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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

Through what structure does the optic nerve pass?

A

Throught the bony orbit nasally

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2
Q

Describe the location and function of the lacrimal gland?

A
  • Lacrimal gland located within orbit, latero-superior to globe
  • Gland function = tear production
    • Basal tears - these are tears that are produced at a constant level, even in the absence of irritation or stimulation
    • Reflex tears - These are tears that are produced in response to irritation
      • Afferent - Cornea - CN VI
      • Efferent - Paraympathetic
    • Neurotransmitters = ACh
    • Crying (emotional) tears
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3
Q

Describe the Lacrimal system.

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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4
Q

What is the function of the tear film?

A
  • Tear film maintains smooth cornea-air surface
  • Oxygen supply to cornea b/c normal cornea has no blood vessels
  • Removal of Debris (Tear film + Blinking)
  • Bactericide
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5
Q

Describe the structure of the tear film.

A
  1. Superficial Oily Layer to reduce tear film evaporation (produced by a row of Meibomian Glands along lid margins)
  2. Aqueous Tear Film made by tear gland
  3. Mucinous Layer on Corneal Surface to maintain surface wetting
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6
Q

Describe the conjunctiva.

A
  • = the thin, transparenttissue covering outer surfaceof eye (including cornea)
  • Begins at outer edge of cornea, covers visible part of eye, and lines inside of eyelids.
  • Nourished by tiny blood vessels nearly invisible to naked eye.
  • Ciliary body produces aqueous humour
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7
Q

Describe the layers of the eye.

A

3 layers:

  1. Sclera - Hard + opaque, protects eye + maintains shape of the eye –> “the white of the eye” - tough + has high water content
  2. Choroid - Pigmented + Vascular –> shields out unwanted scattered light + supplies blood
  3. Retina - Neurosensory Tissue converting light –> impulses to the brain via the optic nerve
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8
Q

Define the cornea.

A

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

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9
Q

Describe the structural features of cornea.

A
  • Front-most part of Anterior Segment
  • Continuous with sclera
  • Low water content
  • Convex Curvature
  • Higher refractive index than air
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10
Q

Describe the function of the cornea. What does prolonged contact-lens wear?

A
  • Powerful refracting surface, providing 2/3 of eye’s focusing/refracting power
    • It has a convex curvature and a higher refractive index than air
  • Acts as a clear window to look through
  • Physical + infection barrier

Cornea relies on tear film + aqueous fluid for nutrients + oxygen supply

Prolonged contact-lens wear –> reduces oxygen supply to cornea + increases risk of corneal infection

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11
Q

Describe the physical 5-layer structure of the cornea.

A
  1. Epithelium - stratified
  2. Bowman’s Membrane (specialised basement membrane)
  3. Stroma - regularity contributes to transparency, thickest
    • ​​Corneal nerve ending provide sensation and nutrients for healthy tissue
    • No blood vessels in normal cornea so transparent
  4. Descemet’s Membrane (specialized basement membrane)
  5. Endothelium – pumps fluid out of corneal and prevents corneal oedema,
    1. Only single layer (simple)
    2. No regenerative ability
    3. Endothelial cell density decreases w/ age
    4. Endothelial cells pump out excess fluid from cornea.
    5. Thus, dysfunction –> corneal oedema+ cloudiness
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12
Q

Define the uvea and describe its composition.

A
  • Vascular coat of eye ball between sclera + retina.
  • Composed of three parts:
    • Iris - Coloured part of the eye:
      • Controls light levels inside eye(like aperture on a camera)
      • Pupil = round opening in centre of iris
      • Iris embedded w/ tiny muscles that dilate + constrict pupil size.
    • Ciliary body
    • Choroid - Between retina + sclera; composed of layers of blood vessels nourishing back of eye.
  • These three portions intimately connected –> disease of one also affects others, though not necessarily to the same degree.
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13
Q

Describe the lens structure and function.

