 degrees is unaffected in an otherwise hemianopic defect.

Answer 116

A

a defect in the visual field

Answer 117

A

 Hemianopia – decreased vision/ blindness in half the visual field of one or both eyes, usually on one side of the vertical line.
 Homonymous heminopia – loss of half of the visual field on the same side in both eyes.
 Heteronymous heminopia: loss of half of the visual field on different sides in both eyes.
- Binasal heminopia – loss of field surrounding the nose.
- Bitemporal heminopia – loss of field closest to the temples.
 Quadrantanopia – decreased vision/ blindness in one quarter of the visual field.

Answer 118

A

Each retina is divided into two halves; temporal and nasal. Axons from the retina form the optic nerve. At the optic chiasm, fibres from the nasal half of each retina decussate and join the uncrossed fibres from the lateral half of the retina (from the other eye) to form the optic tract. These fibres project to the lateral geniculate nucleus (LGN) of the thalamus, where they synapse with second order neurons. The second order neurons pass in the optic radiations to the striate cortex.
Some fibres bypass the LGN and project to the superior colliculus in the midbrain. Here the information is interpreted in conjunction with the afferent information from the somatosensory cortex, frontal eye field and the spinal cord. Efferent fibres project to the brainstem nuclei (pretectal nuclei > Edinger-Westphal nuclei) to bring about the pupillary light reflex and reflex movements of the eye, and to the spinal cord to initiate neck movement.
Other fibres bypass the LGN and project to the suprachiasmatic nucleus of the hypothalamus, and are involved in circadian rhythms.

Answer 119

A

the potential presence of blind spots (scotomas), which could indicate ocular pathology

Blind spot in temporal visual field at about 15 degrees eccentricity (due to optic disc)

Answer 120

A

automated perimetry (Goldmann perimetry)

 This measures the response to the presence of objects in different areas in the visual field.
 The patient’s head is held still, usually with a chin rest inside a large bowl-like instrument.
 The patient stares at a source of light straight ahead.
 Random lights of different intensities are flashed in the peripheral visual field.
 The patient presses a button to indicate their response when they perceive the computer-generated light suddenly appearing in their field of vision.
 If they can’t see objects in an appropriate portion of their field of view, they may have a blind spot indicating vision loss.

GOLDMANN VISUAL FIELDS

Spots of different size and intensity allow isopters to be plotted
Isopters are lines joining points of equal sensitivity (circles around the macula I think?)

HUMPHREY VISUAL FIELD ANALYSER

Standard one that use now
Show visual field defects

Answer 121

A

some distance away

- tumour needs to be quite large before it can interfere with the chiasm

Answer 122

A

Hormone secreting:
 Patients with this type of tumour normally seek medical advice prior to occurrence of visual field defects.
 This is because the excessive secretion of hormones will bring about other systemic symptoms, which would result in the patient going to the doctor.
Non-hormone secreting tumour:
 Vision loss will occur prior to any other systemic symptoms.

Answer 123

A

1-2 months

the quality of life of the patient is dependent on the pre-surgical severity

Answer 124

A

refractive error: light rays entering the eye are not properly brought into focus on the retina

Answer 125

A

6/6 vision

Answer 126

A

Myopia (short-sightedness):
 This can be caused by the corneal surface being too curved, or by the eyeball being too long.
 The image of distant objects focus in front of the retina, instead of on the retina.

Hyperopia/Hypermetropia (long-sightedness):
 This can be caused by the corneal surface not being curved enough, or by the eyeball being too short.
 The image of near objects focus behind the retina, instead of on the retina.

Presbyopia:
 This is the normal ageing of the lens which leads to a change in the refractive state of the eye.
 As the lens ages it becomes less able to alter its curvature and this causes difficulty with near vision.
(long-sightedness caused by loss of elasticity of the lens)

Answer 127

A

• Myopia is a common condition in which the refracting power of the eye at rest is too great in relation to the axial length of the eye; the focused image of an object lies anterior to the retina.
• Physiological Myopia
 Results from a mismatch between the refracting power of the eye and the axial length of the globe when neither of these components lies outside the normal range.
 Onset begins in the second decade and may progress through the third decade.
 Physiologic myopia is not thought to be heritable, but there appears to be an increased frequency of the disorder among higher socioeconomic groups and among those with greater academic training.
 Physiologic myopia usually ranges from about 0.5 to about 8.0 D, where the eye appears normal on physical and radiographic evaluation.
 Treatment – concave (minus)/ diverging lenses/spectacles.

