Neurology 12 - Structure and Function of the Eye Flashcards
Where does the eye sit?
- In the bony orbit
- Made of the sphenoid bone, frontal bone, zygomatic bone
List the parts of the eye
- The space between the eyelids is called the palpebral fissure
- The outer corner of the eyelid is called the lateral canthus
- Pupil, iris, sclera
- Medial canthus is the medial corner. It contains the caruncle (which forms the third eyelid in some animals)
- The dark outer ring between the sclera and the eyelid is the limbus (also where the corneal stem cells sit in)
List the components of the lacrimal system
- Lacrimal glands produces the largest component of tears
- Absorbed by the tear punctum (medial lid margin), canaliculi, tear sac and tear duct
- Exits into the nose cavity
Describe tear production
- Basal tears
- Reflex tears (in response to irritation)
What is the function of the tear film?
- Maintains smooth cornea air surface
- Provides oxygen supply
- Removal of debris (tear film and blinking)
- Bactericide
List the three layers of the tear film
- Superficial Oily 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 of the eye?
- 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.
What gives the eye its shape?
Vitreous humour
List the layers of the coat of the eye
- Sclera (hard and opaque)
- Choroid (pigmented and vascular)
- Retina (neurosensory tissue)
What is the diameter of the eye?
24mm
What is the sclera?
- The white of the eye
- Tough opaque tissue that serves as the eyes protective outer coat
- High water content
What is the cornea?
- A continuation of the sclera
- Covers the front of the eye, refracting surface providing 2/3 of the eyes focusing power. It has convex curvature
- Transparent
- Low water content
List the functions of the cornea
- Refraction (2/3)
- Physical barrier
- Infection barrier
Describe the structure of the cornea (5 layers)
- Epithelium
- Bowmans membrane
- Stroma (regularity contributes towards transparence - corneal nerve endings, provides sensation and nutrients for healthy tissue)
- Descements membrane
- Endothelium
What is the function of the endothelium of the cornea?
- 1 layer of cells
- No regeneration power and so decreases with age
- Endothelial cell dysfunction can result in oedema and cloudiness
What happens if the cornea is hydrated?
It will go white
What is the uvea? Describe its structure.
- Vascular coat of the eyeball, lies between the sclera and retina
- Composed of the iris, ciliary body and choroid
- These three parts are connected, so a disease of one part affects the other parts
What is the choroid?
- Between the retina and sclera (posterior part of the uvea)
- Composed of layers of blood vessels that nourish the back of the eye
What is the iris?
- The coloured part of the eye
- Controls light levels similar to the aperture on a camera
- Round opening in the centre is called the pupil
- Embedded with muscles that dilate and constrict pupil size
Describe the structure of the lens of the eye
- Outer acellular capsule
- Regular inner elongated cell fibres (transparent)
- May loose transparency with age (cataract)
List the functions of the lens of the eye
- Transparency (regular structure)
- Refractive power (1/3 power, higher refractive index than aqueous and vitreous fluid)
- Elasticity allows for accommodation
What are lens zonules?
- Suspends the lens
- A fibrous ring
- Passive connective tissue
- Connects the lens to the ciliary muscle
What is the result of contraction of the ciliary muscle?
Pushing - the lens contracts
What is the retina?
- Very thin layer of tissue that lines the inner part of the eye
- Responsible for capturing light rays that enter the eye
- Light impulses are then sent to the brain for processing via the optic nerve
What is the function of the optic nerve?
- Transmits eletrical 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 of the eye?
- Located in the centre of the retina, temporal to the optic nerve
- Small and highly sensitive, responsible for detailed central vision
- Fovea is the centre of the macula
What is the function of the macula?
- Allows us to appreciate detail and perform tasks that require central vision
- Eg. reading
What are the segments of the eye?
- Anterior segment is the ocular structure anterior to the lens
- Posterior segment is the ocular structure posterior to the lens
What is the first cause of reversible blindness?
