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MD1 Neuroscience > Visual System > Flashcards

Visual System Flashcards

1
Q

What forms the supraorbital margin?

A

Frontal bone (with supraorbital notch for the supraorbital nerve)

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2
Q

What forms the infraorbital margin?

A
  • Zygomatic bone (laterally)
  • Maxilla (medially)
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3
Q

What forms the roof of the orbit?

A

Frontal bone, lesser wing of sphenoid

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4
Q

What forms the floor of the orbit?

A

Maxilla, zygomatic, palatine

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5
Q

What forms the lateral wall of the orbit?

A

Zygomatic, lesser wing of sphenoid

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6
Q

What forms the medial wall of the orbit?

A

Maxilla, lacrimal bone, ethmoid, body of sphenoid

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7
Q

Where will blowout fractures to the orbit normally occur?

A

To the lacrimal bone or ethmoid bone, as they are extremely thin and fragile.

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8
Q

What proportion of the eyeball is sclera?

A

5/6th of the eyeball

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9
Q

What does the sclera do?

A

Maintains the shape of the globe, offering resistance to internal and external forces

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10
Q

What two characteristics make the sclera strong?

A
  1. Made of collagen
  2. Laid down in whirls
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11
Q

What does the sclera provide attachment for?

A

Extraocular eye muscles

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12
Q

What are the 3 layers to the eye?

A
  1. Sclera and cornea
  2. Vascular supply
  3. Neural part
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13
Q

What proportion of the eyeball is cornea?

A

1/6th

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14
Q

What is the principal refracting component of the eye?

A

Cornea

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15
Q

What makes the cornea optimum for vision?

A

Being avascular and transparent

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16
Q

Why is the cornea transparent?

A

Because the collagen fibrils are uniform in diameter, evenly spaced and run in bundles (lamellae) parallel to one another.

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17
Q

What does scleral opacity vary depending on?

A
  • Composition of the stroma
  • Hydration
  • Size and distribution of collagen
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18
Q

What will happen to the order of collagen fibrils in the cornea if it is damaged?

A

The order will not necessarily return and can result in a scar

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19
Q

What is the anterior chamber of the eye?

A

Junction between iris and cornea

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20
Q

What happens at the anterior chamber of the eye?

A

Aqueous humour drains out of eye

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21
Q

What are the key structures of the anterior chamber of the eye?

A
  • Cornea
  • Trabecular meshwork
  • Canal of Schlemm
    • Aqueous humour drains into venous system via canal of Schlemm
  • Ciliary body
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22
Q

What happens at the Canal of Schlemm?

A

Aqueous humour drains into the venous system

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23
Q

What forms the middle coat of the eye?

A

Uvea

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24
Q

What forms the middle layer of the eye?

A

Ciliary body, sclera and conjunctiva

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25
Q

What is the function of the ciliary body?

A
  • Formation of aqueous humour (Ciliary epithelium)
  • Tethers lens (Ciliary processes)
  • Accommodation (Ciliary muscle)
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26
Q

What are the important structures within the ciliary body?

A
  • Ciliary processes (Ciliary epithelium): form aqueous humour and form attachment for lens
  • Ciliary muscle
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27
Q

What does the ciliary body produce?

A

Aqueous humour

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28
Q

What does the aqueous humour do?

A
  • Important for maintaining the health of the lens and cornea.
  • Creates intraocular pressure.
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29
Q

What structures does accommodation involve?

A
  1. Ciliary muscles
  2. Zonules to attach to lens
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30
Q

What are zonules?

A

Ligaments that attach between ciliary processes and lens.

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31
Q

Where is the ciliary muscle found?

A

Within the ciliary body

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32
Q

What is the ciliary muscle innervated by?

A

Parasympathetic NS

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33
Q

What kind of muscle is ciliary muscle?

A

Non-voluntary muscle (smooth muscle)

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34
Q

How does the ciliary muscle accommodate?

A

The circular fibres change the tension on the zonules, deforming the lens

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35
Q

What does relaxation of the ciliary muscle result in?

A

A thin lens that is deformed (distant objects)

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36
Q

What does contraction of the ciliary muscle result in?

A

A relaxed, fat lens (closer objects)

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37
Q

What is presbyopia?