• Pathological Myopia
 Pathologic myopia is a heritable condition in which the eye is abnormally long; the refracting apparatus is usually normal.
 Refractive error in pathologic myopia is usually greater than about 8.0 D.
 Patients with pathologic myopia are predisposed to retinal pathologies.
 Pathologic myopia may be associated with systemic disorders. Dilated fundus examination should be performed at frequent intervals, and patients should be alerted to symptoms of retinal detachment.
 Treatment – concave (minus)/ diverging lenses/spectacles.

Answer 128

A

• The refracting power of the eye is insufficient to bring the focused image of an object onto the retina; the image lies posterior to the retinal plane.

Hyperopia is the normal condition in infants and young children.
Adolescent and adult hyperopia is not usually associated with anatomic abnormalities of the posterior segment.
Many patients with hyperopia are able to overcome their refractive deficiency by accommodating even when viewing at distance.
The ability to accommodate diminishes with age.
In addition to blurred vision, hyperopia may incite headaches in young adults because increasing effort is required to focus at intermediate distances.
Treatment - convex (plus)/ converging lenses/spectacles.

Answer 129

A

Colour vision can be estimated by the patient looking at a red object (e.g. a red pen) with each eye.
If there is an optic nerve or tract lesion on one side the colour looks pink, dull or washed out with that eye. This is ‘red desaturation’.

Answer 130

A

a condition in which the corneal surface is asymmetric: light is refracted differently along different axes - light is refracted to multiple areas of the retina

it may be myopic in one plane and hypermetropic or emmetropic in the other plane

Answer 131

A

90 degrees - a configuration geometrically labelled an ellipsoid cap

with cylindrical and spherical spectacle lenses or with rigid contact lenses

Answer 132

A

an array of corneal configurations, usually owing to corneal ectasis, such as keratoconus (degenerative changes of the cornea that make it a more conical shape), or corneal scarring

irregular astigmatism is not correctable with spectacles but may be correctable with rigid contact lenses

Answer 133

A

functional reduction in visual acuity of an eye caused by disuse during visual development

it almost always develops before age 2

Answer 134

A

Blindness can occur in the affected eye if amblyopia is not detected and treated before age 8.

The brain must simultaneously receive a clear, focused, properly aligned, overlapping image from each eye for the visual system to develop properly.
This development takes place mainly in the first 3 years of life but is not complete until about 8 years of age.
Amblyopia results when there is persistent interference with the image from one eye but not the other.
The visual cortex suppresses the image from the affected eye. If suppression persists long enough, vision loss can be permanent.

Answer 135

A

 Strabismus can cause amblyopia because misalignment of the eyes results in different retinal images being sent to the visual cortex. Because the visual pathways are developed in adults, presentation of 2 different images results in diplopia rather than suppression of one image.
 Anisometropia (inequality of refraction in the 2 eyes, most often resulting from astigmatism, myopia, or hyperopia) results in different focus of the retinal images, with the image from the eye with the greater refractive error being less well focused.
 Deprivation amblyopia is caused by obstruction of the visual axis at some point between the surface of the eye and the retina (eg, by a cataract), this interferes with or completely prevents formation of a retinal image in the affected eye

Answer 136

A

spectacles or contact lenses, cataract removal, patching

Answer 137

A

if there is an imbalance in the extraocular muscles of the two eyes, the eyes will point in different directions
such a misalignment (in the horizontal plane) or lack of coordination between the two eyes is called strabismus
in most cases, strabismus is congenital

Answer 138

A

Esotopia (convergent squint) – the directions of gaze of the two eyes cross, and the person is said to be cross-eyed.
Exotopia (divergent squint) – the directions of the gaze diverge, and the person is said to be wall-eyed.