Cataract
What is the location and function of the anterior chamber of the eye?
- Between the cornea and the lens
- Filled with clear aqueous fluid
- Supplies nutrients
What is the posterior chamber of the eye?
Behind the posterior part of the iris, anterior to the lens of the eye
Describe the pathway of intraocular fluid
- Ciliary body secretes aqueous fluid
- Intraocular aqueous fluid flows into the anterior chamber between the ciliary epithelium and the lens of the eye
- Aqueous humour supplies nutrients
- Fluid is drained out of the eye by the trabecular meshwork
- Exits through the canal of schlemm (80%), while 20% diffuses to uveal-scleral outflow
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
What is the normal intraocular pressure?
12-21mmHg
What is glaucoma?
- Medical condition of sustained raised intraocular pressure
- Retinal ganglion cell death and enlarged optic disc cupping
- Causes visual field loss and blindness
List the types of glaucoma
- Primary open angle glaucoma (commonest - trabercular meshwork dysfunction)
- Closed angle glaucoma (acute or chronic)
What causes closed angle glaucoma? What are the risk factors and how is it treated?
- Increased pressure pushing the iris/lens complex forwards, which blocks the trabecular meshwork
- Risk factors include small eye (hypermertopia) narrow angle at trabecular meshwork
- May present sudden painful red eye with acute drop in vision
- Can be treated with peripheral laser iridotomy to create a drainage hole on the iris
What is the blind spot of the eye?
- Where the optic nerve meets the retina (optic disk is the anatomical landmark)
- There are no light sensitive cells
What is the most sensitive part of the retina?
- Fovea
- Has the highest concentration of cones but the lowest concentration of rods
What is central vision used for? How is it assessed?
- Day vision, colour vision, fovea has highest concentration of cone photoreceptors
- Reading, facial recognition
- Assessed by visual acuity assessment
- Loss of foveal vision - poor visual acuity
What is peripheral vision used for? How is it assessed?
- Shape, movement and night vision
- Navigation vision
- Assessed by visual field - unable to navigate environment, the patient may need a white stick
List the layers of the retina
- Photoreceptors (detection of light)
- Bipolar cells (local signal processing to improve contrast sensitivity and regulate sensitivity)
- Inner layer (retinal ganglion cells - transmission of signals from the eye to the brain)
- 1st, then 2nd then 3rd order neurons
How does the focea appear on a optical coherence tomography test?
- Fovea forms a pit at the centre of the macula
- This is due to the absense of the overlying ganglion cell layer
What are the types of photoreceptors?
- Rod photoreceptors are longer outer segments, with more photo-sensitive pigment - 100 times more sensitivie to light than cones, they have slow response to light and are important in night vision (120million)
- Cone photoreceptors are less sensitive to light but have a faster response, they are responsible for day light fine vision and colour vision (6million)
COmpare scotopic and photopic vision
- Scotopic is rod vision (motion)
- Photopic is cone vision (colour and details)
Where are the highest concentration of rod photoreceptors in the retina?
- 20-40 degrees away from fovea
- Cone photoreceptors highest in the fovea
What spectrum of light can the human eye see?
450nm to 680/700nm
How is colour perception tested?
Ishihara test
Describe the dark adaptation
- Increase in light sensitivity in the dark is dark adaptation
- Cone adaptation 7 minutes
- Rod adaptation 30 minutes following regeneration of rhodopsin
Describe the process of light adaptation
- Adaptation fromdark to light
- Takes over 5 minutes
- Bleaching of photo-pigments
- Neuroadaptation
- Inhibition of rod/cone function
- Constriction of pupil with light is pupil adaptation
Which is the commonest form of colour vision deviciency in humans?
Red-green confusion
What is refraction?
- Occurs when the idea light is passing from one medium to the other
- Velocity changes as light passes through one medium to another
- Ratio of the speed in a vacuum (air) compared with in a medium
- This calculates the index of refraction (n)
- As light goes from one medium to another, the path changes
What does light do when it meets a new medium?
