Structure and function of the eye Flashcards
Label the bony orbit and the surface anatomy of the eye
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.
Describe the lacrimal system
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
What causes tear production?
– Basal Tears – Reflex Tears – in response to irritation • Afferent – Cornea – CN V1 (Ophthalmic Branch of Trigeminal Nerve) • Efferent – Parasympathetic • Neurotransmitter - Acetylcholine – Crying (Emotional) Tears
What is the function of the tear film?
• 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
What are the layers of the tear film?
– 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
What is the conjunctiva?
- 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.
Label a cross section of an eye and describe the three layers of the eye?
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
Describe the sclera
- 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
What is the cornea and its function?
- 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
Describe the structure of the cornea?
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
Describe the UVEA
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.
Describe 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.
Describe the Iris
- 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.
Describe the structure and function of the lens
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
What is the lens zonules?
Lens is suspended by a
fibrous ring known as lens
zonules, consists of passive
connective tissue. It connects the lens to the ciliary muscles
What is the retina?
- 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.
What is the optic nerve?
- 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.
What is the macula?
- 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.
What are the anterior and posterior segments of the eye?
Anterior Segment – Ocular
structure anterior to the lens. Aqueous humour
Posterior Segment – Ocular
structure posterior to the lens. Vitreous humour
What is the anterior chamber?
– Between Cornea and Lens
– Filled with Clear Aqueous
Fluid (produced by the ciliary epithelium of the ciliary bodies)
– Supplies nutrients
Describe the ciliary bodies?
• 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
Describe the Schlemm’s canal?
Modified vein that sits around the trabecular meshwork where the aqueous is actively absorbed.
Define glaucoma
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
What are the types of glaucoma
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
Describe what can be seen through an ophthalmoscope
See diagram
Where is blind spot?
Where the optic nerve meets the retina there are no
light sensitive cells. It is a blind spot.
Describe the fovea?
- 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
What is the difference between central and peripheral vision?
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
Describe retinal structure
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
What are the two main classes of photoreceptors in the retina?
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
Describe the distribution of photoreceptors
The highest concentration of rod photoreceptors lie 20-40 degrees away from fovea
See graph
How do you test colour blindness test?
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
What is light dark adaption?
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
What are the two types of lenses?
Convex lens - converging lens, always have focal point after the lens
Concave lens - diverging lens, always have focal point before the lens
Define emmetropia
Adequate correlation between axial length
and refractive power
Parallel light rays fall on the retina (no
accommodation)
Define Ametropia
Mismatch between axial length and refractive
power
Parallel light rays don’t fall on the retina (no accommodation) – Nearsightedness (Myopia) – Farsightedness (Hyperopia) – Astigmatism – Presbyopia
Describe myopia
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`
Describe hyperopia
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
Describe Astigmatism
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
What is the near response triad?
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
What is Presbyopia
• 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
What are the different types of optical correction?
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
Describe the mechanism of accomodation
– 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