Answer 139

A

children are able to avoid diplopia (double vision) by involuntarily suppressing one of the images
left and right images are sometimes suppressed alternately, in which case excellent vision may develop in each eye, but binocular vision does not develop in either situation
more frequently, one eye is constantly suppressed, preventing normal visual development in that eye

Answer 140

A

should be treated in early childhood

- involves the use of prismatic glasses, surgery to the extrocular muscles to realign the eyes or cycloplegic refraction

Answer 141

A

Without treatment, conflicting images are sent to the brain from the two eyes, degrading depth perception (poor stereoacuity), and, more importantly, causing the person to suppress input from one eye.
The dominant eye will be normal but the suppressed eye will become amblyopic, meaning that is has poor visual acuity.
If medical intervention is delayed until adulthood, the condition cannot be corrected.

Answer 142

A

cataract, or opacification (clouding) of the crystalline human lens

Answer 143

A

It is usually a condition of the elderly (over 65 years of age). Nearly all patients older than 50 demonstrate some degree of degenerative lens changes when examined by slit lamp.
In most cases, there is normal ageing changes in which progressive yellowing of the lens nucleus (nuclear sclerosis) and hydration of the lens cortex are seen.
Genetic predisposition to senile cataract (cataract in old age) has been hypothesized but not proved.
Prolonged exposure to ultraviolet radiation has been shown to be cataractogenic.
Congenital and traumatic cataracts can occur from a variety of changes.

Answer 144

A

 Cataract surgery
 The lens is removed and replaced with an artificial plastic lens.
 Although the artificial lens cannot adjust its focus like the normal lens, it provides a clear image, and glasses can be used for near and far vision.

Answer 145

A

progressive loss of vision associated with elevated intraocular pressure

Answer 146

A

Aqueous humor is produced in the ciliary body and passes from the posterior chamber through the pupil into the anterior chamber.
Pressure in the aqueous humor plays a crucial role in maintaining the shape of the eye.
As this pressure increases (due to increased production or decreased drainage of aqueous humor), it compresses the retinal blood vessels, causing degeneration of the optic nerve.
The optic nerve axons are compressed, and vision is gradually lost from the periphery inward.

Answer 147

A

Unfortunately, by the time a person notices a loss of more central vision, the damage is advanced and a significant portion of the eye is permanently blind.

Answer 148

A

Early detection and treatment with medication (eye drops) or surgery (trabeculectomy - small hole in sclera, followed by a trap-door in sclera) to reduce intraocular pressure are essential.

Answer 149

A

This is the most common form of glaucoma.
The anterior chamber angle anatomy appears normal, but aqueous outflow is reduced.
High intraocular pressures result from reduced outflow of aqueous humour through the trabecular meshwork/ canal of Schlemm.
Progressive visual field loss begins in the periphery and occurs so insidiously that affected individuals may be unaware until late in the disease course.
Intraocular tension measurement is an effective screening method.
Medical treatment attempts to reduce aqueous production by the ciliary body or to increase outflow through the trabecular meshwork (trabeculectomy) or uvea.

• Common risk factors include age, race (black Africans are greater risk), positive family history and myopia.
- Diagnosis is only made if the IOP is measured. Visual fields are performed and show a normal blind spot with scotomas.

Answer 150

A

This is an ophthalmic emergency.
This occurs due to reduced aqueous drainage as a result of the ageing lens pushing the iris forward against the trabecular meshwork.
People most at risk are hypermetropes (extreme long-sightedness) and women.
The attack is more likely to occur under reduced light conditions when the pupil is dilated.
AACG causes pressure build-up behind the iris, causing sudden onset of a red painful eye and blurred vision.
Patients become unwell with nausea and vomiting and complain of headache and severe ocular pain.
Prompt treatment – eye drops to reduce the intraocular pressure- is required.

Answer 151

A

• Prostaglandin Analogue (Laatanoprost/Xalatan) – reduce intraocular pressure by increasing the outflow of aqueous humor.
 These drugs increase the uveoscleral outflow mainly.
• Beta-blocker (Timolol) – reduce intraocular pressure by decreasing the production of aqueous humor.
 It increases the peripheral resistance and so there is less blood supply to the ciliary body, thus there is reduction in the production of aqueous humor.
• Carbonicanhydrase Inhibitor (Brinzolamide) – reduce intraocular pressure by decreasing aqueous humor production/secretion.
 Inhibition of this enzyme in the ciliary processes slows the formation of bicarbonate, and reduces sodium and fluid transport.

