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Neuroscience and Mental Health > 12. Structure and function of the eye > Flashcards

12. Structure and function of the eye Flashcards

1
Q

Where are corneal stem cells located?

A
  • In the limbus

* Border between the cornea and sclera

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2
Q

What are basal tears?

A

Tears that continuously keep the cornea wet and nourished

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3
Q

What are reflex tears?

A
  • Tears produced in response to irritation
  • Afferent - cornea - opthalmic branch (V1) of the trigeminal
  • Efferent - parasympathetic
  • Neurotransmitter - acetylcholine
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4
Q

Where are tears produced?

A
  • Lacrimal gland

* Superio-laterally to the eye

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5
Q

Where do tears flow after lubricating the eye?

A
  • Drains through the two puncta - opening on medial lid margin
  • Flows through the superior and inferior canaliculi
  • Gathers in the tear sac, then the tear duct
  • Exits into the nasal cavity
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6
Q

How is a reflux of tears back into the eyes prevented?

A

Valve where the canaliculi meet the tear sac

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7
Q

What is the function of a tear film?

A
  • Maintains smooth cornea-air surface
  • Oxygen supply to cornea (normal cornea has no blood vessels)
  • Removal of debris (with blinking)
  • Bactericide
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8
Q

What are the 3 layers of tear film?

A

1) Superficial oily layer - reduces tear film evaporation, produced by Meibomian Glands along lid margins
2) Aqueous tear film
3) Mucinous layer - on corneal surface to maintain surface wetting - produced by Goblet cells

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9
Q

What is the conjunctiva?

A
  • Thin, transparent tissue that covers the outer surface of the eye
  • Covers the visible part of the eye and lines the inside of the eyelids
  • Nourished by tiny blood vessels
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10
Q

What are the 3 layers of the coat of eye (inside the head)?

A
  • Sclera - fibrous, hard and opaque (high water content)
  • Choroid - pigmented and vascular
  • Retina - neurosensory tissue
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11
Q

What is the function of the sclera?

A
  • Protection

* Maintaining the shape of the eye

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12
Q

What is the function of the choroid?

A
  • Provides circulation

* Shields out unwanted scattered light

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13
Q

What is the cornea and its function?

A
  • Front-most part of the anterior segment
  • Continuous with the sclera
  • Transparent, convex
  • Strong tissue
  • Low water content - dehydrated by the inner layer of the cornea - corneal endothelium
  • Powerful refracting surface - 2/3 of the eye’s focusing power
  • Physical and infection barrier
  • Relies of tears for nutrients and oxygen (as well as aqueous humour from inside)
  • Corneal nerve endings provide sensation, and also nutrients from neurotrophins
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14
Q

What are the 5 layers of the cornea?

A

1) Epithelium - protection, regenerates quickly
2) Bowman’s membrane
3) Stroma - thickest layer, filled with dehydrated collagen (transparent)
4) Descemet’s membrane
5) Endothelium

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15
Q

What is the function of the endothelial cells in the cornea and how do they change with age?

A

• Pump out excess fluid from the cornea
- prevents corneal oedema and haziness (cloudiness)
• No capacity to regenerate - cell count declines with age

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16
Q

What happens to the cornea when it’s hydrated?

A
  • Become opaque

* Eventually turns white

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17
Q

What is the uvea?

A 
• Vascular coat of eyeball
• Lies between the sclera and retina
• Composed of three parts:
- iris
- ciliary body
- choroid
• The parts are intimately connected
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18
Q

Does a disease of one part of the uvea affect the other parts and why?

A
  • Yes, due to close connections

* Not necessarily affected to the same degree

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19
Q

What nourishes the outer and inner retina?

A
  • Outer - choroid

* Inner - radial artery

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20
Q

What is the iris and its function?

A
  • Coloured part of the eye
  • Controls light levels inside the eye
  • Affects the focal plane too - helps with focusing
  • Embedded with tiny muscles that dilate and constrict
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21
Q

Describe the structure of the lens?

A

• Located behind the iris
• Outer acellular capsule
• Regular inner elongated cell fibres - transparency
(- may lose transparency with age - cataract)
• Suspended by a fibrous ring known as lens zonules - consists of passive connective tissue

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22
Q

What is the function of the lens?

