Structure and function of the eye Flashcards
Where does the eye sit?
Within the eye socket - orbit
What are the different parts of the eye?
- pupil
- iris
- upper and lower eyelid
- lateral canthus ( outer corner)
- medial canthus
- sclera (white part)
- palpebral fissure (area between open eyelids)
- caruncle (red inner bit in eye)
- limbus (border between cornea and sclera)
LEARN PART OF EYE SOCKET
Netters
What are the types of tears?
- basal tears
- reflex tears
- crying (emotional) tears
Describe the pathway of tears produced as a reflex e.g. if irritated
Afferent – Cornea – CN V1 (Ophthalmic Branch of Trigeminal Nerve)
Efferent – Parasympathetic
Neurotransmitter – Acetylcholine
What produces tears and describe their pathway
Lacrimal gland (outer corner of eye) (LOOK AT ANATOMY)
How do tears pass from the lacrimal gland to the nasal cavity?
- Tears drain through the two puncta and open on medial lid margin
- Tears flow through the superior and inferior canaliculi
- They gather is the tear sac
(LOOK AT ANATOMY)
What is the importance of the tear film?
- Tear film maintains smooth cornea-air surface for clear vision
- Oxygen Supply to Cornea – Normal cornea has no blood vessels
- Removal of Debris (Tear film and Blinking)
- Bactericide
What are the 3 layers of the tear film?
- Superficial Oily Layer
- Aqueous Tear Film
- Mucinous Layer on the Corneal Surface
What is the importance of the lipid layer of the tear film? What produced it?
- Protecting the tear film from rapid evaporation
- Meibomian Glands, situated along the eyelid margins
What is the importance of the aqueous tear film layer?
What makes it?
- It delivers oxygen and nutrient to the surrounding tissue
- It contains factors against potentially harmful bacteria
- Lubricates the eye
- Made by tear gland
What is the importance of the mucinous layer of the tear film?
- Ensures that the tear film sticks to the eye surface
- The mucin molecules act by binding water molecules, to the hydrophobic corneal epithelial cell surface.
What is the 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 causes bloodshot eyes?
Bleeding in the conjunctiva spreads around
What is the normal diameter of the eye (AP)?
24 mm in adults
What are the 3 layers of the eye and their texture?
Sclera – Hard and Opaque (outer)
Choroid – Pigmented and Vascular
Retina – Neurosensory Tissue
What is the sclera and its role?
The outer fibrous opaque layer called the sclera, responsible for protecting the eye, and maintaining the shape of the eye.
What is the choroid and its role?
The middle pigmented vascular layer called the choroid, responsible for providing circulation to the eye, and shielding out unwanted scattered light.
What is the retina and its role?
The innermost neurosensory layer called the retina, responsible for converting light into neurological impulses, to be transmitted to the brain via the Optic Nerve.
Describe the composition of the sclera and its properties
- high water content
- strong
- tough and opaque
- eye’s protective layer
Describe the composition of the cornea and its properties
What dehydrates the cornea?
- lower water content
- transparent, dome shaped
- powerful refracting surface
- physical and infections barrier
- relies on tear film and aqueous fluid for oxygen
- dehydrated by the inner layer of the cornea (corneal endothelium)
What is the relationship between the sclera and the cornea?
The sclera is continuous with the cornea - the cornea is the front part
How does prolonged contact lens use affect the cornea?
- Reduces oxygen supply to the cornea, and compromises corneal tissue health
- Excessive and prolonged contact wear increases the risk of serious corneal eye infection
What are the 5 layers of the cornea?
- Epithelium
- Bowman’s Membrane
- Stroma (thickest)
- Descemet’s Membrane
- Endothelium
What does the endothelial layer do and what happens to it with age?
- pumps fluid out from the cornea to prevent oedema and haziness
- endothelial count declines with age
What happens to the cornea when it is hydrated?
- it becomes opaque
- eventually becomes white
What is the uvea?
- The uvea is the vascular coat of eyeball and lies between the sclera and retina
- It is composed of three parts: iris, ciliary body and the choroid
- These three portions are intimately connected
What is the choroid?
- 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
What is the iris?
- The coloured part of the eye
- It controls light levels inside the eye
- The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size
What happens to the size of the pupil when the iris muscles dilate?
It widens
Describe the structure of the lens
What happens to the trasnparency of the lens with age - what disease is this?
- Outer Acellular Capsule
- Regular inner elongated cell fibres (transparency)
- May loose transparency with age – cataract
What is the function of the lens?
- Refractive Power (1/3 of the eye’s)
- Accommodation: Elasticity (muscles constrict -> lens becomes smaller + thicker -> short sight)
How is the lens suspended and what is it attached to?
By a fibrous ring known as lens zonules. It anchors the lens to the ciliary body.
Why do elderly people have short sightedness?
The lens loses its elastic properties so when the muscle constricts, the lens remains wide and thin.
Where does the optic nerve connect to the back of the eye?
Near the macula
What is the visible part of the optic disc called?
optic disc
easy to see when looking at the back of the eye
What is the macula?