Answer 152

A

This causes a painless progressive visual field loss.
This is usually secondary to trauma or diabetes.
The shadow corresponds to the area of detached retina.
Following a tear in the retina, fluid (from the vitreous space) collects in the potential space between the sensory retina and the pigment epithelium.
Symptoms - sudden onset of floaters often associated with flashes of light prior to the detachment.
Treatment – laser surgery to scar the edge of the retinal tear, thereby attaching the retina to the back of the eye.

Answer 153

A

• Retinitis pigmentosa (RP) is characterized by progressive vision loss due to a gradual degeneration of photoreceptors.
• There is no known cause, but there is clearly a strong genetic component.
• The photoreceptor cells appear to die by apoptosis:
 Loss of rods may lead to early night blindness and constricted visual fields.
 Loss of cones may affect central visual acuity.
• Symptoms – the first sign is usually a loss of peripheral vision and night vision.
• Clinically, retinal atrophy is accompanied by constriction of retinal vessels and optic nerve head atrophy (“waxy pallor” of the optic disk) and the accumulation of retinal pigment around blood vessels, thus accounting for the “pigmentosa” in the disease’s name.
• Treatment - oral supplements of vitamin A palmitate. This slows the course of the disease, although the disease is slowly progressive over decades already.

Answer 154

A

In this condition, patients only lose their central vision.
The cause is unknown but risk factors include increasing age, smoking, hypertension, hypercholesterolemia and ultraviolet exposure.
There are two types of AMD: dry and wet

Non-exudative (Dry) Macular Degeneration
• There is painless and progressive loss of vision.
• With age, lipofuscin deposits (drusen) are found between the retinal pigment epithelium (RPE) and Bruch’s membrane.
• Drusen may be hard or soft and there may be focal RPE detachment.
• Not all patients with these changes will be affected visually but some develop distortion and blurring of their central vision.
• Extensive atrophy of RPE can occur (geographic atrophy).

Exudative (Wet) Macular Degeneration
• This accounts for only 10% of the cases.
• It occurs with the development of abnormal subfoveal choroidal neovascularisation in the region of the macula and causes severe central visual loss.

• Treatment:
 Laser surgery can sometimes minimise further vision loss, but the disease currently has no known cure.
 Vitamins C and E – this slows the progression of the disease.
 β carotene - this slows the progression of the disease.
 Zinc and copper - this slows the progression of the disease.

Answer 155

A

 Perception is knowledge based and partly learned.
 Perception is inferential - We perceive the whole person and not half a person.
 It is categorical – We like to categorize what we see.
 Perception is relational - what we see as small or large depends on the context.
 Perception is adaptive – we tend to perceive significant things better than insignificant.

Answer 156

A

 Bottom-up processing – The perceptual system is assumed to analyse a stimulus into a set of features and then the brain matches/compares it to other sets already existing in the brain. If a match occurs, then recognition occurs.
 Top-down processing – The context creates expectancy and sets up what is known as “perceptual set”. We “see” what we expect, or want to see, and recognition occurs.
 Both mechanisms together – In trying to read a page of poor handwriting we may use both processes: puzzling out what each letter looks like, as well as guessing meaning from the context.

Answer 157

A

 We control perception by paying attention to different aspects of our environment.
 Attention is the directing and focusing of perception.
 It may be:
- Selective – we attending more to stimuli that are changing, repeated, intense and personally meaningful.
- Divided or focused – Our ability to divide attention is limited, although it can be improved with practice.
- Negatively affected by stress and fatigue.

Answer 158

A

• Stigma involves a negative evaluation of and associated lowering of respect for individuals because of some personal characteristic, which may be physical or behavioural.

Stigma – branding or marking.
Enacted Stigma – societal reaction produces discriminatory experiences (non-sufferers treat the stigmatised individual differently).
Felt Stigma – expected societal reactions can change self-identity (individual feels embarrassed or shamed regardless of what others do or feel).
Curiosity Stigma

Answer 159

A

• Disability is typically assessed by measures of activities of daily living (ADL), which assess the person’s ability to perform everyday self-care or mobility activities.
• These measures assess activities that virtually everyone would wish to perform and, therefore, do not include activities that may be important for particular individuals.
• There are two main methods of assessment;
 Self-report
 Observation

