Structure and Function of the Eye Flashcards
How is the orbit structured?
- 7 bones contribute to the framework of the orbit (frontal, lacrimal, zygomatic, maxilla, ethmoid, spheroid and palatine)
- there are holes in the orbit (e.g. optic canal, ethmoidal foramina)
What are the three types of tear production?
- basal tears - nourish, lubricate and protect the cornea.
- reflex tears - in response to irritation
- crying (emotional) tears
Reflex tears
- Afferent – Cornea – CN V1 (Ophthalmic branch of Trigeminal Nerve)
- Efferent – Parasympathetic
- Neurotransmitter – Acetylcholine
The lacrimal system - how are tears produced?
- 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 produces tears?
- the lacrimal gland
What are the functions 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 is the structure of the tear film?
It has 3 layers:
- Lipid layer
- Water layer
- Mucin Layer
What is the function of the lipid layer of the tear film?
Superficial Oily Layer to reduce tear film evaporation (produced by a row of Meibomian Glands along the lid margins)
What is the function of the water later of the tear film?
Aqueous Tear Film (Tear Gland)
What is the function the Mucin layer of the tear film?
Mucinous Layer on the Corneal Surface to maintain surface wetting
Which layer in the tear film protects the tear film from rapid evaporation? – A) Lipid Layer – B) Water Layer – C) Mucinous Layer – D) All Three Layers
A)
Conjunctiva
- 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.
What is the diameter of the eye in adults?
~24 mm (anterio-posterior diameter)
What are the 3 layers that coat the eye? What are their properties?
– Sclera – Hard and Opaque (outermost)
– Choroid – Pigmented and Vascular
– Retina – Neurosensory Tissue (innermost)
Sclera
- commonly known as “the white of the eye”
- the tough, opaque tissue that serves as the eye’s protective outer coat.
- High water content
Cornea
- transparent
- dome-shaped window covering the front of the eye
- powerful refracting surface, providing 2/3 of the eye’s focusing power (Convex curvature, Higher refractive index than air)
- Low water content
– Front-most part of Anterior Segment
– Continuous with the Scleral Layer - physical barrier
- infection barrier
What gives the eye its focusing / refractive power?
- 2/3 is the cornea
- 1/3 is the lens
What is the structure of the cornea?
- front-most part in the anterior segment of the eye
- continuous with the scleral layer
- has 5 layers:
- Epithelium
- Bowman’s membrane
- Stroma
- Descemet’s membrane
- Endothelium
Stroma (layer of the cornea)
- regularity contributes towards transparency
- Corneal nerve endings provides sensation and nutrients for healthy tissue
- No blood vessels in normal cornea
Endothelium of the cornea
- pumps fluid out of corneal and prevents corneal oedema
- Only 1 layer of endothelial cells
- No regeneration power
- Endothelial cell density decreases with age
- Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness
What conditions cause red eyes? How can you differentiate them?
Conjunctivitis and Uveitis (inflammation of the iris); also conjunctival hyperaemia.
Bacterial conjunctivitis will come with pus, viral without. there is also fungal and allergic.
Diffuse redness is usually conjunctivitis; in uveitis you see a red ring around the cornea.
What happens when you hydrate the cornea?
- it becomes white / milky
What are the components of the uvea?
- 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.
What is the uvea?
- Vascular coat of eye ball and lies between the sclera and retina
- composed of three parts (iris, ciliary body and choroid)
Choroid
- lies between the retina and sclera
- composed of layers of blood vessels that nourish the back of the eye.
Iris
- coloured part of the eye
- 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.
- It is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
Lens
- gives the eyes 1/3 of the refracting power (the other 2/3 is the cornea)
- Outer Acellular Capsule
– Regular inner elongated cell
fibres – transparency
– May loose transparency with age – Cataract
What are the functions of the lens?
- transparency (regular structure)
- accommodation (elasticity)
- refractive power ( 1/3 power; Higher refractive index than aqueous fluid and vitreous)
What are lens zonules?
The lens is suspended by a fibrous ring known as lens zonules, consists of passive connective tissue.
Retina
- 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.
Optic nerve
- The 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.
Macula
- The macula is located roughly in the centre of the retina, temporal to the optic nerve.
- 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 reading.
- ~ 6mm diameter
How is the eye divided into anterior and posterior segments?
- anterior: ocular structures in front of the lens
- posterior: ocular structures behind the lens
How many chambers are there in the eye?
2
Anterior chamber of the eye
- Between Cornea and Lens
– Filled with Clear Aqueous Fluid
– Supplies nutrients
What structure in the eye secretes aqueous humour?
The ciliary bodies
Ciliary bodies
- Ciliary body secretes aqueous fluid in the eye
- Intraocular Aqueous Fluid flows anteriorly into the Anterior Chamber along the green arrow
- Aqueous Fluid supplies nutrient
- Trabecular Meshwork drains the fluid out of the eye
- drained via canal of Schlemm -> into the nose
Normal Intraocular Pressure
12- 21mmHg
now up to 24 mmHg? NICE
Aqueous flow (drainage)
- 80 - 90% TM canal of Schlemm
- rest uveal scleral outflow (is this correct?)
“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
Glaucoma
– Medical Condition of Sustained Raised Intraocular Pressure (risk factor)
– Retinal Ganglion Cell Death and Enlarged Optic Disc Cupping
– Visual Field Loss, Blindness
What are the different types of glaucoma?
