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Neurology > Structure and function of the eye > Flashcards

Structure and function of the eye Flashcards

1
Q

Label the following diagram:

A
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2
Q

Label the following diagram:

A
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3
Q

What are the different types of tears produced by the lacrimal system?

A

Basal tears

Reflex tears

Crying (emotional) tears

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4
Q

What are basal tears?

A

Tears produced at a constant level, even in the absence of irritation or stimulation.

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5
Q

What are reflex tears?

A

Increased tear production in response to ocular irritation.

Afferent- cornea- CNV1 (ophthalmic branch of trigeminal nerve)

Efferent- parasympathetic

Neurotransmitter- acetylcholine

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6
Q

Describe the process of tear production.

A

Tear produced by the lacrimal gland (located within orbit, laterosuperior to the globe).

Tear film drains through the 2 puncta (tiny openings on upper and lower medial lid margins).

Tear flows through the superior and inferior canaliculi.

Both canaliculi converge as one single common canaliculus.

Tear gathers in the tear sac.

Tear exits the tear sac through the tear duct into the nasal cavity.

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7
Q

What is the purpose of the tear film?

A

Maintains smooth cornea-air surface.

Oxygen supply to cornea- normal cornea has no blood vessels.

Removal of debris (tear film and blinking).

Maintains clear vision.

Bactericide.

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8
Q

What are the 3 layers of the tear film?

A

Superficial oily layer to reduce tear film evaporation (produced by a row of Meibomian glands along the lid margins).

Aqueous tear film (tear gland)- delivers oxygen and nutrient to surrounding tissue; contains factors against potentially harmful bacteria.

Mucinous layer on the corneal surface to maintain surface wetting- mucin molecules bind water molecules to hydrophobic corneal epithelial cell surface.

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9
Q

What is the conjunctiva?

A

Thin, transparent tissue that covers the outer surface of the eye.

It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids.

It is nourished by tiny blood vessels that are nearly invisible to the naked eye.

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10
Q

Label the following diagram:

A
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11
Q

What is the average anteroposterior diameter of the eye in adults?

A

24mm

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12
Q

What are the 3 layers of the coat of the eye?

A

Sclera- hard and opaque, fibrous, protects eye and maintains shape.

Choroid- pigmented and vascular, provides circulation and shield out unwanted scattered light.

Retina- neurosensory tissue, converts light into neurological impulses to be transmitted to the brain via the optic nerve.

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13
Q

What is the sclera?

A

White of the eye

Tough, opaque tissue

Protective outer coat

High water content

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14
Q

What is the cornea?

A

Transparent, dome-shaped window covering the front of the eye

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

Low water content

Front-most part of anterior segment

Continuous with scleral layer

Convex curvature

Higher refractive index than air

Physical barrier

Infection barrier

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15
Q

What can prolonged contact lens wear result in?

A

Reduced oxygen supply to the cornea, compromising corneal tissue health.

If excessive, increases risk of serious corneal eye infection.

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16
Q

What are the 5 layers of the cornea?

A

Epithelium

Bowman’s membrane

Stroma

Descemet’s membrane

Endothelium

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17
Q

What is the role of the stroma in the cornea?

A

Regularity contributes towards transparency.

Corneal nerve endings provide sensation and nutrients for healthy tissue.

No blood vessels.

Thickest layer within the cornea.

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18
Q

What is the role of the endothelium in the cornea?

A

Pumps fluid out of cornea and prevents corneal oedema

Only 1 layer of endothelial cells

No regeneration power

Endothelial cell density decreases with age

Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness

Most posterior layer

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19
Q

What is the uvea?

A

Vascular coat of eye ball.

Lies between sclera and retina.

Composed of 3 parts: iris, ciliary body, choroid.

These 3 portions are intimately connected and a disease of one part also affects the others, though not necessarily to the same degree.

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20
Q

What is the choroid?

A

Lies between retina and sclera.

Composed of layers of blood vessels that nourish the back of the eye.