- 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 e.g. needed for reading
What is the fovea?
Centre of the macula
What is 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. Characterised by sustained high pressure.
What are the risk factors for glaucoma?
Age, family history, accidents, intraocular pressure (pressure only modifiable factor)
How is the eye divided?
Anterior and posterior segment - anterior is everything in front of the lens.
What are the two chambers of the eye and where are they?
They are formed by the lens - the anterior and posterior chamber
What fills the anterior and posterior segments?
Anterior - clear aqueous fluid called aqueous humour, hence said to be optically empty. Useful to look for infections of inflammation as debris and cells can be seen. Posterior has vitreous humour.
What is the ciliary body and what does it do?
- Ring shaped tissue, surrounding the lens.
- It secretes aqueous fluid into the anterior chamber
What is the ciliary body?
The ciliary body is a part of the eye that includes the ciliary muscle and the ciliary epithelium
What are the two layers of the iris?
The thin posterior pigmented epithelial layer, and the thick anterior layer, composed of stromal tissue & smooth muscles.
How does the aqueous humour flow? (uveal scleral outflow)
- It flows anteriorly towards the anterior chamber
- The fluid flows out of the eye, via the trabecular meshwork, situated at the junction between the ciliary body, and the cornea
- It is passive flow
What is normal IOP?
12 to 21 mmHg
Describe the flow of aqueous humour via the canal of schlemm
This canal is invisible and sits around the cornea, deep in the sclera. It is a modified vein that absorbs aqueous humour and pushes it into the venous system.
To reduce IOP what two things can be done?
- increase AH outflow
- reduces flow of AH into anterior chamber
What is the first line treatment for glaucoma and what do they act on?
Prostaglandin analogues
They act on uveal-scleral flow (this flow is responsible for up to 20% of aqueous humour reabsorption).
What is the through which damage occurs in glaucoma?
- Results in gradual and accumulative damage to the optic nerve tissue, in the posterior segment of the eye
- Loss of ganglion nerve fibres -> hollowing out of the optic nerve head (enlarged optic disc cupping)
- Patient with untreated glaucoma looses peripheral vision progressively
- Untreated glaucoma will lead to blindness
What is primary open angle glaucoma?
- most common
- trabecular meshwork dysfunction
- patients generally have no symptoms till advanced staged
What is closed angle glaucoma?
- Acute or chronic
- Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork (vicious cycle)
What are the risk factors for closed angle glaucoma?
How may it present?
How may it be treated?
- Small eye (hypermetropia), narrow angle at trab. meshwork
- May present with sudden painful red eye with acute drop in vision
- Treated with peripheral laser iridotomy to create a drainage hole on iris
Which cells are affected in glaucoma?
retinal ganglion cells
Describe the composition of the fovea and its importance
- It has the highest concentration of cones, but a low concentration of rods
- There is a 1:1 ratio between photoreceptors and ganglion cells
- Only your fovea has the concentration of cones to perceive in detail
- If an image does not fall on the fovea, you will know its there but won’t be able to see detail
What is central vision?
- Macular vision
- 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)
How can central vision be assessed?
Assessed by visual acuity assessment – loss of foveal vision results in poor visual acuity
What is peripheral vision?
- For detecting shape and movement in the environment, night vision
- Navigation vision – patients with loss of peripheral vision have problems navigating the world
How is peripheral vision assessed?
What may happen if patients have extensive visual field loss?
- Visual field assessment
- Patient may need white stick even with perfect visual acuity
Describe the structure of the neuroretina (layers) and what they do
Outer Layer (right at the back)– 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 is the outer thin layer and inner thick layer of the retina called?
Outer thin layer -> retinal pigment epithelium
Inner thicker layer -> neuroretina
What is the function of the retinal pigment epithelium?
Transports nutrient from the choroid to the photoreceptor cells, and removes metabolic waste from the retina.
What is the neuroretina?
The Neuroretina is made up of photoreceptors and neurons.
It can be roughly divided into the 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.
What does the macula look like?
What does the fovea form in the macula?
How can the fovea/macula be examined?
Macula: Yellow, pigmented region at the centre of the retina of about 6mm in diameter
- The fovea forms the pit at the centre of the macula due to absence of the overlying ganglion cell layer
- Clinically can be assessed with an Optical Coherence Tomography scan
What are rods - features?
When used, speed and sensitivity and number?
- 100 times more sensitive to light than cones
- Slow response to light
- Responsible for night vision (Scotopic Vision)
- 120 million rod
What are cones - features?
When used, speed and number?
- Less sensitive to light, but faster response
- Responsible for day light fine vision and colour vision (Photopic Vision)
- 6 million cones
Where are rods and cones found?
Rods:
- periphery mostly
Cones:
- central part of the eye
What is scotopic vision?
Peripheral and night vision, recognises motion
What is photopic vision?
Central and day vision, recognises colour and details
Where are the highest concentration of rods?
20-40 degrees away from the fovea
Describe the frequency spectrum of rod vision
Has only one single peak light sensitivity, at 498 nano-meters
What are the different types of cones and which frequencies do they detect?
