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Y2 LCRS 2 - Neuroscience and mental health > Structure and function of the eye > Flashcards

Structure and function of the eye Flashcards

1
Q

What are the bones making up the orbit of the eye?

A
  • Frontal
  • Sphenoid
  • Lacrimal
  • Ethmoid
  • Zygomatic
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2
Q

What is the average anterior-posterior diameter of the adult eye?

A

24mm

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3
Q

What are the two segments of the eye? What are the separated by? What fluid do their contain

A

Two segements?

  • Anterior segment
    • Contains aqueous humour
  • Posterior segment
    • Contains vitreous humour

Separated by LENS

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4
Q

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

A

Outer to inner:

  1. Sclera
  2. Choroid
  3. Retina
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5
Q

Describe the structure and function of the sclera.

A

Structure:

  • Hard and opaque
  • High water content - therefore opaque
  • Continous with the cornea

Function:

  • Protects eye and maintains its shape
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6
Q

Describe the characteristics of the cornea.

A

Structure:

  • Transparent
  • Dome-shaped
  • Covers front of eye
    • Most anterior part of the anterior segment
  • Low water content - therefore transparent

NOTE: If you hydrate the cornea (i.e. increase the water content) then it will become opaque

Function:

  • Powerful refracting surface due to its convex (dome) shape
  • Provides 2/3rds of the eye’s refractive (focusing) power
  • Physical barrier
  • Infection barrier
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7
Q

What are the 5 layers of the cornea?

A

Outer to inner:

  1. Epithelium
  2. Bowman’s Membrane
  3. Stroma
  4. Descemet’s Membrane
  5. Endothelium
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8
Q

Describe the structure of the stroma.

A
  • Layer 3 of cornea
  • Thickest layer
  • Highly regular arrangement of collagen fibrils for transparency
    • The fibrils arrangement and water content determines transparency and opacity
  • ​No blood vessels
  • Has corneal nerve endings provide sensation and nutrients to cornea
    • Nutrients provided via neurotrophins (growth factors)
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9
Q

Describe the characteristics of the endothelium.

A

Structure:

  • Layer 5 of cornea
  • Only one layer of endothelial cells

Function:

  • Pumps fluid out of cornea, preventing corneal oedema

Characteristics:

  • Endothelial cells have no regeneration power
  • Endothelial cell density decreases with age
  • Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness
    • Makes sense because hydrating the cornea makes it opaque
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10
Q

Describe the structure of the choroid.

A
  • Pigmented and dark
  • Composed of layers of blood vessels that nourish the back of the eye
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11
Q

What is the uvea?

A

Vascular coat of eye ball - refers to the entire network of vasculature of the eye

Lies between the sclera and retina

Composed of 3 parts - shown in red on diagram:

  • Iris
  • Cililary body
  • Choroid

These three portions are intimately connected - therefore a disease of one part also affects the other portions though not necessarily to the same degree

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12
Q

What are the layers of the iris?

A

Anterior - stromal layer

Posterior - epithelial layer

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13
Q

Describe the structure and function of the lens.

A

Structure:

  • Outer acellular capsule
    • Basement membrane - composed of collagen
  • Regular inner elongated cell fibres
    • Regular arrangement gives transparency
    • The lens may lose transparency with age → cataract

Function:

  • Provides 1/3rd of the eye’s refractive power
    • Higher refractive index than aqueous humour and vitreous humour

Its transparency and elasticity are also useful properties for its function

  • Transparent - lets light in
  • Elastic - for accomodation
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14
Q

What are lens zonules?

A

Lens is suspended by a fibrous ring known as lens zonules

  • Lens zonules consist of passive connective tissue??
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15
Q

Describe the structure and function of the retina.

A

Structure:

  • Very thin layer of neurosensory tissue
  • Lines the inner part of the eye.

Function:

  • It is responsible for capturing the light rays that enter the eye
    • Much like the film’s role in photography
  • These light impulses are then sent to the brain for processing, via the optic nerve
  • SUMMARY: Converts light into action potentials sent down optic nerve via photoreceptors
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16
Q

What is the function of the optic nerve (CNII)?