A
  • Transparency
  • 1/3 refractive power, higher refractive index than aqueous and vitreous fluid
  • Accommodation - elasticity (muscles constrict, smaller and thicker lens, shorter sight)
  • Normally flat and tort - tension along the stretched lens zonules when the ciliary muscles are relaxed
  • Lens loses elastic properties with age - problems with short-sightedness with age as muscle contraction doesn’t make a difference
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23
Q

What is the function of the optic nerve?

A
  • Transmits electrical impulses from the retina to the brain

* Connects to the back of the eye near the macula

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24
Q

What is the visible part of the optic nerve called?

A

Optic disc

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25
Q

What is the macula?

A
  • Located roughly in the centre of the retina, temporal (lateral) to the optic nerve
  • Small, highly sensitive part of the retina
  • Responsible for detailed central vision
  • Fovea is the very centre of the macula
  • Important for tasks such as reading
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26
Q

What is glaucoma?

A
  • Disease of the back of the eye
  • Neurones die
  • Due to too much aqueous humour
  • One of the leading causes of irreversible blindness
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27
Q

What are the risk factors of glaucoma?

A
  • Age
  • Family history
  • Accidents
  • Intraocular pressure (only modifiable risk factor)
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28
Q

What 2 segments can the eye be divided to and what do they refer to?

A

• Anterior segment
- anatomical structures of the eye, in front of the lens
• Posterior segment
- anatomical structures of the eye, behind the lens

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29
Q

What are 3 of the anatomical spaces inside the eye?

A 
• Anterior chamber
- within the anterior segment
- filled with aqueous humour ('optically empty' as it's completely transparent)
• Posterior chamber
- within the anterior segment
- directly posterior to the iris but anterior to the lens
• Posterior cavity
- vitreous chamber
- filled with vitreous humour
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30
Q

What are debris and cells inside the anterior chamber a sign of?

A

Infection and inflammation

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31
Q

What are the 2 layers of the iris?

A
  • Thin posterior pigmented epithelial layer

* Thick anterior layer - stromal tissue + smooth muscles

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32
Q

Where is aqueous fluid secreted into the anterior chamber from?

A

Ciliary body (ciliary epithelium)

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33
Q

Where does the fluid flow out from the anterior chamber?

A

• 80% flows out through the canal of Schlemm, around the cornea, deep in the sclera
- modified vein that absorbs aqueous humour and pushes it into the venous system
• also flows out via the Trabecular Meshwork
• Situated at the junction between the ciliary body . and cornea

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34
Q

What is uveal-scleral flow?

A
  • Passive gradient flow for the absorption of aqueous humour, between the choroid and sclera
  • 20% of aqueous humour reabsorption
  • Pressure dependent
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35
Q

What is the first line of treatment to reduce IOP in glaucoma?

A
  • Prostaglandin analogues

* Increase uveal-scleral flow

36
Q

Describe the progression of damage in glaucoma

A
  • Sustained high eye pressure
  • Accumulative damage to the optic nerve tissue
  • Loss of ganglion nerve fibres - hollowing out of the optic nerve head
  • Peripheral vision progressively lost => blindness (if untreated)
37
Q

What is the most comment type of glaucoma?

A

Primary Open Angle Glaucoma (trabecular meshwork dysfunction)

38
Q

What is Closed Angle Glaucoma, what are the risk factors and how is it treated?

A
  • Increased pressure pushing the iris/lens complex forwards
  • Blocks the trabecular meshwork - vicious cycle
  • May present with sudden painful red eye with acute drop in vision
  • Can be acute or chronic
  • Small eye (hypermetropia)
  • Narrow angle at trabecular meshwork

• Peripheral laser iridotomy to create a drainage hole on iris

39
Q

What are the type of cells primarily affected in glaucoma?

A

Retinal ganglion cells

40
Q

What is the optic nerve blind spot?

A
  • No light sensitive cells where the optic nerve meets the retina
  • Optic disc - corresponding anatomical landmark for the physiological blind spot
41
Q

What is the most sensitive part of the retina?

A

The fovea at the centre of the macula

42
Q

Describe the proportion of cells in the fovea?

