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Y2 LCRS2 Neuroscience and Mental Health > Structure and function of the eye > Flashcards

Structure and function of the eye Flashcards

1
Q

What are the inner and outer corners of the eye referred to as?

A

Medial and lateral canthus

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2
Q

What name is given to the border between the cornea and the sclera?

A

Corneal limbus

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3
Q

Where is the palpebral fissure? What is the caruncle?

A

Palpebral fissue - edge of the top eyelid

Caruncle (remnant of third eyelid)

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4
Q

What is the clinical significance of the corneal limbus?

A

Common site for corneal epithelial neoplasm (this is a darker edge, conains corneal epithelial stem cells)

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5
Q

Where does the tear duct end? Why does nothing come out of the punctum when you sneeze?

A

Inferior nasal meatus of the nasal cavity

Nothing comes out when you sneeze and tears also do not reflux because there is a valve between the canaliculli and the tear sac.

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6
Q

What is the afferent and efferent pathway of lacrimation (in response to irritation)?

A
  • Afferent – Cornea – CN V1 (ophthalmic)
  • Efferent – Parasympathetic
  • Neurotransmitter = ACh
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7
Q

Recall the drainage of tear film.

A
  • Tear made by Lacrimal Gland
  • Drains through two puncta = opening on medial lid margin
  • Flows through superior + inferior canaliculi
  • Gather in tear sac
  • Exits sac via tear duct (nasolacrimal duct, opens into inferior meatus) into nose cavity
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8
Q

Recall 3 functions of tear film

A
  1. Bactericide
  2. O2 and nutrient supply to cornea because a normal cornea has no blood vessels
  3. Smooth cornea-air surface mainenance
  4. Removal of debris (tear film+blinking)
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9
Q

Recall the layers of tear film from deep to superficial

A

Mucinous, Aqueous, Superficial oily

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10
Q

Describe the fx of each layer of tear film, including where each layer is produced.

A
  • Superficial Oily =reduce tear film evaporation (produced by a row of Meibomian Glands along lid margins)
  • Aqueous Tear Film (made by tear gland) – very thick layer
  • Mucinous Layer on corneal surface to maintain surface wetting – (produced by goblet cells) viscous layer which physically protects the surface of the eye
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11
Q

Which layer in the tear film protects the tear film from rapid evaporation?–

A) Lipid Layer

B) Water Layer

C) Mucinous Layer

D) All Three Layers

A

A

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12
Q

Where is the mucous layer of tear film produced?

A

Goblet cells in conjunctiva

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13
Q

Where is the oily layer of tear film produced? What happens when these get infected?

A

Meibomian glands within eyelids

  • Infection –> red bump on eyelid=stye
  • If the infection becomes encapsulated and cystic then it is called a chalazion (which needs to be taken out surgically)
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14
Q

What are the features of the conjunctiva?

A
  • Thin + transparent
  • Begins at outer edge of cornea and lines the INSIDE OF THE EYELIDS (so you can’ t put your finger behind your eye)
  • Nourished by tiny blood vessels nearly invisible to the naked eye
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15
Q

What surrounds the retina?

A

Uvea

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16
Q

What is the uvea made up of?

A

Predominantly vascular

  1. Choroid
  2. Ciliary body
  3. Iris
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17
Q

What does the choroid lie between?

A

Retina and sclera

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18
Q

What is the choroid made up of?

A

Vasculature supplying the lateral parts of the retina

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19
Q

What is the main component of the sclera?

A

Collagen

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20
Q

Recall and describe the most significant osmotic gradient that is maintained in the eye

A

Sclera = high H2O,

cornea = low H2O

Sclera has protective fx

Cornea must remain transparent

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21
Q

How is the transparency of the cornea maintained?

A

Corneal endothelium actively removes water

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22
Q

In emmetropia, what provides the refractive power of the eye?

A

Cornea = 2/3 of power, lens = 1/3

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23
Q

Recall one advantage and one disadvantage of the barrier function of the cornea?

A

Ad: prevents infection Disad: prevents drug entry

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24
Q

What are the 5 structures that make up the cornea, working superficial to deep?

A

Epithelium

Bowman’s membrane

Stroma

Descemet’s membrane

Endothelium

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25
Q

Which element of the cornea cannot regenerate?

A

Endothelium (one single layer, endothelial cell density decreases with age)

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26
Q

What characteristic of the lens makes it transparent?

A

Regular structure

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27
Q

Recall the pathophysiology of age-related cateracts

A

Hydration of lens –> loss of transparency

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28
Q

Describe the shape of the lens

A

Aspheric: anterior and posterior surfaces have differing curvature

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29
Q

Recall the attachments of the lens

A

Attached all around by “zonules of Zinn”, anchored to ciliary body = allow the eye to focus by controlling the lens

These are made of passive connective tissue

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30
Q

Recall what happens when the ciliary muscles contract?