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21
Q

What is the iris?

A

Coloured part of eye

Controls light levels inside the eye- similar to aperture on a camera

Round opening in the centre is the pupil

Iris is embedded with tiny muscles that dilate and constrict the pupil size

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22
Q

Describe the structure of the lens.

A

Outer acellular capsule

Regular inner elongated cell fibres- transparency

May lose transparency with age- cataract (opaque lens)

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23
Q

What is the function of the lens?

A

Transparency- regular structure.

Refractive power- 1/3 power, higher refractive index than aqueous fluid and vitreous.

Accommodation- elasticity.

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24
Q

What are the lens zonules?

A

Lens is suspended by a fibrous ring- lens zonules.

Consists of passive connective tissue.

Anchors lens to ciliary body.

Surface of the lens is normally held flat and taut by tension along stretched lens zonules.

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25
Q

What is the retina?

A

Very thin layer of tissue that lines the inner part of the eye.

Captures light rays that enter the eye- like film in photography.

Light impulses are sent to the brain for processing via optic nerve.

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26
Q

What is the role of the optic nerve?

A

Transmits electrical impulses from the retina to the brain.

Connects to the back of the eye near the macula.

Visible portion is the optic disc.

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27
Q

Why does the blind spot exist?

A

Where the optic nerve meets the retina, there are no light-sensitive cells.

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28
Q

What is the macula and its role?

A

Located roughly in the centre of the retina, temporal to the optic nerve.

Small and highly sensitive part of retina.

Responsible for detailed central vision.

Fovea is the very centre of the macula.

Appreciate detail and perform tasks that require central vision, such as reading.

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29
Q

What divides the anterior and posterior segments of the eye?

A

The lens.

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30
Q

What are the features and functions of the anterior chamber of the eye?

A

Between cornea and lens.

Filled with clear aqueous fluid.

Supplies nutrients to surrounding tissue.

Within anterior segment.

Smaller than posterior chamber.

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31
Q

What is the ciliary body?

A

Ring-shaped tissue surrounding the lens.

Secretes aqueous fluid in the eye.

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32
Q

Describe the path of the intraocular fluid.

A

Cliary body secretes aqueous fluid in the eye.

Intraocular aqueous fluid flows anteriorly into the anterior chamber.

Aqueous fluid supplies nutrients to surrounding tissues.

Trabecular meshwork (canal of Schlemm) drains 80-90% fluid out of the eye.

Uveal-scleral outflow.

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33
Q

What is the normal intraocular pressure?

A

12-21mmHg

34
Q

What is glaucoma?

A

Medical condition of sustained raised intraocular pressure (risk factor).

Retinal ganglion cell death and enlarged optic disc cupping.

Visual field loss- peripheral vision goes first, blindness if untreated.

Results in gradual and accumulative damage to optic nerve tissue in the posterior segment of the eye.

35
Q

What are the types of glaucoma?

A

Primary open angle glaucoma (commonest).

Closed angle glaucoma- can be acute or chronic.

36
Q

What is primary open angle glaucoma?

A

Trabecular meshwork dysfunction.

Generally asymptomatic until advanced stages of disease.

37
Q

What is closed angle glaucoma?

A

Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork- vicious cycle.

May present with sudden painful red eye with acute drop in vision.

Can be treated with peripheral laser iridotomy to create a drainage hole on the iris.

38
Q

Give 2 risk factors for closed angle glaucoma.

A

Small eye (hypermetropia- long sightedness).

Narrow angle at trabecular meshwork.

39
Q

What is the fovea?

A

Most sensitive part of retina.

Has highest concentration of cones, but low concentration of rods.

Only the fovea has the concentration of cones to perceive in detail.

40
Q

What is central vision responsible for?

A

Detailed day vision, colour vision- fovea has the highest concentration of cone photoreceptors.

Reading, facial recognition.

Assessed by visual acuity assessment.

Loss of foveal vision = poor visual acuity.