A
  • Highest concentration of cones, but a low concentration of rods
  • 1:1 ratio between photoreceptors and ganglion cells
  • This is why stars out of the corner of your eye seem brighter than when looking directly
  • If an image doesn’t fall on the fovea, you won’t be able to see the detail
43
Q

What is central vision (for)?

A
  • aka macular vision
  • Detail day vision, colour vision
  • Reading, facial recognition
44
Q

What is peripheral vision (for)?

A
  • Detecting shape and movement in the environment
  • Night vision
  • Navigation vision
45
Q

How can you assess central and peripheral vision?

A
  • Central - visual acuity assessment

* Peripheral - visual field assessment

46
Q

What are the 3 layers of the retina?

A
  • Outer: photoreceptors (1st order neurones) - detection of light
  • Middle: bipolar cells (2nd order neurones) - local signal processing to improve contrast sensitivity, regulate sensitivity
  • Inner: retinal ganglion cells (3rd order neurones) - transmission of signal from the eye to the brain
47
Q

What is the function of the retinal pigment epithelium?

A
  • Transports nutrients from the choroid to the photoreceptor cells
  • Removes metabolic waste from the retina
48
Q

What is the pigmented region at the centre of the retina called (yellow patch)?

A

Macula Lutea

about 6mm diameter

49
Q

How can you clinically assess the macula and fovea?

A

Optical Coherence Tomography scan

50
Q

Describe the rod photoreceptors

A
  • Longer outer segment with photo-sensitive pigment
  • 100x more sensitive than cones
  • Responsible from night vision (Scotopic Vision)
  • 120 million rods
  • Allows to to find position in space
  • Responsible for detecting movement and distance judgement
51
Q

Describe the cone photoreceptors

A
  • Less sensitive to light, but faster response

* Responsible for day light fine vision and colour vision (Photopic Vision)

52
Q

What is scotopic vision?

A

• Peripheral and night vision
(• more photoreceptors, more pigment, higher spacial and time summation)
• Rods distributed all over the retina
• Highest density just outside the macula

53
Q

Where are the highest concentration of rod receptors?

A

In the retina, 20-40 degrees away from the fovea

54
Q

What is photopic vision?

A

Central and day vision

55
Q

What are the sensitivities of rods?

A

Only a single peak light sensitivity

56
Q

What are the different cone photo-pigment sub-types?

A
  • S-cones - sensitive to short-wavelength (blue)
  • M-cones - sensitive to medium wavelength (green)
  • L-cones - sensitive to long wavelength (red)
57
Q

Which cones does yellow light stimulate?

A
  • M- and L-cones equally

* Yellow light has a wavelength between the peak sensitivity wavelengths of M- and L-cones

58
Q

What is the most common form of colour vision deficiency?

A
  • Deuteranomaly
  • Type of Anomalous Trichromatism
  • M-cone sensitivity peak shifts towards that of the L-cone curve => red-green confusion
59
Q

What is the prevalence of colour vision deficit?

A
  • Males - 8%

* Females - 0.5%

60
Q

What is dichromatism and monochromatism?

A
  • Dichromatism - 2 cone photo-pigment sub-types are present

* Monochromatism - complete absence of colour vision (no functional day vision)

61
Q

What can be used to test for red-green colour perception deficiencies?

A

Ishihara Test

62
Q

What happens to the sensitivity of the rod receptors in daylight?

A

Greatly suppressed

63
Q

How do the photoreceptors adapt from the light to the dark?

A

• The retina increases its light sensitivity in the dark
• Biphasic process
• Retina switches from photopic vision to scotopic vision
- cone adaptation - 7 mins
- rod adaptation - 30 mins - regeneration of rhodopsin

64
Q

Describe the adaptation of photoreceptors from the dark to the light

A
  • Occurs over 5 mins
  • Bleaching of photo-pigments mediates the process
  • Neuro-adaptation: inhibition of rod/cone function
  • Pupil adaptation: (minor) constriction of pupil with light
  • Pupil acts as an adjustable aperture to regulate light intake
65
Q

What is ametropia?

A
  • Refers to vision disorders characterised by the eyes inability to correctly focus the images of objects on the retina
  • Forms include myopia (nearsightedness), hyperopia (farsightedness) and astigmatism
66
Q

How do you work out the index of refraction?