A

Transmits electrical impulses (action potentials) from the retina to the brain

Photoreceptors of retina → optic nerve → brain

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17
Q

Describe the characteristics of the optic nerve in the eye.

A
  • Optic disc = visible portion of optic nerve
    • Visible when you look through a fundoscope
  • Vision blind spot = where the optic nerve attaches to the eye and hence there are no photoreceptors
  • Macula is found temporal to the optic nerve
    • i.e. Macula is near and lateral to optic nerve

NOTE: With both eyes open, we don’t notice the blind spots becuase the other eye compensates for that part of the visual field

18
Q

Describe the location and characteristics of the macula

A

Location:

  • Roughly in the centre of the retina

Charactersitics:

  • Small + highly sensitive
    • Therefore, it is the part of the retina responsible for detailed central vision
    • Allows us to appreciate detail and perform tasks that require central vision such reading
  • Centre of macula is fovea​
19
Q

Describe the characteristics of the fovea.

A
  • Your fovea is the most sensitive part of the retina.
  • It has the highest concentration of cones, but a low concentration of rods
    • This is why stars out of the corner of your eye are brighter than when you look at the directly
  • But only your fovea has the concentration of cones to perceive in detail
    • Macula allows detailed central vision
    • Fovea gives the most detailed vision (highest resolution)
20
Q

COME BACK TO THIS What is retinal detachment?

A

Vitreous Humour liquifies and detaches from retina with age

If there is a tear in the retina, the vitreous humour floods behind the retina and detaches it from the other coatings, leading to blindness

21
Q

What are the different types of tears?

A

3 Types of Tears:

  • Basal – constantly produced
  • Reflex – in response to irritation
    • Afferent: CN V1 (ophthalmic branch of trigeminal)
    • Efferent: Parasympathetic nerve
      • ACh neurotransmitter
  • Emotional – crying
22
Q

Describe the lacrimal system

A

Lacrimation = tear production

  • Tears secreted by lacrimal gland
  • Drains through the two puncta
    • Puncta = openings on medial lid margin
  • Drainage route:
    • Puncta → canaliculi → tear sac → tear duct → nasal cavity
23
Q

Describe the functions of tear film.

A

Functions:

  • Tear film maintains smooth cornea-air surface
  • Oxygen Supply to Cornea
    • Normal cornea has no blood vessels
    • Gaseous exchange takes place across the tear interface
  • Removal of Debris
    • This is due to both the tear film AND blinking
      • ​Debris can be captured in the tear film
      • Blinking creates a fresh new tear film
  • Bactericide
24
Q

Describe the structure of tear film.

A

Three Layers:

  • Superficial oily (lipid) layer:
    • Outermost
    • To reduce tear film evaporation
    • Produced by a row of Meibomian Glands along the lid margins
  • Aqueous tear film:
    • Produced by lacrimal gland
    • Contains bactericides
  • Mucinous Layer:
    • Innermost
    • On the corneal surface to maintain surface wetting
      • This facilitates diffusion
25
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
    • Reflects back upon itself to line the inside of the eyelids
  • It is nourished by tiny blood vessels that are nearly invisible to the naked eye
26
Q

What are the two chambers of the eye?

A

Anterior chamber

  • Between cornea and lens
    • More specifically between cornea and iris
  • Filled with Clear aqueous fluid (i.e. aqueous humour)
    • ​Aqueous humour supplies nutrients - to the avascular ocular tissues

Posterior chamber

  • Between iris and lens
  • Also contains aqueous humour
27
Q

What does the ciliary body do?

A

Secretes aqueous humour

28
Q

Describe the flow and drainage of aqueous humour.

A

Flow:

  • Intraocular aqueous humour flows anteriorly into the anterior chamber (along the green arrow)

Drainage:

  • Drains out of eye via trabecular meshwork and into canal of Schlemm
    • ​80-90%
  • Uveal-scleral outflow
    • Uvula → sclera - drained into vessels which are different from the other pathway
    • Proportion - the remainder of fluid not drained into the trabecular meshwork
29
Q

What is glaucoma?