A

Lens Structure:

  • Outer Acellular Capsule
  • Capsule encases regular inner elongated cell fibres– transparency
  • May lose transparency w/ age –> Cataract (opaque lens) – quite common

Function

  • Transparency due to regular structure
  • Refractive Power = 1/3 of overall power
    • Higher refractive index than aqueous fluid + vitreous
  • Accommodation – elasticity
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14
Q

Describe the Lens zonules (suspensory ligaments).

A
  • Lens suspended by fibrous ring known as lens zonules, anchoring lens to ciliary body.
  • Consists of passive connective tissue
  • Surface of lens normally held flat + tort by tension along stretched lens zonules.
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15
Q

Describe the function of the optic nerve

A
  • Transmits electrical impulses from retina –> brain.
  • Connects to back of eye near macula.
  • Visible portion of optic nerve = optic disc.
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16
Q

Describe the blind spot test.

A

Where optic nerve meets retina, no light sensitive cells present –> blind spot.

  1. On paper, draw a dot, then an “X” 10 cm to L
  2. Close R eye + hold paper at arm’s length.
  3. Look at dot + move paper towards you –> X disappears into blind spot!
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17
Q

Describe the macula

A
  • Located roughly in centre of retina, temporal (closer to temples) to optic nerve.
  • Small + highly sensitive part of retina allows for detailed central vision+ perform tasks that require central vision e.g. reading.
  • Fovea= very centre of macula. Allows us to appreciate detail and perform tasks that require central vision such reading
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18
Q

Describe the anterior and posterior segments if the eye.

A

Anterior segment - between cornea and lens (filled with aq. fluid) - supplies nutrients

Posterior segment - posterior to lens

19
Q

Describe the function ciliary body

A
  • Secretes aqueous fluid/humour in eye
  • Intraocular Aqueous Fluid flows anteriorly into Anterior Chamber
  • Aqueous Fluid supplies nutrient
  • Trabecular Meshwork(between ciliary body + cornea) drains fluid out of eye
  • Normal IOP = 12-21mmHg
20
Q

What does glaucoma primarily affect?

A

Glaucoma primarily affects the retinal ganglion cells

Leads to retinal ganglion cell death + enlarged optic disc cupping = loss of ganglion nerve fibres –> hollowing out of optic nerve head

21
Q

Describe the types of glaucoma.

A

Patients with untreated glaucoma lose peripheral vision progressively.

Types

  • Primary Open Angle Glaucoma (on the left) - commonest
    • ​Trabecular Meshwork Dysfunction
    • Generally asymptomatic until advanced stages
  • Closed Angle Glaucoma - acute or chronic
    • ​Increased IOP –> iris/lens pushed forward –> blocking trabecular meshwork - vicious cycle
    • Risk factors = small eye (hypermetropia), narrow angle at trabecular meshwork
    • May present with sudden painful red eye with acute drop in vision
    • Can be treated with peripheral laser iridotomy –> create drainage hole on iris
22
Q

What are the risk factors of glaucoma?

A
  • HIGH PRESSURE DOES NOT MEAN GLAUCOMA àIT JUST MEANS HIGHER RISK OF GLAUCOMA
  • Risk factors:
  • Family history
  • Age
23
Q

Describe the fovea test.

A
  • Fovea is most sensitive part of retina.
  • Fovea has highest [cone cells], but low [rods]
    • Explains why stars out of corner of eye brighter than when look directly.
  • But only fovea has high-enough [cones] to perceive in detail –> try to read letters using peripheral vision, but can’t
24
Q

Describe the difference in central and peripheral vision.

A
  • Central
    • ​DetailDay Vision,Colour vision - fovea has highest conc. of cone cells
      • Reading, Facial recognition
    • Assessed by Visual Acuity Assessment
      • Loss of Foveal Vision –> poor visual acuity
  • Peripheral (by rod cells)
    • 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
25
Q

Describe the retinal structure.

