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Neuro > Structure and function of the eye > Flashcards

Structure and function of the eye Flashcards

1
Q

When are tears produced

A

Basal tears
Reflex tears- response to irritation
Crying (emotional) tears

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2
Q

Lacrimal system

A

Tear produced by the lacrimal gland (superior and on lateral side of the eye)

Tear drains through the two puncta, opening on medial lid margin

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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3
Q

Tear film function

A

Maintains smooth cornea-air surface- important for clear vision and removing debris in blinking

Oxygen Supply to Cornea – Normal cornea has no blood vessels

Bactericide

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4
Q

Layers of tear film and their function

A
  1. Superficial Oily Layer: reduces tear film evaporation (produced by Meibomian Glands along lid margins)
  2. Aqueous Tear Film (Tear Gland)- delivers oxygen and nutrients to surrounding tissue
  3. Mucinous Layer on the Corneal Surface- ensures tear film sticks to surface of the eye
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5
Q

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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6
Q

Layers of the Coat of the eye and their function

A
  • Sclera – Hard and Opaque- protects and maintains shape- high water content
  • Choroid – Pigmented and Vascular- provides circulation and shields unwanted light
  • Retina – Neurosensory Tissue- converts light to impulses which are transmitted to the brain via the optic nerve
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7
Q

Cornea- location and function

A

The cornea is the front-most part of the anterior segment and is continuous with the scleral layer

Transparent, dome-shaped window (convex curvature) covering the front of the eye

It is a powerful refracting surface, providing 2/3 of the eye’s focusing power

The cornea acts as both a physical barrier and an infection barrier

The cornea relies on tear film and aqueous fluid, for nutrients and oxygen supply
Low water content – dehydrated by the inner layer of the cornea – corneal endothelium

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8
Q

Layers of the cornea

A
  1. Epithelium
  2. Bowman’s Membrane
  3. Stroma – thickest layer
  4. Descemet’s Membrane
  5. Endothelium
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9
Q

Uvea components

A
  • The Iris
  • The ciliary body
  • The choroid
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10
Q

Uvea function and location

A

The uvea is the vascular coat of eyeball and lies between the sclera and retina.

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11
Q

The choroid- location and function

A

The posterior part of the uvea is the choroid

The choroid lies between the retina and sclera

It is composed of layers of blood vessels that nourish the back of the eye

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12
Q

The iris- location and function

A

The coloured part of the eye is called the iris

It controls light levels inside the eye similar to the aperture on a camera

The round opening in the centre of the iris is called the pupil

The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size

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13
Q

Lens structure

A

It is composed of an outer acellular thin capsule, encasing a core of regular elongated cell fibres

The lens may loose its transparency with age, resulting in an opaque lens, known as Cataract.

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14
Q

Lens Function

A

Responsible for one third of the refractive power of the eye

located behind the iris

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15
Q

Lens zonules

A

Fibrous ring that suspends the lens- consisting of passive connective tissue

Connects the lens to the ciliary body

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16
Q

Retina- location and function

A

The retina is a very thin layer of tissue that lines the inner part of the eye

It is responsible for capturing the light rays that enter the eye

These light impulses are then sent to the brain for processing, via the optic nerve

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17
Q

Optic nerve- location and function

A

Optic nerve transmits electrical impulses from the retina to the brain

It connects to the back of the eye near the macula

The visible portion of the optic nerve is called the optic disc

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18
Q

The Macula- location and function

A

Located roughly in the centre of the retina, temporal (lateral) to the optic nerve

It is a small, highly sensitive part of the retina responsible for detailed central vision

The fovea is the very centre of the macula

The macula allows us to appreciate detail and perform tasks that require central vision such readin

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19
Q

Segments of the eye- and what they contian

A

Anterior segment- In front of the lens
Contains the anterior chamber- filled with aqueous fluid which supplies nutrients to tissue

Posterior segment- behind the lens
Contains posterior chamber

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20
Q

What happens to the anterior chamber when infected or inflamed

A

Usually ‘optically empty’ because it is filled with aqueous humour (which is completely transparent).