A
  • Amplitude of accommodation varies with age
  • Prebyopia refers to the loss of accommodation with age.
  • Caused by reduction in flexibility of the lens capsule and zonules
  • Treated by the wearing of plus lenses
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38
Q

What is the iris?

A

Aperture of the eye

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39
Q

What two muscles comprise the iris?

A
  • Sphincter pupillae: constricts pupil: innervated by parasympathetic NS
  • Dilator pupillae: dilates pupil: innervated by sympathetic NS
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40
Q

What is the choroid?

A

3 layers of blood vessels underneath the retina that supply nutrients to the retina.

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41
Q

Which blood vessels in the choroid are the most important?

A

Most important is the choriocapillaris, which sits just below the retina.

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42
Q

What are the important components of the retina?

A
  • Optic nerve/optic disc
  • Fovea/foveola
  • Macular
  • Posterior pole
  • Orra serrate
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43
Q

What are the 2 specialised regions of the retina?

A

Fovea and optic nerve

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44
Q

What is the fovea?

A

An avascular area of high visual acuity due to a high density of cones (no rods) where everything is shifted to the side except photoreceptors.

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45
Q

What forms the optic nerve?

A

The axons of ganglion cells as they exit the retina to pass visual information to higher cortical areas.

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46
Q

What is the lamina cribrosa?

A

A band of 3-10 sheets of dense connective tissue that forms a sieve at where the optic nerve exits the eye through which axons must pass. Disease can damage this area and push on axons traversing it.

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47
Q

If a patient notices a “curtain come down” over their vision, what is a potential reason?

A

Arterial occlusion

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48
Q

Which artery supplies the inner retina?

A

Central retinal artery (branch of ophthalmic artery)

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49
Q

What are the ciliary arteries?

A
  • Long posterior ciliary
  • Short posterior ciliary
  • Anterior ciliary
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50
Q

Do the anterior ciliary arteries pierce the globe?

A

No. They supply structures at the front of the eyeball.

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51
Q

What do the posterior arteries travel in?

A

The choroid

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52
Q

Which artery supplies the outer retina?

A

Posterior ciliary artery

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53
Q

Which artery supplies the photoreceptors?

A

Posterior ciliary artery

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54
Q

What do the short posterior arteries supply?

A

Photoreceptors closer to optic nerve and nerve itself

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55
Q

What do the long posterior arteries supply?

A

Photoreceptors all the way around the retina

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56
Q

What are the eyelids comprised of?

A
  • Skin
  • Glands and eyelashes
  • Conjunctiva
  • Muscles:
    • Orbicularis oculi
    • Levator palpebrae superiosis
  • Lacrimal apparatus:
    • Lacrimal gland and ducts
    • Nasolacrimal sac and duct
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57
Q

What are the muscles of the orbit?

A
  • Orbicularis oculi
  • Levator palpebrae superiosis
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58
Q

What does levator palpebrae superiosis do?

A

Elevates the upper lid

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59
Q

What type of muscle is levator palpebrae superiosis?

A

Striated muscle

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60
Q

What is levator palpebrae superiosis innervated by?

A

CNIII (oculomotor)

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61
Q

What does orbicularis oculi do?

A

Depresses upper lid (sphincter muscle)

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62
Q

What type of muscle is orbicularis oculi?

A

Striated muscle

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63
Q

What is orbicularis oculi innervated by?

A

CNVII (facial)

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64
Q

Do levator palpebrae superiosis and orbicularis oculi work together?

A

No. NEVER!

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65
Q

What are the 3 basic layers to the eyeball? What is their function?

A
  1. Outer coat: cornea and sclera
    1. Function: strength
  2. Middle coat: uvea
    1. Function: nutrition
  3. Inner coat: retina
    1. Function: vision
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66
Q

What fundamentally limits visual acuity?

A
  1. Neural factors
  2. Optical factors
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67
Q

What is visual acuity?

A

Ability to resolve fine detail.

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68
Q

How is visual acuity tested?

A

By recognition of letters on a Snellen or LogMAR chart.

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69
Q

What result on a visual acuity test is considered legally blind?

A

6/60

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70
Q

What optical factors affect visual acuity?

A
  • Pupil size
  • Clarity of optical media
    • Cataracts, corneal opacities…
  • Refractive errors → blur
  • Myopia, hypermetropia, astigmatism, presbyopia
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71
Q

What is the best visual acuity at phototopic levels?

