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Neuroscience and Mental Health > Structure and Function of the Eye > Flashcards

Structure and Function of the Eye Flashcards

1
Q

Where are the lacrimal glands located?

A

They are located in the upper lateral region of each orbit, in the lacrimal fossa of the orbit.

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2
Q

What are the three types of tears?

A

Basal Tears – produced at a constant level in the absence of irritation
Reflex Tears – increased tear production in response to irritation
Emotional Tears – crying

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3
Q

Describe the innervation of the cornea mediating the sensory pathway of the tear reflex

A

The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)

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4
Q

Describe the drainage of tears.

A

• Tear produced by the Lacrimal Gland
• Tear drains through the two puncta (two tiny openings in the upper and lower medial lid margins
• Tear flows through the superior and the
inferior canaliculi
• Tear gathers in the Tear Sac
• Tear exits the Tear Sac through the tear duct into the nose cavity

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5
Q

State the 4 overall functions of the tear film

A
  • Tear film maintains smooth cornea-air surface
  • Oxygen Supply to Cornea (since normal cornea has no blood vessels)
  • Removal of Debris (along with blinking)
  • Bactericide
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6
Q

What are the three layers of the tear film?

A

Superficial oily layer
Aqueous tear film
Mucinous layer

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7
Q

What produces the superficial oily layer, and the aqueous layer?

A

Superficial layer is produced by a row Meibomian glands along the lid margins.
Aqueous layer is produced by the Lacrimal gland.

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8
Q

Briefly state the importance of each layer, in terms of their role

A

Superficial Oily Layer => Reduces tear film evaporation
Aqueous Tear Film => Oxygen and nutrients; Bactericide
Mucinous Layer => Ensures that tear film sticks to the eye.

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9
Q

Describe 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.
  • It is nourished by tiny blood vessels that are nearly invisible to the naked eye.
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10
Q

What is the average anterior-posterior diameter of the eye in adults?

A

24mm

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11
Q

What are the three layers that enclose the contents of the eye? Describe their properties and function.

A

Sclera

  • Hard and opaque (protective)
  • Maintains the shape of the eye
  • High water content
  • At the front of the eye it becomes the cornea; at the back it becomes the dura mater covering the optic nerve

Choroid

  • Pigmented and vascular
  • Provides circulation to the eye
  • By absorbing light it limits total internal reflection within the eye.
  • Dark brown in colour because of the presence of choroidal pigment cells.
  • At the front the choroid becomes the ciliary body and the iris.

Retina

  • Neurosensory
  • Converts light into neurological impulses
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12
Q

What are the two segments of the eye separated by?

A

Lens separates anterior and posterior segments

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13
Q

Describe the cornea, to include its roles

A
  • Front-most part of Anterior Segment
  • Transparent, dome-shaped window
  • Low water content

Main roles:

  • Provides 2/3 of the eye’s refractive power
  • Physical barrier protecting from infection
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14
Q

State the 5 layers that make up the cornea (outer to inner)

A
  1. Epithelium
  2. Bowman’s Membrane
  3. Stroma
  4. Descemet’s Membrane
  5. Endothelium (1 cell thick)
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15
Q

Briefly state the importance of the the endothelium layer of the cornea

A

Allows leakage of solutes and nutrients from the aqueous humor to the more superficial layers of the cornea while at the same time pumping water in the opposite direction, from the stroma to the aqueous
Note that:
=> No regeneration power so endothelial cell density decreases with age
=> Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness

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16
Q

What is the collective term for the choroid, iris and ciliary body that are intimately connected?

A

Uvea

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17
Q

Describe the role of the iris and how it carries out this role? What are the two layers of the iris?

A
  • Controls the amount of light reaching theretina via a round opening in the centre of the iris is called the pupil.
    The iris contains two intraocular muscles which act in concert to control the size of the pupil:
  • Innermost is a flat ring of circularly arranged smooth muscle fibers = pupillary sphincter
  • Surrounding the pupillary sphincter is a layer of radially organized myoepithelial cells which form the pupillary dilator.

Two layers:

  • Anterior stromal layer containing muscle fibres
  • Posterior epithelium
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18
Q

What are the two components that comprise the ciliary body and what are they each responsible for?

