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Year 2: Neuroscience and Mental Health > Structure and function of the eye > Flashcards

Structure and function of the eye Flashcards

1
Q

Label the bony orbit and the surface anatomy of the eye

A

Kenhub/netters

1) Upper and lower eye lid make up the palpebral fissure.
2) Lateral canthus
3) Pupil
4) Iris
5) Sclera
6) Medial canthus
7) Caruncle
8) Limbus - border of the iris from the sclera
9) Tear film covers the eye - lubricates the eye but also has visual function.

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2
Q

Describe the lacrimal system

A

Lacrimal System
– Tear produced by the Lacrimal Gland
– 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

What causes tear production?

A 
– Basal Tears
– Reflex Tears – in response to irritation • Afferent – Cornea – CN V1 (Ophthalmic
Branch of Trigeminal Nerve)
• Efferent – Parasympathetic
• Neurotransmitter - Acetylcholine
– Crying (Emotional) Tears
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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 – Normal cornea has no blood
vessels
• Removal of Debris (Tear film and
Blinking)
• Bactericide
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5
Q

What are the layers of the tear film?

A

– Superficial Oily (lipid) Layer to reduce tear film evaporation (produced by a row of Meibomian Glands along the lid margins)
– Aqueous Tear Film (Tear Gland)
– Mucinous Layer on the Corneal. Surface to maintain surface wetting

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6
Q

What is the conjunctiva?

A
  • The conjunctiva is the 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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7
Q

Label a cross section of an eye and describe the three layers of the eye?

A

Netters/ken hub

  • Cornea
  • Iris
  • Ciliary body (produces aqueous humour)
  • rectus muscle
  • optic disk
  • optic nerve
  • vitreous humour
  • lens
  • aqueous humour
3 layers of the eye
– Sclera – Hard and
Opaque
– Choroid – Pigmented
and Vascular (so it can provide nutrients for the retina which is very energy demanding)
– Retina – Neurosensory
Tissue
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8
Q

Describe the sclera

A
  • The sclera, commonly known as “the white of the
    eye,” is the tough, opaque tissue that serves as the eye’s protective outer
    coat.
  • High water content
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9
Q

What is the cornea and its function?

A
  • The cornea is the transparent, dome-shaped window covering the front of the eye. It is a powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through
  • Low water content
  • It is continuous with the scleral layer
  • Physical and infection barrier
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10
Q

Describe the structure of the cornea?

A 
Structure – 5 Layers
– 1 – Epithelium
– 2 – Bowman’s Membrane
– 3 – Stroma – regularity
contributes towards
transparency
• Corneal nerve endings
provides sensation and
nutrients for healthy tissue
• No blood vessels in normal
cornea
– Descemet’s Membrane
– Endothelium – pumps fluid
out of corneal and prevents
corneal oedema, 
• Only 1 layer of endothelial
cell
• No regeneration power
• Endothelial cell density
decreases with age
• Endothelial cell dysfunction
may result in corneal
oedema and corneal
cloudiness. This is what happens if you hydrate the cornea
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11
Q

Describe the UVEA

A 
Vascular coat of eye ball and lies between the sclera and retina.
Uvea is composed of three parts.
• Iris
• ciliary body
• choroid.
These three portions are
intimately connected and a
disease of one part also affects the other portions though not necessarily to the same degree.
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12
Q

Describe the choroid

A

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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13
Q

Describe the Iris

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. So if the pupil is dilated the focus is on the subject but the foreground and background are blurry.
  • 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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14
Q

Describe the structure and function of the lens

A

Structure
– Outer Acellular Capsule
– Regular inner elongated cell fibres – transparency
– May loose transparency with age – Cataract

Function:
– Transparency
    • Regular structure
– Refractive Power
    • 1/3 power
    • Higher refractive index than aqueous fluid and vitreous
– Accommodation
    • Elasticity
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15
Q

What is the lens zonules?