A

6/6

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72
Q

What type of photoreceptors are present at the fovea?

A

Cones

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73
Q

How far away from the fovea is demonstrable visual acuity loss?

A

5 minutes of arc away from fovea.

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74
Q

What is the best visual acuity at scotopic levels?

A

6/60

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75
Q

Where are rods (and rod pathways) found in the retina?

A

Between 5-15˚ away from the fovea

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76
Q

Where is the best peripheral vision in the retina?

A

At approx. 8 degrees off centre of the fovea.

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77
Q

What is visual acuity in the fovea determined by?

A

Number of cones

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78
Q

What are the 6 neurons of the retina?

A

Rods, cones, horizontal cells, bipolar cells, amacrine cells and ganglion cells.

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79
Q

What are the 2 synaptic layers of the retina?

A
  • Outer plexiform layer
  • Inner plexiform layer
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80
Q

What does light pass through before hitting photoreceptors?

A

All retinal layers

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81
Q

What are the properties of rods?

A
  • Night vision
  • Scotopic
  • Very sensitive
  • One type only
  • No colour vision
  • 100 million
  • Absent from fovea
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82
Q

What are the properties of cones?

A
  • Day vision
  • Photopic
  • Less sensitive
  • Three types (RGB)
  • Allow colour vision
  • 5 million
  • Densest in fovea
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83
Q

What is needed for the finest detail to be resolved?

A
  • A good optical system
  • Small, closely packed detectors
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84
Q

What is the “through” pathway?

A

Photoreceptors → bipolar cells → ganglion cells → optic nerve

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85
Q

What provides lateral interactions and modulations to the through pathway?

A
  • Horizontal cells
  • Amacrine cells
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86
Q

What are the first, second and third order neurons in the through pathway?

A
  1. Photoreceptor
  2. Bipolar cell
  3. Ganglion cell
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87
Q

What are the 10 different types of bipolar cells?

A
  • 1x rod bipolar cell
  • 9x cone-bipolar cells
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88
Q

What are bipolar cells important for?

A

Spatial vision & colour vision

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89
Q

What are OFF bipolar cells?

A

Bipolar cells that hyperpolarise when light falls on the retina.

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90
Q

What are ON bipolar cells?

A

Bipolar cells that depolarise when light falls on the retina.

91
Q

In which layer are bipolar cells found?

A

Inner nuclear layer

92
Q

What do horizontal cells receive input from?

A

Photoreceptors

93
Q

What do horizontal cells provide output onto?

A

Photoreceptors

94
Q

Which neurotransmitter do horizontal cells use?

A

Inhibitory neurotransmitter GABA.

95
Q

What effect do horizontal cells have on photoreceptors?

A

They hyperpolarise.

96
Q

What are amacrine cells?

A

Axonless cells that laterally inhibit bipolar cells by releasing inhibitory neurotransmitters glycine or GABA.

97
Q

What is in the ganglion cell layer?

A

Cell bodies of ganglion cells and some displaced amacrine cells.

98
Q

What are ganglion cells?

A

Ganglion cells are the main output neuron of the retina.

99
Q

What are the different types of ganglion cell?

A

ON, OFF, M and P

100
Q

What do ganglion cells release?

A

Glutamate

101
Q

Which cell of the retina is the only neuron to fire APs?

A

Ganglion cells

102
Q

True or false: vision is just about detection.

A

False. Vision is not just about detection, but about comparison and contrast.

103
Q

How do ganglion cells respond to light?

A

By increasing or decreasing their action potential firing rate.

104
Q

What is the receptive field of a ganglion or bipolar cell?

A

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

105
Q

What type of receptive field do ganglion cells have?

A

Concentric-surround

106
Q

What does the level of stimulation of a ganglion cell’s receptive field depend on?

A

Where in the receptive field there is stimulation - the further from the centre, the lower the stimulation.

107
Q

What do photoreceptors contain that responds to light?

A

Photopigments

108
Q

Which photopigment do rods contain?

A

Rhodopsin

109
Q

Which photopigment do cones contain?

A

One of three different cone-opsins

110
Q

What do opsins bind to?

A

Vitamin A (all-trans Retinal)

111
Q

What happens to photoreceptors when they are stimulated by light?