If infected or inflamed we see debris and cells..

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21
Q

Ciliary body

A

Ring-shaped tissue, surrounding the lens.

It secretes aqueous fluid into the Anterior Chamber. Aqueous fluid supplies vital nutrients for the surrounding tissue.

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22
Q

Layers in the iris

A

The thin posterior pigmented epithelial layer, and the thick anterior layer, composed of stromal tissue & smooth muscles.

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23
Q

Aqueous Flow

A

Aqueous humour flows from the ciliary body, into the anterior chamber, and reabsorbed by the trabecular meshwork.

There is another route of aqueous humour exit from the anterior chamber; the canal of Schlemm. This canal is invisible and sits around the cornea, deep in the sclera

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24
Q

Glaucoma

A

Glaucoma is an eye condition, characterised by sustained high eye pressure

Results in gradual and accumulative damage to the optic nerve tissue, in the posterior segment of the eye

25
Q

Types of Glaucoma

A

Primary Open Angle Glaucoma – Commonest- Trabecular Meshwork Dysfunction

Closed Angle Glaucoma – can be acute or chronic Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork – vicious cycle

26
Q

Central vision

A

Detail Day Vision, Colour Vision – Fovea has the highest concentration of cone photoreceptors

Reading, Facial Recognition (patients with loss of central vision have problems with reading)

27
Q

Peripheral vision

A

For detecting shape and movement in the environment, Night Vision

Navigation Vision – patients with loss of peripheral vision have problems navigating the world

28
Q

Renal Structure

A

Outer Layer – Photoreceptors (1st Order Neuron) – Detection of Light

Middle Layer-Bipolar Cells (2nd Order Neurons) – Local Signal Processing to improve contrast sensitivity, regulate sensitivity

Inner Layer – Retinal Ganglion Cells (3rd Order Neurons) – Transmission of Signal from the Eye to the Brain

29
Q

Retinal pigment epithelium- location and transport

A

Outer layer situated in front of the choroid

Transports nutrient from the choroid to the photoreceptor cells, and removes metabolic waste from the retina

30
Q

Neuroretina

A

Outer Layer consisting of photoreceptors, the middle layer of intermediate neurons, and the inner layer of ganglion nerve cells, with axon running along the optic nerve into the brain.

31
Q

Photoreceptors

A

Rod Photoreceptor

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

Cone Photoreceptor

  • Less sensitive to light, but faster response
  • Responsible for day light fine vision and colour vision (Photopic Vision)
  • 6 million cones
32
Q

Photoreceptor distribution

A

Rods distributed all over the retina, with highest density just outside the macula

Cones densely packed in macula, with very few elsewhere

No photoreceptors in the blindspot

33
Q

Scotopic Vision

A

Peripheral and Night Vision

34
Q

Phototopic vision

A

Central and Day Vision

35
Q

Which cones detect each wavelength

A
  • S-Cones: with photo-pigment sensitive to short wavelength – blue
  • M-Cones: with photo-pigment sensitive to medium wavelength – green
  • L-Cones: with photo-pigment sensitive to long wavelength – red

Yellow light stimulates both M-cones and L-cones equally

36
Q

Colour vision deficiency

A

Colour Vision deficits can be caused by a shift in the photo-pigment peak sensitivity. This is Anomalous Trichromatism

Colour Vision deficits can be caused by the absence of one or more of the 3 cone photo-pigments. Dichromatism: 2 cone photo-pigment sub-types are present. Monochromatism: there is complete absence of colour vision.