A

Caused by sustained raised intraocular pressure compressing back of eye

↓

Retinal ganglion cell death & enlarged optic disc cupping

↓

Visual field loss & blindness

NOTE:

Optic disc cupping:

  • The fibres which go into the optic nerve essentially all go round the outside
    • This is the proper ‘disc’ part of the optic disc
  • This means there is a space in the middle which is the ‘cup’
  • As the nerve fibres within the optic nerve begin to die as a consequence of glaucoma, the ‘cup’ becomes larger
30
Q

What are the two types of glaucoma?

A

Primary open angle glaucoma - commonest

Closed angle glaucoma

31
Q

What causes primary open angle glaucoma?

A

Trabecular meshwork dysfunction - therefore aqueous humour not being drained properly

32
Q

What causes closed angle glaucoma?

A

Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork

  • Can be acute or chronic
  • Vicious cycle
    • As the aqueous humour is not being drained, the pressure increases even more
    • This pushes the iris/lens even more forward causing even more blockage of the trabecular meshwork, reducing drainage
33
Q

State the risk factors, presentation and treatment for closed angle glaucoma.

A

Risk factors:

  • Small eye (hypermetropia)
    • i.e. Shorter eyeball than ususal
  • Narrow angle at trabecular meshwork

Presentation:

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

Treatment:

  • Peripheral laser iridotomy to create a drainage hole on the iris
34
Q

What is the difference between central and peripheral vision?

A

Central vision:

  • Functions:
    • Detail 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

Peripheral vision:

  • Functions:
    • 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
35
Q

Describe the structure of the retina.

A

There are 2 Parts to the retina:

  • Retinal pigment epithelium
    • Transorts metabolies between choroid and photoreceptors
  • Neuroretina

3 layers of the neuroretina:

  • Outer Layer/ 1st Order Neurones:
    • Photoreceptors - detect light
  • Middle Layer/ 2nd Order Neurones:
    • Bipolar cells – local signal processing to improve contrast sensitivity, regulate sensitivity
  • Inner Layer/ 3rd Order Neurones:
    • Retinal ganglion cells – Transmit AP (signal) from eye to brain via optic nerve
36
Q

Describe the structure of the macula and fovea in terms of retinal cells.

A

Macula:

  • Also known as the macula lutea (yellow patch)
    • Pigmented region at the centre of the retina of about 6 mm in diameter

Fovea:

  • Forms the pit at the centre of the macula due to absence of the overlying ganglion cell layer
  • Fovea has the highest concentration of photoreceptors for fine vision

Clinically can be assessed with an OCT scan (Optical Coherence Tomography) - allows you to see each of the retina’s distinctive layers

37
Q

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

A
  • Rods
  • Cones
38
Q

How do the photoreceptors work?

A

They synthesise their photopigment in inner photoreceptor segment then transport to outer segment discs

Photopigment in rods:

  • Photopigment: rhodopsin
  • With cofactor: 11 cis retinal cofactor
  • Photons induce change in cofactor, inducing change in rhodopsin
  • Activates G-protein pathway, generating AP
  • Rhodopsin reacts most efficiently to 498nm light

Photopigment in cones:

  • Photopigment: photopsin - 3 types
    • S-cone: BLUE spectrum
    • M-cone: GREEN spectrum
    • L-cone: RED spectrum

These photopigments are technically GPCRs which when activated by light (photons) results in a signal transduction pathway which ultimately leads to AP generation

39
Q

Describe the structure and function of the rods.

A
  • 120 million rods
  • Longer outer segment with photo-sensitive pigment
  • 100 times more sensitive to light than cones
    • i.e. requires less light to function
  • Slow response to light
  • Responsible for:
    • Night vision
    • Peripheral vision
    • Scotopic vision
      • This is the vision of the eye under low light conditions
      • Happens completely through rod cells
    • Recognising motion
40
Q

Describe the structure and function of the cones.

A
  • 6 million
  • Less sensitive to light, but faster response
    • i.e. require more light to function
  • Responsible for:
    • Day Vision
    • Central vision
    • Photopic vision
      • Vision under well lit conditions
      • Mediated by cone cells
    • Recognising colour and details
41
Q

Where is the highest concentration of cones and rods found?

A

Cones - on the fovea

Rods - 20-40 degrees from fovea

42
Q
A

Y2 LCRS 2 - Neuroscience and mental health (23 decks)

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