A

They hyperpolarise

112
Q

What do photoreceptors use as their neurotransmitter?

A

Glutamate

113
Q

Do photoreceptors fire action potentials?

A

No. They respond to light with graded changes in membrane potential.

114
Q

What determines the amount of neurotransmitter released by the photoreceptor?

A

Graded changes in the resting membrane potential.

115
Q

How do photoreceptors function in the dark?

A
  • cGMP gates a sodium channel causing continuous influx of sodium ions.
  • Causes depolarization of the cell.
116
Q

How do photoreceptors function in the light?

A
  1. In the light, cGMP breaks down to GMP
  2. cGMP no longer gates the sodium channels
  3. Flow of Na ions ceases
  4. Cell is hyperpolarized
117
Q

How does phototransduction work?

A
  1. Light activates rhodopsin
  2. Initiates a cascade of events that ultimately leads to the closure of cGMP gated sodium channels.
  3. Rh → Transducin → PDE → breaks down cGMP
  4. Closure of sodium channels→hyperpolarization
118
Q

What is the hyperpolarising signal form the photoreceptor communicated to?

A

ON and OFF bipolar cells at the same synapse.

119
Q

What creates black on white sensation?

A

OFF bipolar cells

120
Q

What creates white on black sensation?

A

ON bipolar cells

121
Q

What creates the centre-surround?

A

The central response - determined by the “Through” pathway (Ph-BC-GCs) - and the surround pathways - determined by inputs from horizontal cells. Horizontal cells create the centre-surround

122
Q

What creates the “surround?”

A
  • Horizontal cells receive input from many photoreceptors and provide output to other photoreceptors
  • Horizontal cells release GABA as their neurotransmitter
123
Q

What is the size of the surround determined by?

A

The extent of electrical coupling between horizontal cells.

124
Q

What is melanoma associated retinopathy?

A
  • Rare complication of melanoma.
  • Antibodies are produced directed against ON bipolar cells.
  • Patient treated with oral prednisolone
125
Q

What are the output neurons of the retina?

A

Ganglion cells

126
Q

What are the 2 different types of ganglion cells? What type of information do they provide?

A
  • M (midget ganglion cells): motion
  • P (parasol ganglion cells): colour vision, visual acuity
127
Q

What are the 5 targets of ganglion cells?

A
  1. Lateral geniculate nucleus (thalamus)
    1. Major target of most GCs
    2. Visual pathway
  2. Pretectum (midbrain)
    1. Pupil responses
  3. Suprachiasmatic nucleus (hypothalamus)
    1. Circadian rhythm
  4. Superior colliculus
    1. Eye movements
  5. Other: various nuclei of thalamus a. Photophobia
128
Q

What is the optic chiasm?

A

Where fibres from right and left optic nerves combine at the base of the brain, anterior to the pituitary.

129
Q

What happens at the optic chiasm?

A

Nasal visual field fibres decussate.

130
Q

Which hemisphere views the right visual field?

A

Left

131
Q

Which hemisphere views the left visual field?

A

Right

132
Q

Where is the pituitary gland located in relation to the optic nerve?

A

Just below the optic nerve/optic chiasm in the Turkish saddle.

133
Q

Where do most ganglion cells terminate?

A

Within the thalamus at the LGN.

134
Q

How is the lateral geniculate nucleus (LGN) organised?

A
  • Segregation of inputs by eye and ganglion cell type
  • Six layers (numbered 1-6)
135
Q

What cell types does the LGN contain and in which layers?

A
  • Magnocellular layers = layers 1-2
  • Parvocellular layers = layers 3-6
136
Q

Which layers of the LGN is the right eye represented in?

A

2, 3 and 5

137
Q

Which layers of the LGN is the left eye represented in?

A

1, 4 and 6

138
Q

How does information get from the LGN to the primary visual cortex?

A

Via the optic radiations (this is the second order neuron)

139
Q

Where is the primary visual cortex located?

A

BA17 in the occipital around the calcarine fissure

140
Q

Where is each half of the visual field represented?

A

On the contralateral visual cortex

141
Q

What is retinotopic organisation?

A

Neighbouring cells within the retina project to neighbouring cells in the LGN & visual cortex.

142
Q

How will a lesion in the right optic nerve affect vision?

A

Loss of vision from the left and right visual fields of the right eye.