- Some of the light reflexts off the boundary and come refracts through the boundary
- Angle of incidence = angle of reflection
- Angle of incidence > or < than angle of refraction depending on the direction of the light
Compare the effect of convex and concave lenses on refraction
- Convex lenses takes light rays to bring them to a focal point
- Concave lenses spread light rays outwards (focal pount is calculated by tracing the refracted rays backwards to a point)
What is emmetropia?
- Adequate correlation between axial length and refractive power
- Parallel light rays fall on the retina (no accomodation)
What is ametropia?
- Mismatch between axial length and refractive power
- Parrallel light rays dont fall on the retna (no accommodation)
- Nearsightedness (myopia)
- Farsightedness (hyperopia)
- Atigmatism
- Presbyopia
What is myopia?
- Parrallel rays converge at a focal pount anterior to the retina
- Etiology is not clear, genetic factor
- Caused by long globe (axial myopia)
- Can be caused by excessive refractive power
- Treated with convex lense
List the symptoms of myopia
- Blurred distance vision
- Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
- Headache
What is hyperopia?
- Parallel rays converge at a focal point posterior to the retina
- Inherited
- Excessive short globe (axial hyperopia - more common)
- Insufficient refractive power (refractive hyperopia)
- Treated with convex lense
List the symptoms of hyperopia
- Visual acuity at near tends to blur relatively early
- Nature of blur varied, and more noticeable if tired
- Asthenoptic symptoms (eyepain, headache in frontal region, burning sensation in the eyes)
- Blepharoconjunctivitis
- Amblyopia (uncorrected hyperopia - lazy eye)
What is astigmatism?
- Parralel rays focus in 2 focal lines rather than a single focal pount
- Hereditary
- Refractive media is not spherical, refracts differently along one meridian than along the meridian perpendicular to it
- More than two focal points
List the symptoms of astigmatism
- Athenopic symptoms (headache, eyepain)
- Blurred vision
- Distortion of vision
- Head tilting and turning
List the treatments for astigmatism
- Regular astigmatism treated with cylinder lenses with or without spherical lenses (convex or concave)
- Irregular astigmatism (rigid cylinder lenses, surgery)
What is the 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 the lense for near vision
What is presbyopia?
- Naturally occuring loss of accomodation (focus for near objects)
- Onset from age 40
- Distant vision intact
- Corrected by reading glasses (convex lenses) to increase refractive power of the eye
How is presbyopia treated?
- Convex lenses in near vision
- Reading glasses
- Bifocal/trifocal
- Progressive power glasses
List the types of optical correction
- Spectacle lenses (monofocal spherical or cylindrical or motifocal)
- Contact lenses (higher quality image, less influence on size of image)
List the disadvantages and complications of contact lenses
- Careful daily cleaning and disinfection
- Expensive
- Risk of infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis
When are intraocular lenses used?
- Replace cataract crystalline lens
- Gives best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses
List the possible surgical corrections and their uses
- Ketatorefractive surgery (RK, AK, PRK, LASIK, ICR, thermokeratoplasty)
- Intraocular surgery (clear lens extraction, phakic introcular lens)
Describe the mechanism of accommodation
- Contraction of circular ciliary muscle inside the ciliary body
- Relaxes the zonules that are normally stretched between the ciliary body attachment and the lens capsule attachment
- Zonules are passive elastic bands - no active contractile muscle
- In the absense of zonular tension, lens returns to its natural convex shape due to its innate elasticityy
- Increase refractive power of the lens
- Mediated by the efferent third cranial nerve
List the colour vision deficiencies
- Deuteranomaly (reduced green light sensitivity)
- Protanomaly (reduced red light sensitivity)
- Protanopia (absent red cones)
- Deuteranopia (complete inability to distinguish colours)
- Tritanopia (blue - yellow colourblindness)
- Tritanomaly (deficiency or abnormality of blue sensitive cones)
- Achromatopsia (total absence of colour vision - black and white)