A
  • Outer layer = Photoreceptors (1st Order) - light detection
  • Middle layer = Bipolar Cells (2nd Order) - Local Signal Processing to improve contrast sensitivity, regulate sensitivity
    • Sound amplifier
  • Inner layer = Retinal Ganglion Cells (3rd Order) - transmission of signal from eye –> brain

Retinal pigment epithelium transports nutrient from choroid –> photoreceptors and removes metabolic waste from retina

26
Q

Define the macula lutea and fovea

A

Macula - Yellow patch at centre of retina of about 6 mm in diameter

Fovea - forms pits at centre of macula due to absence of overlying ganglion cell layer

  • Clinically assessed with OCT scan (Optical Coherence Tomography)
27
Q

Describe the difference in Rod and Cone photoreceptors

A
  • Rod
    • Longer outer segment with photo-sensitive pigment
    • 100X more sensitive to light than cones
    • Slow response to light
    • Responsible for night vision (Scotopic Vision)
    • 120M rods
  • Cone
    • Less light-sensitive, but faster response
    • Responsible for day light fine vision and colour vision (Photopic Vision)
    • 6M cones
    • 3 different types of cone cells for different colours:
      • S for blue, M for green, L for red
28
Q

Describe the distribution of photoreceptors.

A
  • Rods widely distributed all over retina; highest density just outside macula.
  • Density of rod photoceptors gently tails off towards periphery.
  • Rod photo-receptors completely absent in macula.
  • Cone photo-receptors distributed only in macula.
  • More rod cells à more pigment à higher Spatial and Time Summation
29
Q

Explain the peak light sensitivities of different photoreceptors. What photoreceptors are stimulated by yellow light?

A
  • Rods vision has single peak light sensitivity at 498 nm
  • S-Cones w/ pigment sensitive to short wavelength –blue
  • M-Cones w/ pigment sensitive to medium wavelength –green
  • L-Cones w/ pigment sensitive to long wavelength –red
  • Yellow light wavelength between peak sensitivities of M and L
    • Yellow light stimulates M + L equally
    • Experience yellow = green + red combo
30
Q

Describe Deuteranomaly.

A
  • Commonest form of colour deficiency
  • Caused by shifting of M-cone sensitivity peak towards L-cone curve –> red-green confusion.
  • Colour vision deficit higher in Males than Females
31
Q

Describe Anomalous Trichromatism.

A

colour vision deficits due to shift in photo-pigment peak sensitivity.

32
Q

Describe Monochromatism and Dischromatism.

A
  • Colour Vision deficits also caused by absence of cone photo-pigments type(s).
    • Dichromatism = two cone photo-pigment sub-types present.
    • Monochromatism = no colour vision.
  • Rod Monochromatism = no cone photo-receptors –> no functional day vision
  • Blue Cone Monochromatism –> normal daylight visual acuity
33
Q

Describe the Isihara Test.

A

colour perception test for R-G deficiencies only

34
Q

Describe Light Dark Adaptation.

A
  • Dark Adaptation
    • Increase in light sensitivity when moving from light to dark (in daylight, rod cells sensitivity suppressed as cone receptors are responsible for daylight vision)
    • Biphasic Process:
      • Cone adaptation = 7 minutes to adapt
      • Rod adaptation = 30 minutes – regeneration of rhodopsin
  • Light Adaptation
    • Adaptation from dark to light
    • Occurs over 5 minutes
    • Bleaching of photo-pigments
    • Neuro-adaptation
    • Inhibition of Rod/Cone function
      • rod function is greatly suppressed and cone function takes over within a minute
  • Pupil Adaptation (minor) = constriction with light
35
Q

Describe refraction (refraction index) and reflection

A
  • Index of Refraction = speed of light in first medium/ speed of light in new medium
  • As light goes from one medium to another –> velocity CHANGES
  • As light goes from one medium to another –> path CHANGES
  • When light reflected, angle of incidence = angle of reflection
  • How do we change refraction? CHANGE THE LENS
    • Concave – diverging
    • Convex – converging - has a real focal point
36
Q

Define Emmetropia.