143
Q

How will a lesion in the optic chiasm affect vision?

A

Bilateral loss of vision from the temporal visual fields

144
Q

How will a lesion in the right optic tract affect vision?

A

Bilateral loss of vision in the left visual fields of each eye

145
Q

How will a lesion in the superior part of the right optic radiation affect vision?

A

Bilateral loss of vision in the left superior visual field

146
Q

How will a lesion of the entire right optic radiation affect vision?

A

Bilateral loss of vision of the entire left visual field

147
Q

How do migraine auras often manifest?

A

As visual auras

148
Q

What effect does light exposure have on migraine?

A

Worsens pain

149
Q

What is migraine pain thought to be due to?

A

Nerves signalling from the dura (blood vessels in the meninges)

150
Q

Which nerve carries pain from the dura?

A

Trigeminal nerve

151
Q

What is the pain pathway for migraine?

A

Dura → trigeminal nerve → brainstem → posterior nucleus of the thalamus

152
Q

Where in the thalamus does the migrain pain pathway terminate?

A

Posterior nucleus of the thalamus

153
Q

Where do ipGCs project to?

A

Neurons in the posterior nucleus of the thalamus that are light sensitive.

154
Q

What are ipGCs?

A

Intrinsically photosensitive GCs.

  • A small population of GCs containing a visual pigment called melanopsin.
  • Melanopsin is similar to visual pigments found in invertebrates.
  • Light activation of melanopsin leads to depolarization of ipGCs.
155
Q

What are the non-vision-related functions of ipGCs?

A
  • Circadian rhythm
  • Sleep regulation
  • Pupil responses
  • General information about light levels
  • Light Allodynia (photophobia associated with migraine, ocular injury or infection)
156
Q

What happens if light is shone into one eye?

A

Both pupils will constrict

157
Q

What does the pupil response to light depend on?

A

The retina being able to detect light and the iris functioning.

158
Q

Which two muscles control the iris?

A

Dilator pupillae Sphincter pupillae

159
Q

What is the function of ipGCs in pupil responses?

A
  • Melanopsin GCs project to the optical pretectal nucleus (OPN)
  • Provides the retinal input to the brainstem that controls pupil responses
160
Q

What is the function of ipGCs in circadian rhythm?

A

ipGCs project to the suprachiasmatic nucleus (SCN) in the hypothalamus, which is important for driving circadian rhythm.

161
Q

What are the inputs to V1?

A

M and P cells

162
Q

How do the M and P cells arrive at V1?

A

Segregated into M and P pathways, inputting into different strata of layer IV in V1

163
Q

Which layers of V1 do M cells terminate in in V1?

A

Layer 4Cα

164
Q

Which layers of V1 do P cells terminate in in V1?

A

Layer 4Cβ

165
Q

What are the outputs of V1?

A
  • Layer 3 and IVβ: other cortical areas
  • Layer 5: superior colliculus and pons
  • Layer 6: LGN
166
Q

Which V1 layer projects to the LGN?

A

Layer 6

167
Q

Which V1 layer projects to the superior colliculus and pons?

A

Layer 5

168
Q

Which sensory modality are M ganglion cells responsible for?

A

Motion

169
Q

Which layer of the LGN do M ganglion cells project to?

A

Magnocellular layers

170
Q

Are M ganglion cells wavelength sensitive?

A

No

171
Q

Where do M cells from the LGN project to?

A

IVCα

172
Q

Where do cells in layer IVCα project to?

A

Layer IVB

173
Q

When is there mixture of M ganglion cells?

A

After entering IVB in V1

174
Q

How does motion work in V1?

A

Some cells in layer IVB show orientation selectivity with preference for the direction of motion. Directional selectivity encodes for motion.

175
Q

What percentage of the cortex is involved in vision?

A

40%

176
Q

What are the two parallel visual streams?

A
  • Dorsal pathway: Where? - M cells
  • Ventral pathway: What? - P cells
177
Q

What is area MT?

A
  • An area in the middle temporal lobe (ventral stream) specialised for processing object motion.
  • Receives retinotopic information from cortical areas including V2 and V3.
  • Receives input from cells in layer IVB of V1.
  • Detects objects moving above a certain velocity.
178
Q

What type of selectivity are almost all cells in area MT?