A

Pressure on zonules decreased, lens gets smaller and thicker

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31
Q

Recall what happens when the ciliary muscles relax?

A

Pressure on zonules increases, lens gets wider and thinner

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32
Q

How can the optic nerve be identified with an ophthalmoscope?

A

Optic nerve head appears as bright optic disc

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33
Q

What is the optic nerve composed of?

A

Axons coming from the retinal ganglion cells

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34
Q

Where is the macula?

A

Roughly in centre of retina

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35
Q

Where is the aqueous humour found?

A

Anterior segment of eye (in front of lens)

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36
Q

Where is the vitreous humour found?

A

Posterior segment of eye (behind lens)

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37
Q

What is the fx of aqueous humour?

A

Nutrient and oxygen delivery

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38
Q

Recall the flow of aqueous humour. Where is it produced?

A

Produced by ciiary body –> anterior chamber –> trabecular meshowrk (lies in limbus) –> Schlemm’s canal/ uveal-scleral flow

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39
Q

Describe the 2 methods of aqueous humour drainage

A
  1. Schlemm’s canal (80-90%)= modified vein
  2. Uveal-scleral flow (~20%) = passive, PG analogues act here (but since US flow only makes up 20% of drainage, max reduction in pressure from prostaglandin analogues is 20%)
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40
Q

Describe the composition of vitreous humour

A

99% water, collagen, GAGs

41
Q

What is the fx of vitreous?

A

Mechanical support

42
Q

What is a normal IOP?

A

12-21mmHg

43
Q

Recall the names of the layers and sublayers of the retina

A

Layers: retinal pigment epithelium and neuroretina Sublayers of neuroretina: outer/ middle/ inner

44
Q

Recall the composition of each of the layers of neuroretina

A

Outer: photoreceptors Middle: bipolar cells Inner: ganglion cells

45
Q

Recall 2 functions of the retinal pigment epithelium

A
  1. Waste removal 2. Transport nutrients from choroid to photoreceptor cells
46
Q

Define glaucoma.

A

Optic neuropathy with characteristic structural damage to the optic nerve, associated with progressive retinal ganglion cell death, loss of nerve fibres and visual field loss

OR “medical condition of sustained raised intraocular pressure(risk factor)”

NB: aqueous and pressure are not in the definition because it is a disease of retinal ganglion cells

47
Q

Recall the pathophysiology of glaucoma and name the two different types.

A

Sustained raised intraocular pressure –> retinal ganglion cell death and optic disc cupping due to loss of ganglion nerve fibres –> hollowing out of optic nerve head

Peripheral vision is lost progressively

  1. Primary open angle glaucoma
  2. Closed angle glaucoma
48
Q

What are the differences between open and closed angle glaucoma?

A

Primary Open Angle Glaucoma (Left) – Commonest

  • Trabecular Meshwork Dysfunction
  • Generally asymptomatic until advanced stages
  • Patient will see normally because the brain is filling out missing patches

Closed Angle Glaucoma – acute or chronic:

  • Increased IOP –> iris/lens pushed forwards –> blocking trabecular meshwork – vicious cycle
  • May present with sudden painful red eye with acute drop in vision
49
Q

What are the risk factors for closed angle glaucoma? How can it be treated?

A
  • Risk factors = small eye (hypermetropia), narrow angle at trabecular meshwork
  • Laser iridotomy –> creates a drainage hole on the iris
50
Q

Recall the differences between rod and cone photoreceptors

A

Rods

  • Longer outer segment
  • 100x more sensitive to light than cones
  • Slow response to light
  • Night vision- “Scotopic vision”
  • 120M rods

Cones

  • Less light sensitive
  • But faster response to light
  • Day light vision and colour vision - “Photopic vision”
  • 6M cones and 3 types: S for blue, M for green, L for red
51
Q

Where is the highest density of rods?

A

20-40 degrees away from the fovea

52
Q

What is the fx of bipolar cells?

A

Fine-tuning of afferent info

53
Q

Where do retinal ganglion cells synapse with 4th order neurons?

A

Lateral genticulate nucleus

54
Q

What are the 2 oritentations of retinal ganglion cell, and what is the relevance of this

A
  1. On-centre: stimulated by light falling on centre of its receptove field and inhibited by light falling on edge 2. off-centre: opposite
55
Q

What is the receptive field of a neuron?

A

Retinal space within which incoming light can alter the firing pattern of a neuron

56
Q

Which type of photoreceptors have lower convergence, and what is the clinical relevance of this?