A

The ciliary body is a ring shaped tissue (surrounding the lens).
It includes the ciliarymuscle, which controls the shape of the lens, and theciliaryepithelium, which actively secretes the aqueous humor

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19
Q

Describe how the retina and choroid contribute to the different parts of the iris and ciliary body.

A
  • Retina gives rise anteriorly to the ciliary body epithelium and the posterior (epithelial) layer of the iris
  • Choroid gives rise anteriorly to the ciliary body stroma and the anterior layer of the iris (stromal layer)
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20
Q

What name is given to the passive connective tissue fibrous strands, forming a ring, that suspend the lens from the ciliary body? Name the ligament they collectively form?

A

Zonules

Suspensory ligament

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21
Q

Which humours are found in the two segments of the eye?

A 
Anterior = aqueous humour 
Posterior = vitreous humour
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22
Q

Describe the passage and drainage of aqueous humour.

A

From the ciliary body epithelium, aqueous humour percolates through the pupil to the anterior chamber from where it drains into the venous system via the trabecular meshwork and canals of Schlemm located in the irideocorneal angle

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23
Q

What is the role of aqueous humour?

A

Supplies metabolic substrates for the lens and cornea which have no blood supply, and maintains eyeball pressure

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24
Q

Describe vitreous humour, including its main functions

A

Vitreous humor is a gel of extracellular fluid (99% water) which contributes to refractive power.

25
Q

What is the normal range for intraocular pressure?

A

11-12 mm Hg

26
Q

Describe how the vitreous humour changes with age.

A

It loses its jelly consistency, liquefies and can become detached from the retina
Vitreous detachment in seen as floaters

27
Q

What are the potentially disastrous consequences of vitreous humour detachment?

A

Detaching from the retina could cause a small tear in the peripheral retina
If there is a small tear, vitreous humor could seep into the sub-retinal space and lead to retinal detachment
If untreated, it can lead to blindness

28
Q

Describe the structure of the lens

A
  • The lens is encapsulated within an elastic connective tissue membrane (acellular) which is attached to the suspensory ligament.
  • There are regular inner elongated fibres, which give the lens its transparency

NOTE: may lose transparency with age (cataract)

29
Q

Outline the functions of the lens

A
  • Provide 1/3 of the refractive power of the eye

- Elasticity allows for accommodation

30
Q

Define glaucoma?

A

Condition of sustained raised intraocular pressure

31
Q

What changes can be seen in the retina in glaucoma?

A

Retinal ganglion cell death

Enlarged optic disc ‘cupping’ (axons of the optic nerve are compressed at the optic disc)

32
Q

What are the consequences of untreated glaucoma?

A

Progressive loss of peripheral vision

Blindness

33
Q

What is the most common type of glaucoma and what is it caused by?

A

Primary open angle glaucoma

It is caused by a functional blockage of the trabecular meshwork

34
Q

State another relatively common type of glaucoma. What is it caused by?

A

Closed angle glaucoma, which can be acute or chronic.
It is caused by the forward displacement of the iris-lens complex – narrowing the trabecular meshwork (vicious cycle since increased pressure further pushes the iris/lens forward)

35
Q

State a risk factor for Closed angle glaucoma. What does it normally present with? What is the treatment?

A

Risk factor = small eye in high hypermetropia
- May present with sudden painful red eye with acute drop in vision

  • Treatment: peripheral laser iridotomy to create a drainage hole on the iris
36
Q

Light is refracted by thecorneaandlensto focus the incoming light rays onto the retina to form a clear image.
What is the term given to perfect focusing ability?

A

Emmetropia (i.e. perfect correlation between the axial length of the eye and it’s refractive index)

37
Q

What is ametropia? What are the two types?

A

Axial length of the eye does not match its refractive power
As a result, parallel light rays DO NOT fall on to the retina perfectly.
Myopia = short-sightedness
Hyperopia = long-sightedness
Astigmatism
Presbyopia

38
Q

How would you characterise myopia, including symptoms? What type of corrective lens is required?

A
  • Parallel rays converge at a focal point anterior to the retina
  • Due to a long globe (axial myopia) or excessive refractive power (refractive myopia)
  • Blurred distance vision, headache

Requires a concave lens

39
Q

How would you characterise hyperopia, including symptoms? What type of corrective lens is required?