A

Directionally selective

179
Q

What columns are present in area MT?

A

Direction-of-motion columns

180
Q

True or false: all neurons in area MT respond to the same type of motion.

A

False. They respond to different types of motion, e.g. drifting spots of lights.

181
Q

Which type of light will activate all cone photoreceptors?

A

White

182
Q

When will red and green cones be co-stimulated?

A

When light is between ~470-590nm.

183
Q

What colour is red always compared with?

A

Green.

Note: green is always compared with red.

184
Q

What colour is blue always compared with?

A

Yellow.

Note: yellow is always compared with blue.

185
Q

How is the colour red seen?

A

P ganglion cells exhibit a colour opponent centre-surround. Some P ganglion cells are excited by red falling on their centre and inhibited by green falling on the surround. The colour perceived is determined by the activity of ganglion cells. If red light falls on the centre of the ganglion cell and no green light on the surround, then the ganglion cell will be maximally excited. If green falls on the surround, the ganglion cell will be maximally inhibited by hyperpolarising horizontal cells.

186
Q

What will happen to the ganglion cell’s excitability if red light falls on the centre and the surround?

A

It will still be maximally stimulated because, whilst the green cones respond to red light, they respond less than red cones.

187
Q

Which part of the visual pathway is affected in blind sight?

A

The circuit from the LGN to area MT

188
Q

Which area of the visual cortex is the ventral stream?

A

Area V4

189
Q

Where does area V4 (the ventral stream) receive input from?

A

The blob and interblob regions of the primary visual cortex via V2

190
Q

What types of selectivity are neurons in V4 (ventral stream)?

A

Orientation selective and colour selective

191
Q

What area of the temporal lobe is a major output area of area V4?

A

Area IT (inferior temporal)

192
Q

What do neurons in area IT respond to? What is this important for?

A

Respond to a wide variety of abstract shapes and colours. Important for visual memory and perception, including perception of faces.

193
Q

What do stimuli that engage neurons for complex recognition rely on?

A

Addition/summation of stimuli along the pathway.

194
Q

What percentage of the population have colour vision deficiencies?

A

8% of males, 0.5% of females.

195
Q

What are the 3 different groups of colour vision deficiency?

A
  • Monochromacy: people have only one type of cone.
  • Dichromacy: sufferers have only two functional cones.
  • Anomalous trichromacy (most common): all three cones function, but one expresses abnormal pigment and doesn’t work the same as normal cones.
196
Q

What are the types of inherited colour vision deficiencies?

A
  • Protan:
    • Protanope-no red cone
    • Protanomal-abnormal red cone
  • Deutan
    • Deutanope-no green cone
    • Deutanomal-abnormal green cone (most common of all colour vision defects)
  • Tritan
    • Tritanope-no blue cone
    • Tritanomal-abnormal blue cone
197
Q

What do protanopes need in order to be able to see traffic lights properly?

A

Need colours to be brighter, so that they can see the change in brightness.

198
Q

What is the preliminary means of diagnosing colour blindness?

A

Pseudoisochromatic plates

199
Q

What are the 2 components involves in the control of gaze?

A
  • Oculomotor system:
    • Moves the eyes in the orbit (whilst head is still)
    • Involves extraocular muscles and neural pathways that coordinate movement of each eye.
  • Head-movement system:
    • Moves the eye sockets as a whole (whilst head moves).
      • Involves vestibular system as well as oculomotor system
200
Q

What are the 5 different types of eye movements?

A
  1. Saccadic eye movements: shifts fovea rapidly to a new visual target
  2. Smooth pursuits: keeps the image of a moving target on the fovea
  3. Vergence: moves eyes in opposite directions
  4. Vestibular ocular: holds image still on the retina during brief head movements.
  5. Optokinetic: holds the image stationery during sustained head rotation or translation.
201
Q

What are the 6 extraocular eye muscles?

A
  • 4 rectus muscles: superior, inferior, medial, lateral
  • 2 obliques: inferior, superior
202
Q

What are the different movements of the eyes?

A

Elevation, adduction, abduction, depression, intorsion and extorsion.

203
Q

What are the primary actions of the rectus extraocular muscles?

A
  • Superior Rectus: Elevation
  • Inferior Rectus: Depression
  • Medial rectus: Adduction
  • Lateral rectus: Abduction
204
Q

What are the primary actions of the oblique extraocular muscles?