37
Q

Ishihara Test

A

Tests red-green colour perception deficiencies

Consists of plates of circle of dots appearing randomly in size

Subjects with normal red-green vision will recognize the correct pattern in the form of a 2-digit number

38
Q

Dark Adaptation

A

The retina increases its light sensitivity in dark

Biphasic Process

Retina switches from photopic vision to scotopic vision

39
Q

Light Adaptation

A

Occurs over 5 minutes

Bleaching of photo-pigments mediates the process

Neuro-adaptation: Inhibition of Rod/Cone function

Pupil Adaptation (minor) constriction of pupil with light

Pupil acts an adjustable aperture to regulate light intake

40
Q

Ametropia

A

Refers to vision disorders characterized by the eyes inability to correctly focus the images of objects on the retina. Its forms include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism.

41
Q

Refraction

A

As light passes from one medium to another its velocity changes causing it to bend

This occurs in the eye- tear film, cornea, aqueous humour, lens, vitreous humour, retina

42
Q

Convex lens

A

Light rays converge towards a focal point

The focal point is at a distance from the central plane of the lens – this distance is proportional to the thickness of the lens

Thicker lens = closer focal point is to the lens

43
Q

Concave lens

A

Light disperses around because it is refracted in a divergent way

Therefore, the focal point is a virtual point

This virtual focal point is in front of the lens – not after

44
Q

Emmetropia

A

Eye with refractive power of 0

No defects

45
Q

Myopia

A

Parallel rays converge at a focal point anterior to the retina

Nearsightedness- can’t see far away objects well

46
Q

Myopia causes and symptoms

A

Causes:

  • Excessive long globe (axial myopia): more common
  • Excessive refractive power (refractive myopia)

Symptoms:

  • Blurred distance vision
  • Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
  • Headache
47
Q

How myopia is corrected

A

Myopia can be corrected by concave glasses. Concave lenses are divergent – so they move the focal point slightly backwards towards the retina – the person can see.

48
Q

Hyperopia

A

Parallel rays converge at a focal point posterior to the retina

Farsightedness- can’t see close objects well

49
Q

Hyperopia causes and symptoms

A

Causes:

  • Excessive short globe (axial hyperopia) : more common
  • Insufficient refractive power (refractive hyperopia)

Symptoms:

  • Visual acuity at near tends to blur relatively early
  • Asthenopic symptoms: eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis (inflammation of eyelids and conjunctiva)
  • Ambylopia- lazy eye- brain starts to exclude information from the hyperopic eye
50
Q

How hyperopia is corrected

A

Corrected with convex lens

Focuses it to make it hit the retina

51
Q

Astigmatism

A

Parallel rays come to focus in 2 focal lines rather than a single focal point

52
Q

Cases and symptoms of astigmatism

A

Causes:
Refractive media is not spherical – more elliptical
The cornea is not evenly shaped in terms of the radius

Symptoms:

  • Asthenopic symptoms ( headache , eyepain)
  • Blurred vision
  • Distortion of vision
  • Head tilting and turning
53
Q

Treatment of astigmatism

A

Regular astigmatism: cylinder lenses with or without spherical lenses(convex or concave)

Irregular astigmatism: rigid CL , surgery

54
Q

Prebyopia

A

Naturally occurring loss of accommodation (focus for near objects)

Onset from age 40 years (as we age, our natural lens loses its elasticity)

Distant vision intact

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

55
Q

Types of optical correction

A

Spectacle lenses
Contact lenses
Intraocular lenses
Surgical correction

56
Q

Accommodation mechanism

A

Contraction of the Circular Ciliary Muscle inside the Ciliary Body

This relaxes the zonules that are normally stretched between the ciliary body attachment and the lens capsule attachment

In the absence of zonular tension, the lens returns to its natural convex shape due to its innate elasticity

This increases the refractive power of the lens

57
Q

When do the ciliary bodies contract

A

When focusing on a near object
Want to have larger lens for higher refractive power

This uses CN III

58
Q

Light from near objects vs far

A

Near- are divergent so need stronger refractive power to focus

Far- parallel so need less refractive power to focus

Neuro (9 decks)

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