A

Cones = finer visual acuity as smaller receptive field

57
Q

What is the main test for colour-blindness? Describe it.

A

Ishihara test - colour perception test for R-G deficiencies only

Plates of circles of dots appearing randomly in size. Patients with normal red-green vision will see the 2 digit patterns and those without will not or will recognise the wrong pattern. The last slide of the test is orange on blue and you can tell if the person is faking.

58
Q

What are the 2 possible causes for colour-blindness?

A
  1. Shift in the peak of photo-pigment sensitivity
  2. Absence of 1 or more of the 3 cone photpigment subtypes
59
Q

Describe the mediation of light dark adaptation and vice versa.

A

Dark adaptation

  • Takes 30 min in total
  • It is a biphasic process - cone adaptation takes 7min and rod adaptation takes 30 min because rhodopsin has to be regenerated.

Light adaptation

  • Occurs over 5min
  • Bleaching or photo-pigments –> neuroadaptation –> inhibition of rod/cone function

Pupil adaptation also occurs

60
Q

Which type of photoreceptor adapts to changes from light to dark more quickly?

A

Cones- 7 mins

(compared to rods -30mins - this is because rhodopsin has to be regenerated)

61
Q

What is the index of refraction? What is the equation?

A

Index of refraction = a ratio comparing the two speeds of light through the medium.

n= (speed of light in vacuum)/(speed of light in medium);

Value will always be equal to or greater than 1 as the denominator will be smaller.

62
Q

Describe the two types of lenses.

A
  • Convex – “converging lens” – always has a real focal point (convergence)which is after the lens
  • Concave – “diverging lens” – focal point is virtual and before the lens
63
Q

What name is given to perfect vision?

A

Emmetropia = eye does not have any refractive error

  • Adequate correlation between axial length and refractive power(0)
  • Parallel light rays falls on the retina (when there is no accomodation in place)
64
Q

What is the mechanical problem in visual impairment?

A

Parallel lens converges to a point anterior/ posterior to the retina, or to more than one point

65
Q

What general term is given to eyes that have a refractive error?

A

Ametropia

66
Q

What 4 types of ametropia (refractive error) can result from mismatch between axial length of the eye and refractive power?

A

Parallel rays don’t fall on retina without accomodation:

  1. Myopia - near sightedness
  2. Hyperopia - farsightedness
  3. Astigmatism
  4. Presbyopia
67
Q

What term is used to describe convergence of the parallel light rays anterior to the retina? List some causes.

A

Myopia (short-sightedness)

Causes:

  • Excessive long globe (axial myopia)
  • Excessive refractive power(refractive myopia)

Measurements: -ve diopre values. E.g. -3 dioptres will mean focal point is 1/3m anterior to the retina.

68
Q

What are the symptoms of myopia? What is the treatment?

A
  • Blurred distance –> squinting
  • Headache

Treatment = CONCAVE lenses to push the focal point back.

69
Q

What term is used to describe convergence of the parallel light rays posterior to the retina? What are the causes?

A

Hyperopia (long-sightedness)

  • Excessive short globe (axial hyperopia)
  • Insufficient refractive power (refractive hyperopia) e.g. cornea is thinner or flatter than normal
70
Q

What are the symptoms of hyperopia?

A
  • Near visual acuity blurs early
  • Blurring more noticeable if the person is tired, printing is weak or light inadequate
  • Blurring copensated by accommodation in young people but this cannot happen all day –>
    • Asthenopic symptoms:
      • eye pain,
      • headache in frontal region,
      • burning sensation in eyes,
      • blepharoconjunctivitis.
71
Q

What sort of glasses are used to correct hyperopia?

A

Convex lense (or the lens can be removed and IOL can be inserted)

72
Q

Describe how amblyopia can result from hyperopia in childhood.

A

Ambylopia = lazy eye

Uncorrected hyperopia > 5 diopters in one eye–> in childhood development the brain learns to take in signals from one eye only –> lazy eye

73
Q

What is astigmatism?

A

Parralel rays focus on not one point but two

74
Q

What causes astigmatism? How is it treated?

A

Elliptical cornea (not spherical)

  • For regular astigmatism –> cylinder lenses
  • For irregular astigmatism –> rigid CL, surgery
75
Q

What are the symptoms of astigmatism?

A
  • Asthenopic symptoms (headache, eye pain)
  • Blurred vision
  • Distortion of vision
  • Head tilting and turning
76
Q

What term is given to a naturally-occuring loss of accommodation? How is it treated?

A

Presbyopia

(treated with convex lenses to increase refractive power because distant vision is intact)

77
Q

What causes presbyopia?

A

Stiffening of lens (occurs with age after 40_

78
Q

Describe the Near Response Triad.