A
  • Superior oblique: intorsion
  • Inferior oblique: extorsion
205
Q

Where do the oblique extraocular eye muscles insert?

A
  • Superior oblique: behind the equator at an oblique angle close to the MR.
  • Inferior oblique: behind the equator close to the MR.
206
Q

Why do multiple muscles elevate and depress the eyes?

A

Because of the insertion of muscles into the eyeball.

207
Q

When are each of the extraocular eye muscles tested during the eye movement test?

A

See image.

208
Q

How is the heirarchical control of eye movements organised?

A
  • Lower motor neurons (cranial nerves)
  • Brainstem eye movement centres (reticular formation)
  • Higher cortical areas (Frontal eye fields)
209
Q

Which nerves innervate the extraocular eye muscles?

A
  • Oculomotor nerve:
    • innervates SR, IR, MR, IO
  • Abducens nerve (CNVI)
    • innervates LR
  • Trochlear nerve (CNIV):
    • innervates SO
210
Q

How are the movements from one eye coordinated with the other?

A
  • Medial longitudinal fasciculus (MLF):
    • White matter tract that connects the various cranial nerve nuclei
  • Reticular formation
    • Pontine paramedian reticular formation
    • Mesencephalic paramedian reticular formation
211
Q

What are the two eye movement control centres in the brainstem?

A
  • Pontine paramedian reticular formation
    • Horizontal gaze centre
    • Coordination of MR and LR of each eye
    • Coordination of CNIII & CNVI
  • Midbrain RF contains
    • Vertical gaze centre
    • Coordination of SO and SR of each eye
    • Coordination of CNIV and CNIII
212
Q

How are horizontal saccades controlled neurally?

A
  • Pontine Paramedian reticular formation
    • Burst neurons
      • Fire at high frequency just before movement.
      • Several types:
        • Provide excitatory connections with ispilateral abducens.
        • Inhibitory burst neurons suppress the activity of the contralateral abducens
    • Omnipause neurons
      • Fires continuously during the saccade.
      • GABAergic
      • Project to contralateral abducens nucleus
213
Q

What is the circuit for neural control of cascades?

A
  • Requires simultaneous excitation of burst neurons and inhibition of omnipause neurons
  • Excitatory Burst neurons receive input from Cortex.
  • Inhibitory burst neurons inhibit contralateral abducens nucleus
214
Q

What are eye movements ultimately driven by?

A

Behaviour

215
Q

Which upper motor neurons are involved in driving the gaze centres?

A
  • Frontal eye fields, and posterior parietal cortex.
  • FEF controls saccades in the contralateral direction
  • Superior colliculus
  • Basal ganglia
216
Q

What is the vestibulo-ocular reflex?

A

The ability to maintain focus on stationary object while moving head without loss of visual focus or dizziness.

217
Q

How are the eyes coordinated with the head movements?

A
  • Vestibular system provides information about the position of the head in space.
  • Coordinates the position of the head and eyes.
  • Semicircular canals & otolith organs (saccule & utricle)
    • Semicircular canals: head position
    • Otolith: linear acceleration
218
Q

What are the components of the vestibular apparatus?

A
  • Semicircular canals & otolith organs (saccule & utricle)
  • Semicircular canals: head position
  • Otolith: linear acceleration
219
Q

How does transduction of head movements work?

A

See image.

220
Q

How are head movements transduced into neural signals in the semicircular canals?

A

See image.

221
Q

What is vestibular information carried by?

A

CNVIII (vestibulocochlear nerve) to vestibular nuclei in the medulla (#6).

222
Q

Which cranial nerves must be recruited to coordinate head and eye movements?

A

To coordinate head and eye movements information from the vestibular nuclei must be coordinated with the CNIII, CNVI nuclei.

223
Q

Describe the entire vestibular-ocular reflex involved in turning the head to the left.

A
  • Turning head left
  • Left horizontal semicircular canal-left vestibular nucleus- contralateral NVI-LR (of Right eye).
  • Another projection from NVI to NIII (via MLF)-medial rectus of LE.
  • To ensure speedy operation there is also a direct projection from left vestibular nucleus to CNIII-MR.
  • Inhibitory input from RIGHT side.

MD1 Neuroscience (23 decks)

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