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Year 2 Collin > Neuro 12 - Structure and function of eye > Flashcards

Neuro 12 - Structure and function of eye Flashcards

1
Q

Optic nerve passes between bone …….

A

Nasally

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2
Q

Describe the location and function of the lacrimal gland

A

Located latero-superior to globe, within the orbit

Function = tear production.

  • basal tears
  • reflex tears in response to irritation (afferent = cornea, CN6; efferent = PNS)
  • Neurotransmitter = ACh
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3
Q

Describe the tearing process

A
  1. Tear made by lacrimal gland
  2. Drains through 2 puncta (opening on medial lid margin)
  3. Flows through superior and inferior canaliculi
  4. Gather in tear sac
  5. Exit sac via tear duct into nose cavity

Tear duct (nasolacrimal duct) - opens into inferior meatus)

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4
Q

Describe the structure and function of tear film

A

Tear film Function:

  • Maintains smooth cornea-air surface
  • Oxygen supply to cornea (normal cornea has no blood vessels)
  • remove debris
  • bactericide

Structure = 3 layered

  1. Superficial oily layer - reduce tear film evaporation (produced by Meibomian glands on lid margin)
  2. Aqueous tear film - made by tear gland
  3. Mucinous layer - on corneal surface - maintains surface wetting
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5
Q

What is the conjunctiva

A
  1. Thin transparent tissue covering outer surface of eye
  2. Begins at outer. edge of cornea - covers visible part of eye –> lines inside of eyelids
  3. Nourished by tiny blood vessels
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6
Q

What is the eyes antero-posterior diameter in adults

A

24mm

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7
Q

Describe the 3 layers of the eye

A
  1. Sclera - hard, opaque, protects eye and maintains shape. AKA white of the eye. Has a high water content
  2. Choroid - pigmented and vascular –> shields out unwanted scattered light and supplies blood
  3. Retina - neurosensory tissue - converts light into impulses
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8
Q

Describe the general structure and function of the cornea

A

Cornea = transparent, dome-shaped window covering front of eye

Structural features:

  1. Front-most part of anterior segment
  2. Continuous with sclera, but low water content (unlike sclera)
  3. Convex curvature
  4. Higher refractive index than air

Function:

  1. Powerful refracting surface, provides 2/3 focussing/refracting power
  2. Acts as clear window to look through
  3. Physical and infection barrier
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9
Q

What does the cornea rely on for nutrients and oxygen supply?

A

Tear film and aqueous fluid

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10
Q

Describe the 5 layered structure of the cornea

A
  1. Epithelium - stratified
  2. Bowmans membrane (specialised basement membrane)
  3. Stroma - thickest part. Contributes to transparency.
    - corneal nerve endings provide sensation and nutrients
    - no blood vessels in normal cornea - therefore transparent
  4. Descemets membrane (specialised basement membrane)
  5. Endothelium - pumps fluid out of cornea; preventing corneal oedema
    - Endothelial layer only 1 cell thick
    - no regenerative ability —> endothelial cell density decreases with age
    - endothelial cells pump out excess fluid from cornea

(-therefore, endothelial cell dysfunction causes corneal oedema and cloudiness)

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11
Q

What is the uvea and what is it composed of?

A

Uvea - vascular coat of eyeball between sclera and retina

3 Parts -

  1. Iris
  2. Ciliary body
  3. Choroid

All 3 parts intimately connected –> disease of one part also affects others

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12
Q

What is the choroid?

A

Layer between retina and sclera, composed of layers of blood vessels which nourish the back of the eye

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13
Q

Describe the iris

A

Coloured part of the eye –> controls light levels inside the eye

Pupil = round opening in centre of iris

Iris has tiny muscles that dilate and constrict pupil size

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14
Q

Describe lens structure

A

Outer acellular capsule –> capsule encases regular inner elongated cell fibres - transparenc (may lose transparency with age - cataracts)

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15
Q

What is the function of the lens

A
  1. Transparency
  2. Refractive power = 1/3 of overall power (higher RI than aq fluid / vitreous humour)
  3. Accommodation - elasticity
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16
Q

What are lens zonules and what do they do?