Answer 160

A

3 year degree course for orthoptist, ophthalmologist is a medical doctor

Answer 161

A

annulus of Zinn/common tendinous ring = apex of orbit

Oval shaped thickening of the periosteum at the orbital apex
Encloses the optic canal and part of the superior orbital fissure

Answer 162

A

line that bisects the eyeball that’s in the coronal plane and separates the anterior and posterior

Answer 163

A

Lying above the annulus of Zinn on the lesser wing of the sphenoid are the origins of the levator palpebrae superioris and superior oblique muscle

Answer 164

A

Yawing (around vertical axis) (side-to-side)
Rolling (sagittal)
Pitching (transverse) (up and down)

Pitching:
-	Elevation 
-	Depression 
Rolling 
-	Intorsion 
-	Extorsion (superior sclera initially moving outwards) 
Yawing 
-	Abduction (to the lateral orbit) 
-	Adduction (to medial orbit)

Answer 165

A

intorsion = rotation inwards 
extorsion = rotation ouwards

Answer 166

A

Extraocular muscles normally pull in more than one plane
Extraocular muscles normally do not act alone
Lateral rectus abducts the eye
Medial rectus adducts the eye
Superior rectus elevates, intorts and adducts the eyeball
Inferior rectus depresses, extorts and adducts the eyeball
Inferior oblique elevates, abducts and extorts the eyeball
Superior oblique depresses, abducts and intorts the eyeball
In relation to insertion on eyeball, all the origins of these muscles on apex is medial
Therefore, all rectus muscles pull medially
Therefore superior, inferior and medial rectus are all going to some extent adduct the eyeball
Not the case for lateral rectus, as it abducts it

Answer 167

A

When eye is abducted to 23* superior & inferior rectus pull eye vertically
When eye is adducted to 51-55* superior & inferior oblique pull eye vertically

Answer 168

A

If the eye cannot abduct there is a problem with the lateral rectus
If the eye cannot adduct there is a problem with the medial rectus

Answer 169

A

place eyes into secondary positions

Abduct eyes to test superior and inferior rectus muscles as when eye is abducted to 23* the rectus muscles pull the eye vertically
Adduct eyes to test inferior (elevate) and superior oblique (depress) muscles as when eye is adducted to 51* the oblique muscles pull the eye vertically

Answer 170

A

Binocular vision importance in terms of depth perception

- If you’re looking at a close object, you need to overlap visual fields more than if you’re looking at a distant object

Answer 171

A

Versions:

The eyeballs move in the same direction
The lines of sight of each eyeball remain parallel
E.g. supraversion (up), infraversion (down), dextroversion (right) and levoversion (left)

Vergences:

The eyeballs move in opposite directions
The lines of sight of each eyeball do not remain parallel
E.g. convergence and divergence
E.g. infravergence and supravergence
(when you look at something further away your eyes diverge compared to a closer object when they converge)

Answer 172

A

oculomotor

Answer 173

A

they all pass through the cavernous sinus and leave the cranial cavity through the superior orbital fissure

Answer 174

A

Somatomotor nuclei
SR (superior rectus), MR, IR, IO & LPS (nucleus divided into subnuclei which each deal with a different muscle – each with one of these muscles)
Receives fibres from superior colliculus – therefore supplied by information from visual cortex
Receives fibres (internuclear neurones) from medial longitudinal fasciculus – therefore connected to nuclei of IV, VI and VIII
Important for co-ordinating eye movements
There’s the main oculomotor nuclei and a Edinger-Westphal nucleus

Answer 175

A

Superior branch:

Superior rectus (contralateral nucleus)
Levator palpebrae superioris (both left and right)

Inferior branch:

Medial rectus (cell body in ipsilateral nucleus)
Inferior rectus (ipsilateral)
Inferior oblique (ipsilateral)
Ciliary muscle for accommodation *
Smooth muscles of the iris for pupil constriction*
parasympathetic fibres from E-W nuclei; all others from main nuclei

Answer 176

A

Aka E-W or accessory parasympathetic nuclei

Visceral motor nuclei parasympathetic
Posterior to main nuclei
Nuclei receive corticonuclear fibres for accommodation reflex
Also receive fibres from pretectal nucleus for direct and consensual light reflexes

Answer 177

A

Fibres synapse in ciliary ganglion
Postganglionic fibres in short ciliary nerves (which pierce the sclera go on to innervate structures in the eye – intraocular muscles)