A
  • Parallel rays converge at a focal point posterior to the retina
  • Due to a short globe (axial hyperopia) or inadequate refractive power (refractive hyperopia)
  • Blurred near vision, eyepain, headache, burning sensation in the eyes

Requires convex lens

40
Q

What is astigmatism?

A

An imperfection in the curvature of the cornea, so that it is oval rather than round
Therefore, light rays are converged to more than one focal point
Hereditary

41
Q

What is the near response triad?

A

Pupillary Miosis (constriction of sphincter pupillae) – increases the depth of field
Convergence – the medial recti of both eyes contract to align both eyes towards a near object
Accommodation – ciliary muscles contract to thicken the lens and increase its refractive power

42
Q

Which nerve is responsible for accommodation? How does contraction of the ciliary muscle lead to thickening of the lens?

A

Oculomotor Nerve (CN III)

Contraction of the ciliary muscles pulls the ciliary body forwards and inwards, easing the tension in the suspensory ligament and lens capsule, allowing the lens to become more spherical

43
Q

What is presbyopia? How is it fixed?

A

The term given to naturally occurring loss of accommodation with age (onset from age 40) but with intact distant vision

Corrected by reading glasses (convex lenses) to increase refractive power of the eye

44
Q

What are the four branches of vessel arcades radiating from the optic disc?

A

Superior Temporal
Inferior Temporal
Superior Nasal
Inferior Nasal

45
Q

The optic nerve connects where?

A

The optic nerve transmits electrical impulses from the retina to the brain; it connects to the back of the eye medial/nasal to the macula/fovea

46
Q

What is the macula and what is it responsible for?

A
  • It is a small and highly sensitive part of the retina responsible for detailed central vision.
  • The fovea is the very centre of the macula and has the highest concentration of cone cells, but a relatively low concentration of rods
47
Q

Describe central vision

A
  • Allows for Detailed Day Vision, Colour Vision, Reading, Facial Recognition – Fovea
  • Assessed by Visual Acuity Assessment
48
Q

Describe peripheral vision.

A
  • Important for navigation, so shape, movement, night Vision
  • Assessed by Visual Field Assessment
49
Q

Describe the structure of the retina.

A

Immediately adjacent to the choroid you have the retinal pigment epithelium.
Then you have the neuroretina, which consists of three layers:
Outer Layer = photoreceptors (rods and cones); 1st order neurones
Middle Layer = bipolar cells; 2nd order neurones
Inner Layer = retinal ganglion cells; 3rd order neurones, axons of which form the optic nerve, and only become myelinated as they leave the eye via the optic disc.

50
Q

What is the function of the retinal pigment epithelium?

A

Transports nutrients from the choroid to the photo-receptor cells and removes metabolic waste from the retina

51
Q

Describe how the fovea appears on a cross-section of the macula.

A

It appears as a foveal pit due to the absence of overlying retinal ganglion cells

52
Q

What is the significance of the optical disc for the visual field?

A

This region lacks photoreceptors and accounts for the blind spot that occurs in the visual fields.

53
Q

State the two classes of photo-receptor and their properties.

A

Rods

  • Longer outer segment with photo-sensitive pigments
  • 100 times more sensitive to light than cones
  • Slow response to light
  • Responsible for night vision (scotopic vision)
  • 120 million rods

Cones

  • Less sensitive to light
  • Faster response to light
  • Responsible for daylight vision and vision and colour vision (photopic vision)
  • 6 million cones
54
Q

Describe the distribution of rods and cones across the retina.

A
  • Rods have the highest density 20-40 degrees away from fovea
  • They decrease in density the further you move away from the macula
  • There are NO rods at the fovea
  • Cones are most dense at the fovea and their numbers fall off sharply beyond 5º of it.
55
Q

Where is the physiological blind spot located?

A

20 degrees temporal to the fovea

56
Q

What are the three types of cone photopigment and which colours do they respond maximally to?

A

S-cone – short wavelength – BLUE
M-cone – medium wavelength – GREEN
L-cone – long wavelength – RED

57
Q

What is the most common colour vision deficiency and what is it caused by?

A

Deuteranomaly

It is caused by the shifting of the M-cone towards the L-cone (in terms of frequency response)

58
Q

What is the term given to shifted peaks?

A

Anomalous trichomatism

59
Q

What test is used to diagnose colour blindness?

A

Ishihara Test

Neuroscience and Mental Health (17 decks)

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