12. Structure and function of the eye Flashcards
Where are corneal stem cells located?
- In the limbus
* Border between the cornea and sclera
What are basal tears?
Tears that continuously keep the cornea wet and nourished
What are reflex tears?
- Tears produced in response to irritation
- Afferent - cornea - opthalmic branch (V1) of the trigeminal
- Efferent - parasympathetic
- Neurotransmitter - acetylcholine
Where are tears produced?
- Lacrimal gland
* Superio-laterally to the eye
Where do tears flow after lubricating the eye?
- Drains through the two puncta - opening on medial lid margin
- Flows through the superior and inferior canaliculi
- Gathers in the tear sac, then the tear duct
- Exits into the nasal cavity
How is a reflux of tears back into the eyes prevented?
Valve where the canaliculi meet the tear sac
What is the function of a tear film?
- Maintains smooth cornea-air surface
- Oxygen supply to cornea (normal cornea has no blood vessels)
- Removal of debris (with blinking)
- Bactericide
What are the 3 layers of tear film?
1) Superficial oily layer - reduces tear film evaporation, produced by Meibomian Glands along lid margins
2) Aqueous tear film
3) Mucinous layer - on corneal surface to maintain surface wetting - produced by Goblet cells
What is the conjunctiva?
- Thin, transparent tissue that covers the outer surface of the eye
- Covers the visible part of the eye and lines the inside of the eyelids
- Nourished by tiny blood vessels
What are the 3 layers of the coat of eye (inside the head)?
- Sclera - fibrous, hard and opaque (high water content)
- Choroid - pigmented and vascular
- Retina - neurosensory tissue
What is the function of the sclera?
- Protection
* Maintaining the shape of the eye
What is the function of the choroid?
- Provides circulation
* Shields out unwanted scattered light
What is the cornea and its function?
- Front-most part of the anterior segment
- Continuous with the sclera
- Transparent, convex
- Strong tissue
- Low water content - dehydrated by the inner layer of the cornea - corneal endothelium
- Powerful refracting surface - 2/3 of the eye’s focusing power
- Physical and infection barrier
- Relies of tears for nutrients and oxygen (as well as aqueous humour from inside)
- Corneal nerve endings provide sensation, and also nutrients from neurotrophins
What are the 5 layers of the cornea?
1) Epithelium - protection, regenerates quickly
2) Bowman’s membrane
3) Stroma - thickest layer, filled with dehydrated collagen (transparent)
4) Descemet’s membrane
5) Endothelium
What is the function of the endothelial cells in the cornea and how do they change with age?
• Pump out excess fluid from the cornea
- prevents corneal oedema and haziness (cloudiness)
• No capacity to regenerate - cell count declines with age
What happens to the cornea when it’s hydrated?
- Become opaque
* Eventually turns white
What is the uvea?
• Vascular coat of eyeball • Lies between the sclera and retina • Composed of three parts: - iris - ciliary body - choroid • The parts are intimately connected
Does a disease of one part of the uvea affect the other parts and why?
- Yes, due to close connections
* Not necessarily affected to the same degree
What nourishes the outer and inner retina?
- Outer - choroid
* Inner - radial artery
What is the iris and its function?
- Coloured part of the eye
- Controls light levels inside the eye
- Affects the focal plane too - helps with focusing
- Embedded with tiny muscles that dilate and constrict
Describe the structure of the lens?
• Located behind the iris
• Outer acellular capsule
• Regular inner elongated cell fibres - transparency
(- may lose transparency with age - cataract)
• Suspended by a fibrous ring known as lens zonules - consists of passive connective tissue
What is the function of the lens?
- Transparency
- 1/3 refractive power, higher refractive index than aqueous and vitreous fluid
- Accommodation - elasticity (muscles constrict, smaller and thicker lens, shorter sight)
- Normally flat and tort - tension along the stretched lens zonules when the ciliary muscles are relaxed
- Lens loses elastic properties with age - problems with short-sightedness with age as muscle contraction doesn’t make a difference
What is the function of the optic nerve?
- Transmits electrical impulses from the retina to the brain
* Connects to the back of the eye near the macula
What is the visible part of the optic nerve called?
Optic disc
What is the macula?
- Located roughly in the centre of the retina, temporal (lateral) to the optic nerve
- Small, highly sensitive part of the retina
- Responsible for detailed central vision
- Fovea is the very centre of the macula
- Important for tasks such as reading
What is glaucoma?
- Disease of the back of the eye
- Neurones die
- Due to too much aqueous humour
- One of the leading causes of irreversible blindness
What are the risk factors of glaucoma?
- Age
- Family history
- Accidents
- Intraocular pressure (only modifiable risk factor)
What 2 segments can the eye be divided to and what do they refer to?
• Anterior segment
- anatomical structures of the eye, in front of the lens
• Posterior segment
- anatomical structures of the eye, behind the lens
What are 3 of the anatomical spaces inside the eye?