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 has a wavelength between the peak sensitivity wavelengths of M-Cones and L-Cones. Yellow light stimulates both M-cones and L-cones equally.)
What is the commonest colour deficiency?
Deuteranomaly
M-cone sensitivity peak shifts towards that of the L-cone curve -> red-green confusion
What causes colour vision defects, commonest type
- Colour vision deficits can be caused by a shift in the photo-pigment peak sensitivity. This is anomalous trichromatism
- Commonest form: deuteranomaly)
- Colour vision deficits can be caused by the absence of one or more of the 3 cone photo-pigments
What is dichromatism and monochromatism?
Dichromatism: 2 cone photo-pigment sub-types are present.
Monochromatism: there is complete absence of colour vision. Monochromatism patients have no functional day vision.
What test is used to determine red-green colour perceptiion deficiencies?
- Ishihara test
- plates with circle of dots
- if deficiency present they cannot find the right pattern
What is light-dark adaptation?
Dark Adaptation (from light -> dark)
- The retina increases its light sensitivity in dark
- Biphasic Process (2 steps)
- Retina switches from photopic vision to scotopic vision
- Cone adaptation: 7 minutes
- Rod adaptation: 30 minutes – regeneration of rhodopsin
Light Adaptation (suppression of light sensitivity going into light)
- 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
What is ametropia?
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.
What do light photons have to pass to get from the air to the retina?
tear film -> cornea -> aqueous humour -> lens -> vitreous humour -> retina
What is a convex lens?
- converging
- light rays move to a focal point
- thicker lens means the closer the focal point is to the lens
- focal point distance from the central plane is proportional to lens thickness
What is a concave lens?
- diverging
- light refracted away
- focal point is virtual
- it is in front of the lens not after
What is emmetropia?
- Adequate correlation between axial length and refractive power
- Parallel light rays fall on the retina (no accommodation)
- The eye can see without any visual correction
NORMAL
What causes ametropia?
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina (no accommodation)
What is myopia?
Parallel rays converge at a focal point anterior to the retina - near sightedness (only see near things well)
What causes myopia, and what are the symptoms?
Causes:
- Excessive long eye 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
How can myopia be corrected?
By concave glasses. Concave lenses are divergent – so they move the focal point slightly backwards towards the retina – the person can see.
What is hyperopia?
Parallel rays converge at a focal point posterior to the retina
What are the causes and symptoms of hyperopia?
Etiology: not clear, inherited
Causes:
- Excessive short globe (axial hyperopia) : more common
- Insufficient refractive power (refractive hyperopia)
Symptoms:
- Visual acuity at near tends to blur relatively early (Nature of blur varies: 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)
- Eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
- Amblyopia (lazy eye)- uncorrected hyperopia
What leads to lazy eye?
If the refractive error is too drastic in one eye, the brain will start to exclude information coming from the hyperopic eye. Only the other eye is used à the affected eye will become ambylopic (+ lazy eye)
How can hyperopia be treated?
Lens
What is astigmatism?
Parallel rays come to focus in 2 focal lines rather than a single focal point
What causes astigmatism?
- Etiology: heredity
- Cause: refractive media is not spherical – more elliptical
- The cornea is not evenly shaped in terms of the radius
- Light refracts differently along one meridian than along meridian perpendicular to it
- Results in 2 focal points
How is astigmatism corrected? (regular)
A cylinder lens is used - light passing through the vertical part is not bent but light perpendicular to the cylinder is bent
What are the symptoms of astigmatism?
- Asthenopic symptoms ( headache , eyepain)
- Blurred vision
- Distortion of vision
- Head tilting and turnin
How is irregular astigmatism treated?
- surgery
- rigid contact lens
What is presbyopia?
Naturally occurring loss of accommodation (focus for near objects, distant vision intact)
When does presbyopia occur?
onsent from age 40 as lens naturally loses elasticity
How is presbyopia corrected?
reading glasses (convex lenses) to increase refractive power of the eye
What are the different types of optical correction?
- spectacle lenses
- contact lenses
- intraocular lenses
- surgical correction
What are the disadvantages and potential complications of contact lenses?
Disadvantages: careful daily cleaning and disinfection, expense
Complication: infectious keratitis, giant papillary conjunctivitis, corneal vascularization, severe chronic conjunctivitis
What is clear lens extraction and introcular lens insertion?
- Same as cataract extraction
- Implantation of artificial lens
- Lose accommodation (patient will need reading glasses)
What is accomodation?
The ability of the eye to change its focus from distant to near objects.
What is the mechanism of accomodation?
- Contraction of the ciliary muscle
- 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
- Lens thickens
Do zonules have contractile muscle?
They are passive elastic bands with no active contractile muscle
How does ciliary muscle contraction cause the lens to bulge?
When the ciliary muscle contracts, it pushes the inner edge of the ciliary body towards the lens, and relaxes the passive Zonules.
This allows the lens to return to its natural thicker curved configuration, thus increasing its refractive power.
Which cranial nerve is responsible for accomodation?
3rd
