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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
Describe the retinal structure.
* 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
Define the macula lutea and fovea
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
Describe the difference in Rod and Cone photoreceptors
* _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
Describe the distribution of photoreceptors.
* 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
Explain the peak light sensitivities of different photoreceptors. What photoreceptors are stimulated by yellow light?
* Rods vision has single peak light sensitivity at **498 nm** * **S**-Cones w/ pigment sensitive to **s**hort wavelength –blue * **M**-Cones w/ pigment sensitive to **m**edium wavelength –green * **L**-Cones w/ pigment sensitive to **l**ong wavelength –red * Yellow light wavelength between peak sensitivities of M and L * Yellow light stimulates M + L equally * Experience yellow = **green + red** combo
30
Describe Deuteranomaly.
* 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
Describe Anomalous Trichromatism.
colour vision deficits due to shift in photo-pigment peak sensitivity.
32
Describe Monochromatism and Dischromatism.
* 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
Describe the Isihara Test.
colour perception test for **R-G deficiencies only**
34
Describe Light Dark Adaptation.
* 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
Describe refraction (refraction index) and reflection
* 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
Define Emmetropia.
* Adequate correlation between axial (eye) length + refractive power (of 0) * Parallel light rays fall on retina (no accommodation)
37
Define Ametropia and types of them.
* **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
Describe Hyperopia
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
Describe myopia.
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: * **B****lurred****distance****vision** à squint to improve uncorrected visual acuity * **Headache** * _Treatment_ = **concave** glasses or contacts or surgery
40
Describe Astigmatism.
* 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
Describe Presbyopia.
* 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
Describe the near response triad.
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
Describe the types of optical correction.
* 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
Describe the accommodation mechanism.
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

Neuroscience and Mental Health (14 decks)

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