A

Lens zonules (aka suspensory ligaments) - fibrous ring that anchors the lens to the ciliary body

Lens zonules consist of passive connective tissue - lens surface normally held flat and tort by tension along stretched lens zonules

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17
Q

What does the retina do

A

Thin layer of tissue that lines the inner part of the eye —> captures light rays entering eye

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18
Q

Describe the optic nerve

A

Connects to the back of the eye near the macula –> transmits electrical impulses from retina to brain

Visible portion of optic nerve = optic disc

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19
Q

Describe the macula

A
  • Located roughly in centre of retina, temporal to optic nerve
  • it is a small, highly sensitive part of retina - allows for detailed central vision & performing tasks requiring central vision (e.g. reading)

Fovea = very centre of macula

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20
Q

Describe the anterior vs posterior chambers/segments of the eye

A
  1. Anterior chamber - between cornea and lens. Filled with clear aqueous fluid. Supplies nutrients
  2. Posterior chamber - in posterior segment
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21
Q

What is the purpose of the ciliary body and how does it work

A

Ciliary body - secretes aqueous fluid/humour in eye —> intraocular aqueous fluid flows anteriorly into anterior chamber supplies nutrients

The trabecular meshwork (between ciliary body and cornea) drains fluid out of eye (via Canals of Schlemm - 80%) OR via uveal-scleral outflow

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22
Q

What is the normal IOP

A

12-21mmHg (>24mmHg = increased risk of glaucoma)

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23
Q

What does glaucoma primarily affect?

A

Affects retinal ganglion cells

24
Q

How does glaucoma work?

A

It is caused by increased IOP

Causes retinal ganglion cell death + enlarged optic disc cupping —> which causes loss of ganglion nerve fibres —> hollowing out of optic nerve head

Patients with untreated glaucoma –> lose peripheral vision progressively

25
Q

Define glaucoma

A

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

26
Q

Describe the 2 types of glaucoma

A
  1. Primary Open angle glaucoma (commonest) - dysfunction in trabecular meshwork –> increased resistance to fluid outflow (buildup IOP). Generally asymptomatic until advanced stages
  2. Closed angle glaucoma - can be acute or chronic. Iris/lens pushed forwards which blocked the trabecular meshwork (vicious cycle). Raises IOP. RFs include small eye (hypermetropia), narrow angle at trabecular meshwork. May present with sudden painful red eye with acute drop in vision. May be treated with peripheral laser iridotomy –> creates a drainage hole in iris
27
Q

What is the most sensitive part of the retina

A

Fovea

Fovea as the highest cone cell conc, low rod cell conc

28
Q

What is the anatomical landmark for the physiological blind spot

A

Optic disk

29
Q

Describe our central vision

A

Detail day vision, colour vision - fovea = highest conc of cone cells. Needed for reading, facial recognition

Assessed by visual ACUITY assessment –> loss of foveal vision = poor visual acuity

30
Q

Describe our peripheral vision

A

Done by rod cells

Needed for 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

31
Q

Explain visual acuity differences in cone and rod cells

A

Cone cells = 1:1:1 (cones:bipolar cells:ganglion cells)

Rod cells = many:x:1

However rod cells are more sensitive to light, at the expense of visual acuity

32
Q

Describe retinal structure

A
  1. Outer layer = photoreceptors (1st order) - light detection
  2. Middle layer = bipolar cells (2nd order) - local signal processing – to improve contrast sensitivity, regulate sensitivity
  3. Inner layer = Retinal ganglion cells (3rd order) - transmission of signal from eye to brain

Retinal pigment epithelium - transports nutrients from choroid to photoreceptors and also removes metabolic waste from retina

33
Q

What is the yellow patch at the centre of the retina about 6mm in diameter

A

Macula lutea

34
Q

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

A

Due to the absence of overlying ganglion cell layer

35
Q

Describe rod cells

A
  1. Longer outer segment (protein synthesis occurs in the inner segment)
  2. 100x more sensitive to light than cone cells but less acuity.
  3. Slower response to light
  4. Responsible for night vision
  5. 120M rods
36
Q

Describe cone cells

A
  1. Less light sensitive than rods, but faster response
  2. Responsible for day light fine vision and colour vision
  3. 6M cones
  4. 3 different types of cone cells - S (blue), M (green), L (red)
37
Q

Describe photoreceptor distribution

A

Rods = widely distributed over retina –> highest density just outside macula (20-40 degrees away from fovea). Density of rod cells gradually declines approaching the periphery.

NO rod photoreceptors in macula.

Cone receptors ONLY in macula.