A

Adaptation to near vision:

  1. Pupillary Miosis (Sphincter Pupillae) –> increase depth of field
  2. Convergence (medial recti from both eyes) to align both eyes to near object
  3. Accommodation (Circular Ciliary Muscle) –> increase refractive power of lens
79
Q

List the 4 types of optical correction available.

A
  • Spectacle lenses - monofocal (spherical or cylindrical) or multifocal
  • Contact lenses
  • Intraocular lenses - replacement of cataract crystalline lens. Best correction for aphakia. Avoids significant magnification and distortion caused by spectacle lenses
  • Surgical correction - keratorefractive surgery (laser), intraocular surgery (clear lens extraction and implantation of artificial lens)
80
Q

What are the advantages of contact lenses? Who are they indicated for?

A

Advantages:

  • better quality of optical image
  • less influence on size of retinal image than spectacle lenses

Indications:

  • irregular corneal astigmatism,
  • high anisometropia,
  • corneal disease
  • cosmetic, athletic activities, occupational,
81
Q

What are the complications of contact lenses?

A

Complications:

  • infectious keratitis,
  • giant papillary conjunctivitis,
  • corneal vascularization,
  • severe chronic conjunctivitis
82
Q

What are the cons of implanting artificial IOL?

A

Lose accommodation ability so the patient will need reading glasses

83
Q

Describe the nervous innervation of the cornea

A

Highly sensitive Ophthalmic branch of trigeminal nerve

84
Q

Describe the nerve supply to the lacrimal gland

A

PNS-mediated CNV

85
Q

Which layer in the tear film protects the tear film from rapid evaporation?–

A) Lipid Layer–

B) Water Layer–

C) Mucinous Layer

D) All Three Layers

A

A

86
Q

What sign in the eyes is indicative of intermittent hypertension?

A

Rupture of tiny vessels in conjunctiva

87
Q

What is the diameter of the eye?

A

Antero-posterior diameter = 24mm

88
Q

Why is the cornea such a powerful refracting surface?

A

Powerful refracting surface, providing 2/3 of eye’s focusing/refracting power

This is due to convex curvature and higher refractive index than air

89
Q

What is the function of the endothelium of the cornea? What happend with age?

A
  • Single cell layer with no regenerative ability
  • Function - pumps out excess fluid from the cornea
  • Pathophysiology - endothelial cell density decreases with age; endothelial cell dyfsunction –> corneal oedema + cloudiness.
90
Q

What provides nurients to the outer and inner part of the retina?

A

Outer part = choroid

Inner part = central retinal artery

91
Q

Describe the structure of the lens.

A
  • Outer acellular capsule
  • Inner elongated cell fibres which help transparency

(Loss of transparency of lens = cataracts)

92
Q

Which chamber of the eye does the lens occupy?

A

Anterior

93
Q

Compare central and peripheral vision. What tests are used for each?

A

Central vision =

  • day vision and colour vision – fovea has highest conc. of cone photoreceptors
  • Reading, Facial Recognition
  • _Assessed by Visual Acuity Assessment_
    • Loss of Foveal Vision –> poor visual acuity (but can still navigate because peripheral vision is present)

Peripheral vision =

  • movement and night vision due to rod cells
  • Navigation Vision
  • _Assessed by Visual Field Assessment_
    • Extensive loss of Visual Field –> unable to navigate in environment, patient may need white stick even with perfect visual acuity
94
Q

Why does the fovea form a pit in the centre of the macula?

A

There is absence of overlying ganglion cells

95
Q

What type of imaging is used to assess the macula and fovea?

A

Optical coherence tomography (OCT)

96
Q

What types of keratorefractive surgery are available?

A

RK, AK, PRK, LASIK, ICR, thermokeratoplasty

PRK - painful because the laser is directed directly into the cornea which has many nerve endings

LASIK - flap is cut in the cornea and laser is directd straigh onto lens BUT the flap never heas so can come off in trauma.

97
Q

Describe the accommodation mechanism. Which nerve is this mediated by?

A

Nerve: efferent oculomotor nerve

  1. Contraction of Circular ciliary muscles in ciliary body
  2. Relaxes zonules, which are normally stretched between ciliary body attachment and lens capsule attachment
    • Zonules are passive elastic bands with no active contractile muscle
  3. When no zonular tension, lens returns to natural convex shape due to innate elasticity
  4. Increases refractive power of lens
98
Q

In accommodation, which one of the following events does not take place?

–A) Relaxation of Circular Ciliary Muscle

–B) Relaxation of Zonules

–C) Thickening of Lens

–D) Increase of Lens Refractive Power

A

A

Y2 LCRS2 Neuroscience and Mental Health (17 decks)

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