41
Q

What is peripheral vision responsible for?

A

Shape, movement, night vision.

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.

42
Q

Describe the structure of the retina.

A

The retina forms the innermost layer of the coat of the eye in the posterior segment.

Outer layer- photoreceptors (1st order neuron), detection of light.

Middle layer- bipolar cells (2nd order neuron), local signal processing to improve contrast sensitivity, regulate sensitivity.

Inner layer- retinal ganglion cells (3rd order neuron), transmission of signal from the eye to the brain.

43
Q

What is the macula lutea?

A

Yellow patch.

Pigmented region at the centre of the retina of about 6mm in diameter.

44
Q

What are the 2 main classes of photoreceptors in the retina?

A

Rods and cones.

45
Q

What are the features of rod photoreceptors?

A

Longer outer segment with photosensitive pigment

100 times more sensitive to light than cones

Slow response to light

Responsible for night vision (scotopic vision)

120 million rods

46
Q

What are the features of cone photoreceptors?

A

Less sensitive to light, but faster response

Shorter outer segment

Responsible for daylight fine vision and colour vision (phototopic vision)

6 million cones

47
Q

How are photopigments synthesised and regenerated?

A

Photopigments are synthesised in the inner photoreceptor segment and are then transported to the outer segment.

Outer segment is made up of stacks of discs.

The distal discs with deactivated photopigments are shed from the tips and phagocytosed by the retinal epithelial cells.

Deactivated photopigments are regenerated inside the retinal epithelial cells, then transported back to the photoreceptors.

48
Q

Describe the distribution of photoreceptors.

A

Rod photoreceptors are widely distributed all over the retina, with the highest density just outside the macula- 20-40 degrees away from fovea.

The density of rod photoreceptors gently tails off towards the periphery.

Rod photoreceptors are completely absent within the macula.

Cone photoreceptors are distributed only within the macula.

49
Q

What is the single peak light sensitivity of rod photoreceptors?

A

498nm wavelength.

50
Q

What are the 3 cone photopigment subtypes?

A

S-cones with photopigment sensitive to short wavelength- blue.

M-cones with photopigment sensitive to medium wavelength- green.

L-cones with photopigment sensitive to long wavelength- red.

51
Q

What is the commonest form of colour vision deficiency?

A

Deuteranomaly.

Caused by shifting of M-cone sensitivity peak towards that of the L-cone curve, causing red-green confusion.

52
Q

What is the prevalence of colour vision deficits?

A

About 8% in males.

About 0.5% in females.

53
Q

What is anomalous trichromatism?

A

Colour vision deficits can be caused by a shift in the photopigment peak sensitivity.

54
Q

What is the Ishihara test?

A

Colour perception test.

Ishihara isochromatic plates can test for red-green deficiencies only.

Consists of plates of circle of dots appearing randomly in size.

Subjects with normal red-green vision will recognise the correct pattern in the form of a 2-digit number.

Patients with colour vision deficiencies will not recognise any pattern, or will recognise the wrong pattern.

55
Q

Describe the process of dark adaptation.

A

Increase in light sensitivity in dark

Biphasic process

Cone adaptation 7 minutes

Rod adaptation 30 minutes- regeneration of rhodopsin

56
Q

Describe the process of light adaptation.

A

Adaptation from dark to light

Occurs over 5 minutes

Bleaching of photopigments

Neuroadaptation

Inhibition of rod/cone function

57
Q

What is refraction?

A

Light is passing through one medium into another.

Velocity changes.

For example, if light comes from air (considered a vacuum), strike a boundary at some angle of incidence measured from a normal line, and enters water.

58
Q

What are the 2 basic types of lens?

A

Converging (convex) lens takes light rays and brings them to a point.

Diverging (concave) lens takes light rays and spreads them outwards.

59
Q

What is emmetropia?

A

Adequate correlation between axial length and refractive power.

Parallel light rays fall on the retina (no accommodation).

60
Q

What is ametropia?