More rod cells = more pigment = higher spacial and time summation

38
Q

Rod vision has single peak light sensitivity at?

A

498nm

39
Q

What are the 3 types of cone cells

A

S-cones = sensitive to short wavelength (Blue)

M-cones - sensitive to medium wavelength (Green)

L-cones - sensitive to long wavelength (red)

Yellow light = between peak sensitivities of M and L (green and red combo, equally stimulated)

40
Q

What is the commonest form of colour vision deficiency?

A

Deuteranomaly

M-cone (green) sensitivity peak shifts towards L-cone peak (red). Result = red-green confusion

(Colour vision deficit more common in males than females)

41
Q

What is anomalous trichromatism

A

Colour vision deficit - due to shifts in photo-pigment peak sensitivity

42
Q

Colour vision deficits may also be caused by the absence of cone photo-pigments. Expand

A

Dichromatism = 2 cone photo-pigment subtypes present

Monochromatism = no colour vision

“Rod monochromatism” = no cone photoreceptors - no functional day vision

“Blue cone monochromatism” - normal daylight visual acuuity

43
Q

What test is done for colour pereception of R-G deficiencies?

A

Ishihara test

44
Q

Contrast dark adaptation and light adaptation

A

Dark adaptation = increase in light sensitivity in dark.
- Biphasic process –> cone adaptation takes 7 mins, rod adaptation takes 30 mins (regeneration of rhodopsin)

Light adaptation = adaptation from dark to light. Occurs over 5 mins. Involves bleaching of photo-pigments, neuroadaptation, inhibition of rod/cone function. Pupil adaptation also occurs - constriction with light

45
Q

Distinguish convex and concave lenses

A

Convex - focusses light to a point

Concave - spreads light outward

46
Q

Describe an emmetropic eye (good eye)

A

Adequate correlation between eye (axial) length and refractive power –> parallel light rays fall on retina (when there is no accommodation)

47
Q

Describe ametropia

A

Mismatch between axial (eye) length and refractive power –> parallel light rays don’t fall on retina.

Types of ametropia:

  1. Near sightedness (myopia)
  2. Farsightedness (hyperopia)
  3. Astigmatism
  4. Presbyopia
48
Q

Describe myopia

A

Light rays converge to a point anterior to retina.

Causes - long globe/eye (common), excessive refractive power

Symptoms - blurred distance vision, headache

Treated with concave glasses

49
Q

Describe hyperopia

A

Light rays converge at a focal point posterior to retina

Causes - excessive short globe (axial hyperopia - common), insufficient refractive power

Symptoms - near visual acuity blurs early

Lazy eye = amblyopia - uncorrected hyperopia

Treatment = convex glasses or remove lens and put in IOLS (cataract extension)

50
Q

Describe astigmatism

A

Parallel rays focus in 2 focal lines - no single focal point. Heredity

Causes - refractive index/ corner not evenly shaped, not spherical –> different refraction along one meridian than along meridian perpendicular to it –> 2 focal points

Symptoms - blurred vision, distortion of vision

Treatment - regular astigmatism = cylinder lenses (+ convex/concave if needed)

Irregular Astigmatism = rigid cylinder lense, surgery

51
Q

What is the “near response triad” for adaptation for near vision?

A
  1. Pupillary miosis (sphincter pupillae) - miosis = constriction of pupil
  2. Convergence (medial recti from both eyes) to align both eyes to near object
  3. Accommodation (circular ciliary muscle) –> increase refractive power of lense
52
Q

What is presbyopia

A

Naturally occurring loss of accommodation from the age of 40.

Distance vision intact.

Treated by convex glasses (increase refractive power)

53
Q

What are the complications of contact lenses

A

Infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis

54
Q

What type of lenses give the best optical correction for aphakia

A

Intraocular lenses

55
Q

What are the cons of clear lens extraction and IOL (intra-ocular lens)

A

Same as cataract extraction –> artificial lens implanted

Cons = lose accommodation, patient will need reading glasses

56
Q

Describe the accommodation mechanism

A

Mediated by oculomotor nerve

  1. Contraction of circular ciliary muscle in ciliary body —> Relaxes zonules (which are normally stretched between ciliary body attachment and lens capsule attachment) - zonules are passive elastic bands with no active contractile muscle
  2. Makes lens thicker, increasing refractive power of lens

Year 2 Collin (114 decks)

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