• Anterior chamber
- within the anterior segment
- filled with aqueous humour ('optically empty' as it's completely transparent)
• Posterior chamber
- within the anterior segment
- directly posterior to the iris but anterior to the lens
• Posterior cavity
- vitreous chamber
- filled with vitreous humourWhat are debris and cells inside the anterior chamber a sign of?
Infection and inflammation
What are the 2 layers of the iris?
- Thin posterior pigmented epithelial layer
* Thick anterior layer - stromal tissue + smooth muscles
Where is aqueous fluid secreted into the anterior chamber from?
Ciliary body (ciliary epithelium)
Where does the fluid flow out from the anterior chamber?
• 80% flows out through the canal of Schlemm, around the cornea, deep in the sclera
- modified vein that absorbs aqueous humour and pushes it into the venous system
• also flows out via the Trabecular Meshwork
• Situated at the junction between the ciliary body . and cornea
What is uveal-scleral flow?
- Passive gradient flow for the absorption of aqueous humour, between the choroid and sclera
- 20% of aqueous humour reabsorption
- Pressure dependent
What is the first line of treatment to reduce IOP in glaucoma?
- Prostaglandin analogues
* Increase uveal-scleral flow
Describe the progression of damage in glaucoma
- Sustained high eye pressure
- Accumulative damage to the optic nerve tissue
- Loss of ganglion nerve fibres - hollowing out of the optic nerve head
- Peripheral vision progressively lost => blindness (if untreated)
What is the most comment type of glaucoma?
Primary Open Angle Glaucoma (trabecular meshwork dysfunction)
What is Closed Angle Glaucoma, what are the risk factors and how is it treated?
- Increased pressure pushing the iris/lens complex forwards
- Blocks the trabecular meshwork - vicious cycle
- May present with sudden painful red eye with acute drop in vision
- Can be acute or chronic
- Small eye (hypermetropia)
- Narrow angle at trabecular meshwork
• Peripheral laser iridotomy to create a drainage hole on iris
What are the type of cells primarily affected in glaucoma?
Retinal ganglion cells
What is the optic nerve blind spot?
- No light sensitive cells where the optic nerve meets the retina
- Optic disc - corresponding anatomical landmark for the physiological blind spot
What is the most sensitive part of the retina?
The fovea at the centre of the macula
Describe the proportion of cells in the fovea?
- Highest concentration of cones, but a low concentration of rods
- 1:1 ratio between photoreceptors and ganglion cells
- This is why stars out of the corner of your eye seem brighter than when looking directly
- If an image doesn’t fall on the fovea, you won’t be able to see the detail
What is central vision (for)?
- aka macular vision
- Detail day vision, colour vision
- Reading, facial recognition
What is peripheral vision (for)?
- Detecting shape and movement in the environment
- Night vision
- Navigation vision
How can you assess central and peripheral vision?
- Central - visual acuity assessment
* Peripheral - visual field assessment
What are the 3 layers of the retina?
- Outer: photoreceptors (1st order neurones) - detection of light
- Middle: bipolar cells (2nd order neurones) - local signal processing to improve contrast sensitivity, regulate sensitivity
- Inner: retinal ganglion cells (3rd order neurones) - transmission of signal from the eye to the brain
What is the function of the retinal pigment epithelium?
- Transports nutrients from the choroid to the photoreceptor cells
- Removes metabolic waste from the retina
What is the pigmented region at the centre of the retina called (yellow patch)?
Macula Lutea
about 6mm diameter
How can you clinically assess the macula and fovea?
Optical Coherence Tomography scan
Describe the rod photoreceptors
- Longer outer segment with photo-sensitive pigment
- 100x more sensitive than cones
- Responsible from night vision (Scotopic Vision)
- 120 million rods
- Allows to to find position in space
- Responsible for detecting movement and distance judgement
Describe the cone photoreceptors
- Less sensitive to light, but faster response
* Responsible for day light fine vision and colour vision (Photopic Vision)
What is scotopic vision?
• Peripheral and night vision
(• more photoreceptors, more pigment, higher spacial and time summation)
• Rods distributed all over the retina
• Highest density just outside the macula
Where are the highest concentration of rod receptors?
In the retina, 20-40 degrees away from the fovea
What is photopic vision?
Central and day vision
What are the sensitivities of rods?
Only a single peak light sensitivity
What are the different cone photo-pigment sub-types?
- S-cones - sensitive to short-wavelength (blue)
- M-cones - sensitive to medium wavelength (green)
- L-cones - sensitive to long wavelength (red)
Which cones does yellow light stimulate?
- M- and L-cones equally
* Yellow light has a wavelength between the peak sensitivity wavelengths of M- and L-cones
What is the most common form of colour vision deficiency?
- Deuteranomaly
- Type of Anomalous Trichromatism
- M-cone sensitivity peak shifts towards that of the L-cone curve => red-green confusion
What is the prevalence of colour vision deficit?