A

(speed of light in a vacuum) / (speed of light in a medium)

denominator will always be smaller - n is always greater or equal to 1

67
Q

Is the angle of incidence the same or different to the angle of reflection and refraction?

A
  • Angle of incidence = angle of refraction

* Angle of incidence > or < angle of refraction depending on the direction of the light

68
Q

How do convex lenses change the direction of light?

A
  • Light passes through
  • Light rays converge towards a focal point
  • Distance of the focal point is proportional to the thickness of the lens
  • Thicker lens - closer focal point
69
Q

How do concave lenses change the direction of light?

A
  • Light passes through
  • Light rays disperse as it is refracted in a divergent way
  • The focal point is a virtual point (before the lens, not after)
70
Q

What is emmetropia?

A
  • Eye with a refractive error of 0 i.e. no visual defects - ideal vision
  • Adequate correlation between axial length and refractive power
  • Parallel light rays fall on the retina
71
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

72
Q

What happens to the focal point in myopia (nearsightedness) and hyperopia (farsightedness)?

A
  • Myopia - anterior to the retina

* Hyperopia - posterior to the retina

73
Q

What are the causes and symptoms of myopia (nearsightedness)?

A
  • Excessive long globe (axial myopia) - eye is too long - more common
  • Excessive refractive power (refractive myopia) - lens and cornea are too powerful
  • Blurred distance vision
  • Squint in an attempt to improve uncorrected visual acuity
  • Headache
74
Q

How can myopia be corrected?

A

Concave glasses (divergent) or contact lenses - move the focal point slightly backwards

75
Q

What are the causes and symptoms of hyperopia (farsightedness)?

A
  • Excessive short globe (axial hyperopia)
  • Insufficient refractive power (refractive myopia)
  • Visual acuity at near tends to blur
  • Nature varies from inability to read fine print, to near vision being clear then suddenly and intermittently blur
  • Asthenopic (eye strain) symptoms - eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
76
Q

What is amblyopia?

A
  • Lazy eye
  • Eye fails to achieve normal visual acuity
  • If refractive error is too drastic in one eye, brain will start to exclude information from the hyperopic eye
  • Only the other eye is used
  • Affected eye becomes ambylopic
77
Q

What is astigmatism?

A
  • Parallel rays come to focus in 2 focal lines rather than a single point
  • Hereditary
  • Caused by elliptical refractive media (not spherical)
  • Cornea is not evenly shaped
78
Q

What are the symptoms and treatment for astigmatism?

A
  • Asthenopic symptoms
  • Blurred vision
  • Distortion of vision
  • Head tilting and turning
  • Regular astigmatism: cylinder lenses with or without spherical lenses
  • Irregular astigmatism: rigid cylinder lenses, surgery
79
Q

What is presbyopia?

A

• Naturally occurring loss of accommodation
• Onset from 40yrs
• Distant vision intact
• Corrected by reading glasses to increase refractive power
(• bifocal glasses, trifocal glasses, progressive power glasses)

80
Q

How is the optical image different in contact lenses, compared to spectacle lenses?

A
  • Higher quality

* Less influence on the size of the retinal image

81
Q

When would contact lenses be given?

A
  • Cosmetic
  • Athletic activities
  • Occupational
  • Irregular corneal astigmatism
  • High anisometropia
  • Corneal disease
82
Q

What are the disadvantages of contact lenses?

A 
• Careful daily cleaning and disinfeciton
• Expense
• Complications
- infectious keratitis
- giant papillary conjunctivitis
- corneal vascularisation
- severe chronic conjunctivitis
83
Q

What are intraocular lenses?

A
  • Implantation
  • Replacement of cataract crystalline lens
  • Gives best optical correction for aphakia
  • Loss of accommodation
84
Q

How can near/farsightedness be corrected by surgery?

A
  • Keratorefractive surgery

* Intraocular surgery (with or without intraocular lens)

85
Q

How does the lens accommodate (for near vision)?

A
  • Circular ciliary muscles contract
  • Zonules relax (that are normally stretched)
  • Lens returns to it’s natural convex shape
  • Increased refractive power of the lens
86
Q

Which nerve is accommodation mediated by?

A

Efferent oculomotor nerve (III)

Neuroscience and Mental Health (17 decks)

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