Answer 178

A

Lens accommodation by innervating ciliary muscles
Innervate the constrictor pupillae
constrict using parasympathetic fibres
pupillary constriction happens when objects become nearer

Answer 179

A

Lines of sight converge (need to overlap fields more to obtain binocular vision)
Pupils constrict (to prevent spherical aberration – so less light waves scattering off the coroid and hitting the retina in the peripheral parts of it and distorting the image that you see)
Change focal length by altering shape of lens

Answer 180

A

(with objects nearer us we want to refract the light waves more)

Ciliary muscles contracts
Tension removed from suspensory ligaments
Tension removed from lens
Lens bulges antero-posteriorly (bends light waves more so they hit the retina)

Answer 181

A

Originate from neuronal cell bodies at the border of pons and midbrain
Located in grey matter surrounding cerebral aqueduct
Receives fibres from superior colliculus – therefore supplied by information from visual cortex
Receives fibres from medial longitudinal fasciculus – therefore connected to nuclei of III, VI and VIII
Receive corticonuclear fibres from both cerebral hemispheres

TROCHLEAR NERVE: MOTOR FUNCTION
- Innervates: superior oblique only (contralateral nucleus)

Answer 182

A

Originates from neuronal cell bodies beneath floor of fourth ventricle
Receives fibres from superior colliculus – therefore supplied by information from visual cortex
Receives fibres from medial longitudinal fasciculus – therefore connected to nuclei of III, IV and VIII
Receive corticonuclear fibres from both cerebral hemispheres
Internuclear neurones to contralateral main oculomotor nucleus via MLF important

Answer 183

A

double vision

don’t precisely overlap our visual fields

Fatigue
Cranial nerve dysfunction (e.g. lesions)
Raised intracranial pressure
Cerebellar (coordinates contractions of skeletal muscles) dysfunction
Blow-out fractures of the orbit

Answer 184

A

SENSORY CODING

The activity of axons in sensory nerves ‘codes’ information from the sense organs
anatomical coding
temporal coding

ANATOMICAL CODING

Different nerves represent different sensory modalities
Distinctions between stimuli of the same modality (e.g. arising from different spatial locations)

TEMPORAL CODING
- Rate of firing of axons represents (‘encodes’) stimulus intensity

Answer 185

A

The systematic study of the relation between the physical characteristics of stimuli and the sensations they produce

Used to measure:
• Absolute threshold
-minimum level of a stimulus that can be detected
• Difference threshold (‘just-noticeable difference’)
-minimum detectable difference between 2 stimuli

Answer 186

A

higher-level cognitive processes such as expectations

Answer 187

A

The adjacency/proximity principle
elements of a visual scene that are close are grouped together
The similarity principle
similar elements are perceived as belonging together
Good continuation
elements that smoothly follow a line tend to belong together
The law of closure
missing information is supplied to close or complete a figure
The principle of common fate
elements on the same movement trajectory belong together

Answer 188

A

Templates
stored visual memories of patterns compared with visual input
Prototypes
flexible, idealised stored patterns compared with visual input
Feature detection models
distinctive features model Geons (certain no. of shapes that any shape can be constructed from)

Answer 189

A

We don’t attend to everything that our sensory system detects even though this can have fatal consequences
Our capacity for conscious processing of information is limited – selective attention is responsible for allocating this limited resource of awareness

Answer 190

A

Can be ‘bottom-up’ or ‘top-down’:

Bottom-up = reflexively/automatically, e.g. in response to a stimulus appearing suddenly
Top-down = consciously controlled movement

Answer 191

A

Density of rods decreases as you go more peripheral of retina – so more peripheral retina becomes less sensitive to light

Very few cones outside of the macula region

Answer 192

A

if there’s a ganglion cell on one side of this line, its axons will stay on the same side as the horizontal line and go to the optic disc

HORIZONTAL RAPHE

Visual field abnormalities that do not cross the horizontal midline are of retinal origin (compare with lesions originating in brain)
E.g. glaucoma – problem with nerve in the eye

Answer 193

A

1% ganglion cells are also photoreceptors – melanopsin based – they’re not involved with vision itself (visual acuity) but they have other roles
They project to all 4 subcortical regions
Concerned with:
circadian rhythms
pupil responses
adaptive responses to overall lighting conditions