A

Refractive error.

Mismatch between axial length and refractive power.

Parallel light rays don’t fall on the retina (no accommodation).

Nearsightedness (myopia).

Farsightedness (hyperopia).

Astigmatism.

Presbyopia.

61
Q

What is myopia?

A

Parallel rays converge at a focal point anterior to the retina.

62
Q

What causes myopia?

A

Excessive long globe (axial myopia)- more common.

Excessive refractive power (refractive myopia).

63
Q

What are the symptoms of myopia?

A

Blurred distance vision.

Squint in an attempt to improve uncorrected visual acuity when gazing into the distance.

Headache.

64
Q

What is hyperopia?

A

Parallel rays converge at a focal point posterior to the retina.

65
Q

What causes hyperopia?

A

Excessive short globe (axial hyperopia)- more common.

Insufficient refractive power (refractive hyperopia).

66
Q

What are the symptoms of hyperopia?

A

Visual acuity at near tends to blur relatively early.

Nature of blur varies from inability to read fine print, to clear near vision that suddenly and intermittently blurs.

Asthenopic symptoms:

  • eye pain
  • headache in frontal region
  • burning sensation in the eyes
  • blepharoconjunctivitis

Amblyopia- uncorrected hyperopia >5D.

67
Q

What is astigmatism?

A

Parallel rays come to focus in 2 focal lines rather than a single focal point.

68
Q

What causes astigmatism?

A

Refractive media is not spherical.

Refract differently along one meridian than along meridian perpendicular to it.

2 focal points (punctiform object is represent as 2 sharply defined lines).

69
Q

What are the symptoms of astigmatism?

A

Asthenopic symptoms (headache, eye pain)

Blurred vision

Distortion of vision

Head tilting and turning

70
Q

How is astigmatism treated?

A

Regular astigmatism: cylinder lenses with or without spherical lenses (convex or concave), Sx.

Irregular astigmatism: rigid CL, surgery.

71
Q

What is the near response triad?

A

Adaptation for near vision.

Pupillary miosis (sphincter pupillae) to increase depth of field.

Convergence (medial recti from both eyes) to align both eyes towards a near object- eyes both adduct medially at same time.

Accommodation (circular ciliary muscle) to increase the refractive power of lens for near vision.

72
Q

What is presbyopia?

A

Naturally occurring loss of accommodation (focus for near objects).

Onset from age 40 years.

Distant vision intact.

Corrected by reading glasses (convex lenses) to increase refractive power of the eye.

73
Q

How is presbyopia treated?

A

Convex lenses in near vision.

Reading glasses

Bifocal glasses

Trifocal glasses

Progressive power glasses

74
Q

What are the different types of optical correction?

A

Spectacle lenses- monofocal (spherical, cylindrical) or multifocal lenses.

Contact lenses.

Intraocular lenses.

Surgical correction.

75
Q

What are the advantages of contact lenses?

A

Higher quality of optical image and less influence on the size of retinal image than spectacle lenses.

76
Q

What are the indications for contact lenses?

A

Cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease.

77
Q

What are the disadvantages of contact lenses?

A

Careful daily cleaning and disinfection.

Expense.

78
Q

What complications may arise from contact lens use?

A

Infectious keratitis

Giant papillary conjunctivitis

Corneal vascularisation

Severe chronic conjunctivitis

79
Q

What is accommodation?

A

Contraction of the circular ciliary muscle inside the ciliary body.

This relaxes the zonules that are normally stretched between the ciliary body attachment and the lens capsule attachment.

Zonules are passive elastic bands with no active contractile muscle.

In the absence of zonular tension, the lens returns to its natural convex shape due to its innate elasticity.

This increases the refractive power of the lens.

Mediated by the efferent third cranial nerve.

80
Q

What are the surgical options for optic correction?

A

Keratorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty.

Intraocular surgery: clear lens extraction (with or without IOL), phakic IOL.

Neurology (17 decks)

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