A

Lens is suspended by a
fibrous ring known as lens
zonules, consists of passive
connective tissue. It connects the lens to the ciliary muscles

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16
Q

What is the retina?

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. Much like
the film's role in
photography.
 - These light impulses are
then sent to the brain for
processing, via the optic
nerve.
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17
Q

What is the optic nerve?

A
  • The optic nerve transmits electrical impulses from the
    retina to the brain. Retinal ganglion neurones
  • 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

What is the macula?

A
  • The macula is located roughly in the centre of the retina, temporal to the optic nerve.
  • Macula Lutea (yellow patch), pigmented region at the centre of the retina of about 6 mm in diameter
  • 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. The macula allows us to appreciate detail and
    perform tasks that require
    central vision such as reading.
19
Q

What are the anterior and posterior segments of the eye?

A

Anterior Segment – Ocular
structure anterior to the lens. Aqueous humour

Posterior Segment – Ocular
structure posterior to the lens. Vitreous humour

20
Q

What is the anterior chamber?

A

– Between Cornea and Lens
– Filled with Clear Aqueous
Fluid (produced by the ciliary epithelium of the ciliary bodies)
– Supplies nutrients

21
Q

Describe the ciliary bodies?

A

• Ciliary Body: sites medial to the ciliary muscle
– Secretes aqueous fluid in the eye
• Intraocular Aqueous Fluid flows anteriorly into the Anterior Chamber along the
green arrow (see diagram)
• Aqueous Fluid supplies nutrient
• Trabecular Meshwork drains the fluid out of the eye out through the angle.
• Once through it is abosrbed via two pathways. Uveal-scleral 10-20% (passive) outflow and TM canal of schlemm 80-90% (active).
• Normal Intraocular Pressure – 12-21mmHg

22
Q

Describe the Schlemm’s canal?

A

Modified vein that sits around the trabecular meshwork where the aqueous is actively absorbed.

23
Q

Define glaucoma

A

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

Glaucoma
– Medical Condition of Sustained Raised Intraocular Pressure (risk factor - remember high pressure does not always mean glaucoma. It just means a higher risk of developing it)
– Retinal Ganglion Cell Death and Enlarged Optic Disc Cupping
– Visual Field Loss,
Blindness

24
Q

What are the types of glaucoma

A

Primary Open Angle Glaucoma (Left) – 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
• 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 to create a drainage hole on the iris

See diagram

25
Q

Describe what can be seen through an ophthalmoscope

A

See diagram

26
Q

Where is blind spot?

A

Where the optic nerve meets the retina there are no

light sensitive cells. It is a blind spot.

27
Q

Describe the fovea?

A
  • Your fovea is the most sensitive part of the
    retina.
  • It has the highest concentration of cones (colour), but a low concentration of rods.
  • Fovea has the highest
    concentration of photoreceptors for fine vision
  • This is why stars out of the corner of your eye are brighter than when you look at it directly.
  • But only your fovea has the concentration of cones to perceive in detail.
  • Fovea forms the pit at the
    centre of the macula due to
    absence of the overlying
    ganglion cell layer
28
Q

What is the difference between central and peripheral vision?

A

Central Vision (Cones)
– 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 (Rods)
– 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

29
Q

Describe retinal 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

See slides

30
Q

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

A

Rod Photoreceptor
• More sensitive to light
than cones
• Responsible for night vision (Scotopic Vision - Peripheral and Night Vision. Recognises motion)

Cone Photoreceptor
• Less sensitive to light, but faster response
• Responsible for day light fine vision and colour vision
(Photopic Vision - Central and Day Vision. Recognises colour and details)

Photoreceptors have different frequencies

31
Q

Describe the distribution of photoreceptors

A

The highest concentration of rod photoreceptors lie 20-40 degrees away from fovea

See graph

32
Q

How do you test colour blindness test?