Answer 194

A

where fibres from inferior retina enter the temporal lobe (the fibres loop forward before they go backward)

Answer 195

A

Binocular vision gives you depth perception and stereopsis (the single perception of a slightly different image from each eye)
Due to decussation in optic chiasm
You get monocular vision if you go out to the far periphery of the visual field
Eyes are aligned so image in nasal field of one eye matches image in temporal field in the other eye – creating binocular visual field

Answer 196

A

inferotemporal visual field defect

Answer 197

A

the decussating fibres (nasal retina) so you’re losing temporal visual field = bitemporal hemianopia

Answer 198

A

HOMONYMOUS HEMIANOPIAS (RETROCHIASMAL LESIONS)

Visual field defect in the temporal side of one eye and the nasal of the other
Right homonymous hemianopia – may be a lesion of the optic tract
Lesion in visual cortex – can sometimes get a degree of sparing of macular vision

Answer 199

A

Just affecting one of the four quadrants
Superior visual field loss – optic radiations in Meyer’s loop (temporal lobe) affected
Inferior visual field loss – optic radiations in parietal lobe affected

Answer 200

A

The nerves that represent corresponding points in the 2 eyes (points which view the same object) get closer and closer together as they pass through the visual pathway
Lesions of the visual cortex, therefore, often produce perfectly congruous (identical in both eyes) visual field defects while those from the optic tracts often produce incongruous defects

Answer 201

A

Bitemporal hemianopia
Homonymous hemianopia
Homonymous quadrantanopia
Homonymous hemianopia with central sparing
Damage to optic nerve -> loss of field in one eye
Damage to retina -> loss of field in one eye
Lesion affecting chiasm typically leads to bitemporal visual field loss
Retrochiasmal lesion (behind the chiasm in the visual pathway) -> homonymous hemianopia
Lesion of Meyer’s loop -> homonymous quadrantanopia
Lesion in visual cortex -> homonymous hemianopia with central sparing

Answer 202

A

Retinal
supplies inner retina
disrupted in glaucoma
Choroidal
supplies photoreceptors
disrupted by retinal detachment

Answer 203

A

OPSIN PROTEIN SIGNAL AMPLIFICATION: THE PHOTOTRANSDUCTION CASCADE
Photon absorption -> G-protein (transducen) dissociation -> phosphodiesterase activation -> closure of cGMP-gated channels
- Cascade allows huge signal amplification allows meaningful response to single photon absorption
- Each activated opsin can activate lots of transducen, each activated transducen can activate lots of phosphodiesterase, each phosphodiesterase can hydrolyse lots of cGMP, which causes a big change in membrane conductance, even with absorption of a single photon of light

Answer 204

A

Rods capture more photons
They have larger signal amplification
They are more sensitive

Answer 205

A

Sign inverting – inhibitory synapses

Sign conserving – excitatory synapses

Answer 206

A

‘on’ bipolar cells have metabotropic glutamate receptors

- ‘off’ bipolar cells have ionotropic glutamate receptors

Answer 207

A

On centre ganglion cells: excited when light intensity in centre of receptive field higher than surround
Off centre ganglion cells: excited when light intensity in centre lower than surround

Answer 208

A

CENTRE:SURROUND – RED:GREEN COLOUR 
Green:
-	400nm 
-	Balance of inhibitory input > excitatory 
-	Ganglion cell inhibited by ‘green’ 
Red:
-	600nm 
-	Balance of inhibitor input < excitatory 
-	Ganglion cell excited by ‘red’

BLUE: YELLOW COLOUR
- Ganglion cell connected to blue cone by ON bipolar and red & green cones by OFF bipolar

Blue:
-	430nm 
-	Balance of inhibitory input < excitatory 
-	Ganglion cell excited by ‘blue’ 
Yellow:
-	570nm 
-	Balance of inhibitory input > excitatory 
-	Ganglion cell inhibited by ‘yellow’

Answer 209

A

Does not simply report the amount of light falling on the photoreceptors
Rather encodes ‘visual information’:
spatial contrast in ‘grey scales’ (centre:surround) and/or colour
movement in a particular direction
local motion
‘brightness’ (luminance)

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Case 3 Flashcards

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