A
  • Adequate correlation between axial (eye) length + refractive power (of 0)
  • Parallel light rays fall on retina (no accommodation)
37
Q

Define Ametropia and types of them.

A
  • Mismatch between axial length + refractive power
  • Parallel light rays don’t fall on retina (no accommodation); types:
    1. Near-sightedness (Myopia)
    2. Farsightedness (Hyperopia)
    3. Astigmatism
    4. Presbyopia
38
Q

Describe Hyperopia

A

Parallel rays converge at focal point posterior to retina

  • Aetiology: not clear, inherited; causes:
    • Excessive short globe (axial hyperopia), more common
    • insufficient refractive power (refractive hyperopia)
  • Symptoms: (originally compensated by accommodation until certain point)
    • Near visual acuity blurs relatively early
      • Blur varies in degree
      • blurred vision more noticeable if person tired, printing is weak or light inadequate
    • Asthenopic symptoms: eye pain, headache in frontal region, burning sensation in eyes, blepharoconjunctivitis
    • Lazy Eye = Amblyopia – uncorrected hyperopia > 5 diopters
  • Treatment = convex glasses/contacts or remove lens + put in IOLs (cataract extraction)
39
Q

Describe myopia.

A

Parallel rays converge at focal point anterior to retina

  • Aetiology not clear, genetic factor; Causes:
    • Excessive long globe/eye (axial myopia), more common
    • Excessive refractive power (refractive myopia)
  • Symptoms:
    • Blurreddistancevision à squint to improve uncorrected visual acuity
    • Headache
  • Treatment = concave glasses or contacts or surgery
40
Q

Describe Astigmatism.

A
  • Parallel rays focus in multiple focal lines, NOT single focal point
  • Aetiology = heredity; causes:
    • Refractive index/cornea not evenly shaped, not spherical à refract differently along one meridian than along meridian perpendicular to it à 2 focal points (punctiform object is represent as 2 sharply defined lines)
  • Symptoms
    • Asthenopic symptoms (headache, eye pain)
    • blurred vision
    • distortion of vision
    • head tilting + turning
  • Treatment
    • For regular astigmatism = cylinder lenses w/ or w/o spherical lenses (convex or concave), Sx
    • Irregular astigmatism = rigid CL, surgery
41
Q

Describe Presbyopia.

A
  • Naturally occurring loss of accommodation (focus for near objects)
  • Onset from age 40 years
  • Distant vision intact
  • Treatment = corrected by convex glasses in near vision à increase refractive power
    • E.g. reading, Bi/trifocal, Progressive power glasses
42
Q

Describe the near response triad.

A

For adaptation for 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
43
Q

Describe the types of optical correction.

A
  • Spectacle lenses
    • Monofocal: spherical or cylindrical
    • Multifocal
  • Contact lenses
    • Higher quality of optical image + less influence on size of retinal image than spectacle lenses
    • Indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
    • disadvantages: careful daily cleaning + disinfection, expense
    • Complication: infectious keratitis, giant papillary conjunctivitis, corneal vascularization, severe chronic conjunctivitis
  • Intraocular lenses:
    • Replacement of cataract crystalline lens
    • Give best optical correction for aphakia, avoid significant magnification, and distortion caused by spectacle lenses
  • Surgical correction:
    • Keratorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty
    • Intraocular surgery: clear lens extraction (with or without IOL), phakic IOL
44
Q

Describe the accommodation mechanism.

A

Mediated by efferent oculomotor nerve

  1. Contraction of Circular Ciliary Muscle in Ciliary Body
  2. Relaxes zonules, which are normally stretched between ciliary body attachment and lens capsule attachment
    1. 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

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