A

Ishihara test - can test for only red-green deficiencies only. The most common form of colour deficiency is red-green confusion.

Patients with colour vision
deficiencies will not recognise any pattern or recognise the wrong pattern

33
Q

What is light dark adaption?

A 
Dark Adaptation
– Increase in light sensitivity in dark
– Biphasic Process
• Cone adaptation 7 minutes
• 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
Pupil Adaptation (minor)
– Constriction of pupil with light
34
Q

What are the two types of lenses?

A

Convex lens - converging lens, always have focal point after the lens
Concave lens - diverging lens, always have focal point before the lens

35
Q

Define emmetropia

A

Adequate correlation between axial length
and refractive power

Parallel light rays fall on the retina (no
accommodation)

36
Q

Define Ametropia

A

Mismatch between axial length and refractive
power

Parallel light rays don’t fall on the retina (no
accommodation)
– Nearsightedness (Myopia)
– Farsightedness (Hyperopia)
– Astigmatism
– Presbyopia
37
Q

Describe myopia

A

Parallel rays converge at a focal point anterior to the retina
Etiology : not clear , genetic factor

Causes
– excessive long globe (axial myopia) : more common
– excessive refractive power (refractive myopia)

Treatment
- you need a concave lens

Symptoms
– Blurred distance vision
– Squint in an attempt to improve uncorrected
visual acuity when gazing into the distance
– Headache`

38
Q

Describe hyperopia

A

Parallel rays converge at a focal point posterior to the retina
Etiology : not clear , inherited

Causes
– excessive short globe (axial hyperopia) : more common
– insufficient refractive power (refractive hyperopia)

Treatment

  • You need a convex lens
  • surgery

Symptoms
– visual acuity at near tends to blur relatively early
• nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur
• blurred vision is more noticeable if person istired , printing is weak or light inadequate
– asthenopic symptoms: eyepain, headache in frontal
region, burning sensation in the eyes, blepharoconjunctivitis
– Amblyopia – uncorrected hyperopia > 5D

39
Q

Describe Astigmatism

A

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

Cause : refractive media is not spherical–>refract differently along one meridian than along meridian perpendicular to it–
>2 focal points (punctiform object is represent as 2 sharply defined lines)

Symptoms
– asthenopic symptoms ( headache , eyepain)
– blurred vision
– distortion of vision
– head tilting and turning

Treatment
– Regular astigmatism :cylinder lenses with or
without spherical lenses(convex or concave), Sx
– Irregular astigmatism : rigid CL , surgery

LOOK UP

40
Q

What is the near response triad?

A

Adaptation for Near
Vision

Near Response Triad
– Pupillary Miosis (Sphincter
Pupillae) to increase depth
of field
– Convergence (medial recti
from both eyes) to align
both eyes towards a near
object
– Accommodation (Circular
Ciliary Muscle) to increase
the refractive power of
lens for near vision
41
Q

What is Presbyopia

A 
• Naturally occurring loss
of accommodation
(focus for near objects)
• Onset from age 40 years
• Distant vision intact
• Corrected by reading glasses (convex lenses)
to increase refractive power of the eye
Treatment
– convex lenses in near vision
• Reading glasses
• Bifocal glasses
• Trifocal glasses
• Progressive power glasses
42
Q

What are the different types of optical correction?

A

Spectacle lenses
– Monofocal lenses : spherical lenses , cylindrical
lenses
– Multifocal lenses

Contact lenses
– higher quality of optical image and less influence
on the size of retinal image than spectacle lenses
– indication : cosmetic , athletic activities ,
occupational , irregular corneal astigmatism , high
anisometropia , corneal disease

Contact lenses
– disadvantages : careful daily cleaning and
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

43
Q

Describe the mechanism of accomodation

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

– Note that zonules are passive elastic bands with no active contractile muscle

– 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

Mediated by the efferent
Third Cranial Nerve

Year 2: Neuroscience and Mental Health (17 decks)

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