– Primary Open Angle Glaucoma – Commonest
– Closed angle glaucoma - can be acute or chronic
(angle refers to angle between the cornea and the iris)
Primary open angle glaucoma
- commonest
- trabecular Meshwork Dysfunction
Closed angle glaucoma
- can be acute to 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
Which type of cells in the eye are primary affected in glaucoma? – A) Photoreceptors – B) Retinal Pigment Epithelial Cells – C) Bipolar Cells – D) Retinal Ganglion Cells
D)
Blind Spot
Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot.
Fovea
- it is at the very center of the macula
- 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.
Whatisthe corresponding anatomic landmark for the physiological blind spot? – A) Macula – B) Fovea – C) Optic Disc – D) Ora Serrata
C)
Central Vision
– 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
– Shape, Movement, Night Vision – Navigation Vision – Assessed by Visual Field Assessment – Extensive loss of Visual Field – unable to navigate in environment patients might need a white stick even with perfect visual activity
How is the retina structured on a cellular layer?
- 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
What way do the photoreceptors face?
They face the back of the eye, the light is refracted from there.
Macula, Fovea and Foveal Pit
- 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
– Clinicallycanbeassessed with an OCT scan (Optical Coherence Tomography)
What are the 2 types of photoreceptors?
- rods
- cones
Rod photoreceptors
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 photoreceptors
- Less sensitive to light, but faster response
- Responsible for day light fine vision and colour vision (Photopic Vision)
- 6 million cones
Scotopic vision
- Rod Vision
- Peripheral and Night Vision
- (More Photoreceptors, More Pigment, Higher Spatial and Time Summation)
– Recognizes motion
Photopic Vision
Cone Vision
– Central and Day Vision
– Recognizes colour and details
Where can one find the highest concentration of Rod Photoreceptors in the retina? – A) Optic Disc – B) Fovea – C) 10-20 degrees away from Fovea – D) 20-40 degrees away from Fovea
D)
Which photoreceptors detect which frequencies?
frequencies = colours (wavelength in nm)
- S cones: blue
- Rods: black
- M cones: green
- L cones - red
What percentage of people have normal colour vision?
92%
Ishihara test
- Colour Perception Test
- Ishihara Isochromatic Plates can test for red-green deficiencies only
- 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
- Patients with colour vision deficiencies will not recognize any pattern or recognize the wrong pattern
What is the test with numbers presented as blops on a background of different coloured blops called?
Ishihara test
What number on Ishihara test should everyone see?
- 25
- if they say they don’t see it they are very likely lying as even patients with achromatopsia can see them
Achromatopsia
- black and white vision
Light dark adaptation
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
Whichisthe commonest form of colour vision deficiency in humans?
– A) Absence of all cone photoreceptors
– B) Absence of M-cone (green) photoreceptors
– C) Abnormal L-cone (red) photoreceptors
– D) red-green confusion
D)
Refraction
- based on light passing from 1 medium to the other
- As light goes from one medium to another, the VEOLCITY changes!
- As light goes from one medium to another, the PATH changes!
What change when light passes from 1 medium to the other?
Velocity
Path
Convex lense
A converging lens (Convex) takes light rays and bring them to a point.
Concave lense
A diverging lens (concave) takes light rays and spreads them outward.
Focal length
length between focal point and lens
Emmetropia
= normal vision
- Adequate correlation between axial length and refractive power
- Parallel light rays fall on the retina (no accommodation)
Ametropia (refractive error)
• Mismatch between axial length and refractive power
• Parallel light rays don’t fall on the retina (no accommodation)
– Nearsightedness (Myopia)
– Farsightedness (Hyperopia)
– Astigmatism
– Presbyopia
Myopia
= nearsightedness
- focal point anterior to the retina
- Etiology : not clear , genetic factors
- Causes:
excessive long globe (axial myopia): more common excessive refractive power (refractive myopia)
- correction with diverging lenses, contact lenses or surgical lens removal
Symptoms of Myopia
– Blurred distance vision
– Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
– Headache
Hyperopia
= farsightedness
- focal point posterior to the retina
- Etiology: not clear, inherited
- Causes:
excessive short globe (axial hyperopia) more common insufficient refractive power (refractive hyperopia)
- correction with converging lenses, cataract lens, contact lens
Symptoms of Hyperopia
- 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 is tired , printing is weak or light inadequate
– asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
Amblyopia
uncorrected hyperopia > 5D
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 of Astigmatism
– asthenopic symptoms ( headache , eyepain)
– blurred vision
– distortion of vision
– head tilting and turning
Treatment of astigmatism
- Regular astigmatism :cylinder lenses with or
without spherical lenses(convex or concave), Sx - Irregular astigmatism : rigid CL , surgery
Near response triad
= Adaptation for near vision
– 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
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 (reading glasses, bi-/trifocal glasses, progressive power glasses)
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
– 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
Accommodation Mechansims
• Accommodation Mechanism
– 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
• Which statement is false for Myopia?
– A) May be associated with large globe
– B) Light ray converges behind the retina
– C) May be associated with increased corneal curvature
– D) Unable to see objects clearly at distance without glasses or other optical correction
B)
Inaccommodation, which one of the following events does not take place?
– A) Relaxation of Circular Ciliary Muscle
– B) Relaxation of Zonules
– C) Thickening of Lens
– D) Increase of Lens Refractive Power
A)