- Males - 8%
* Females - 0.5%
What is dichromatism and monochromatism?
- Dichromatism - 2 cone photo-pigment sub-types are present
* Monochromatism - complete absence of colour vision (no functional day vision)
What can be used to test for red-green colour perception deficiencies?
Ishihara Test
What happens to the sensitivity of the rod receptors in daylight?
Greatly suppressed
How do the photoreceptors adapt from the light to the dark?
• The retina increases its light sensitivity in the dark
• Biphasic process
• Retina switches from photopic vision to scotopic vision
- cone adaptation - 7 mins
- rod adaptation - 30 mins - regeneration of rhodopsin
Describe the adaptation of photoreceptors from the dark to the light
- Occurs over 5 mins
- Bleaching of photo-pigments mediates the process
- Neuro-adaptation: inhibition of rod/cone function
- Pupil adaptation: (minor) constriction of pupil with light
- Pupil acts as an adjustable aperture to regulate light intake
What is ametropia?
- Refers to vision disorders characterised by the eyes inability to correctly focus the images of objects on the retina
- Forms include myopia (nearsightedness), hyperopia (farsightedness) and astigmatism
How do you work out the index of refraction?
(speed of light in a vacuum) / (speed of light in a medium)
denominator will always be smaller - n is always greater or equal to 1
Is the angle of incidence the same or different to the angle of reflection and refraction?
- Angle of incidence = angle of refraction
* Angle of incidence > or < angle of refraction depending on the direction of the light
How do convex lenses change the direction of light?
- Light passes through
- Light rays converge towards a focal point
- Distance of the focal point is proportional to the thickness of the lens
- Thicker lens - closer focal point
How do concave lenses change the direction of light?
- Light passes through
- Light rays disperse as it is refracted in a divergent way
- The focal point is a virtual point (before the lens, not after)
What is emmetropia?
- Eye with a refractive error of 0 i.e. no visual defects - ideal vision
- Adequate correlation between axial length and refractive power
- Parallel light rays fall on the retina
What is 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
What happens to the focal point in myopia (nearsightedness) and hyperopia (farsightedness)?
- Myopia - anterior to the retina
* Hyperopia - posterior to the retina
What are the causes and symptoms of myopia (nearsightedness)?
- Excessive long globe (axial myopia) - eye is too long - more common
- Excessive refractive power (refractive myopia) - lens and cornea are too powerful
- Blurred distance vision
- Squint in an attempt to improve uncorrected visual acuity
- Headache
How can myopia be corrected?
Concave glasses (divergent) or contact lenses - move the focal point slightly backwards
What are the causes and symptoms of hyperopia (farsightedness)?
- Excessive short globe (axial hyperopia)
- Insufficient refractive power (refractive myopia)
- Visual acuity at near tends to blur
- Nature varies from inability to read fine print, to near vision being clear then suddenly and intermittently blur
- Asthenopic (eye strain) symptoms - eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
What is amblyopia?
- Lazy eye
- Eye fails to achieve normal visual acuity
- If refractive error is too drastic in one eye, brain will start to exclude information from the hyperopic eye
- Only the other eye is used
- Affected eye becomes ambylopic
What is astigmatism?
- Parallel rays come to focus in 2 focal lines rather than a single point
- Hereditary
- Caused by elliptical refractive media (not spherical)
- Cornea is not evenly shaped
What are the symptoms and treatment for astigmatism?
- Asthenopic symptoms
- Blurred vision
- Distortion of vision
- Head tilting and turning
- Regular astigmatism: cylinder lenses with or without spherical lenses
- Irregular astigmatism: rigid cylinder lenses, surgery
What is presbyopia?
• Naturally occurring loss of accommodation
• Onset from 40yrs
• Distant vision intact
• Corrected by reading glasses to increase refractive power
(• bifocal glasses, trifocal glasses, progressive power glasses)
How is the optical image different in contact lenses, compared to spectacle lenses?
- Higher quality
* Less influence on the size of the retinal image
When would contact lenses be given?
- Cosmetic
- Athletic activities
- Occupational
- Irregular corneal astigmatism
- High anisometropia
- Corneal disease
What are the disadvantages of contact lenses?
• Careful daily cleaning and disinfeciton • Expense • Complications - infectious keratitis - giant papillary conjunctivitis - corneal vascularisation - severe chronic conjunctivitis
What are intraocular lenses?
- Implantation
- Replacement of cataract crystalline lens
- Gives best optical correction for aphakia
- Loss of accommodation
How can near/farsightedness be corrected by surgery?
- Keratorefractive surgery
* Intraocular surgery (with or without intraocular lens)
How does the lens accommodate (for near vision)?
- Circular ciliary muscles contract
- Zonules relax (that are normally stretched)
- Lens returns to it’s natural convex shape
- Increased refractive power of the lens
Which nerve is accommodation mediated by?
Efferent oculomotor nerve (III)
