Structure and function of the eye Flashcards
Which bones form the medial wall of the bony orbit?
orbital plate of Ethmoid bone
Lacrimal bone
orbital surface of the maxilla
lesser wing of the spenoid bone
Which bones make up the roof and floor of the bony orbit?
Roof – orbital plate of frontal bone
Floor – orbital plate of maxillary bone
Which bones make up the lateral wall of the bony orbit?
greater wing of the sphenoid bone + zygomatic bone
Where does the eye sit?
Within the orbit
What is the palpebral fissure?
Just the opening between eyelids
Epileptic space between medial and lateral canthi of two open eye lids
What is the limbus?
Border between the sclera and cornea
Also where the corneal stem cells sit in, 36-48hr stem cell turnover of epithelial corneal cells, move to centre of the eye
What is the sclera?
The white bit
Describe eye from most central to outward
Pupil
Iris
Limbus
What is caruncle?
Made of skin covering sabaceous and sweat glands
Medial aspect of the eye
What is the average anterior-posterior diameter of the orbit?
24 mm
What are the three layers of the eye? Describe their properties and function.
Sclera • Hard and opaque • Maintains the shape of the eye Choroid • Pigmented and vascular • Provides circulation to the eye • Shields out unwanted scattered light Retina • Neurosensory • Converts light into neurological impulses
What is the anterior and posterior segment of the eye separated by?
The lens
What are the two anatomical spaces within the anterior segment?
Anterior chamber (cornea to iris) Posterior chamber (iris to lens)
What types of humour does each segment produce?
Ant. segment - aqueous humour
Post. segment - vitreous humour
Which part of the eye is visible at the back of the eye?
Optic disc
What are the zonules?
fibrous strands that hold the lens in place in line with the pupil, connected to the ciliary muscles, also called SUSPENSORY LIGAMENTS
Summarise the anterior segment?
Aq fluid produced by ciliary body and it passes into ant. chamber and then through angle via trabecular meshwork
The drainage important in maintenance of intraocular pressure
Aq supplies nutrients to cornea and other tissue in ant. chamber
No blood vessels in middle of eye, need a clear window for passage of light
THUS needs tissue needs to be bathed in fluid to receive nutrients and remove waste
Summarise the posterior segment?
Between lens and retina
vitreous humour is 99% water, trapped inside jelly matrix providing mechanical support to eye
collagen and GAGs in the vit. humour
Regular structure allows it to be transparent
What happens to vit. humour as we get older?
Loses its jelly consistency liquefies detaches from the retina SEEING FLOATERS normally harmless, small tear in peripheral retina sometimes if this does happen then vit. can get into subretinal space and cause detachment of retina which can lead to BLINDNESS so needs prompt attention
Where is the lacrimal gland located and what are the 3 types of tears it produces?
latero-superior to the globe
Basal tears - constant level even in absence of stimulation or irritation
Reflex tears - increased tear production in response to irritation - Afferent to cornea to CN v1, efferent is parasympathetic and ACh
Emotional tears = crying
Describe the passage of tears
Tear produced by lacrimal gland
Tear drains through two puncta, opening on medial lip margin
tear flows through the superior and the inferior canaliculi
tear gathers in the tear sac
Tear exits the tear sac via tear duct in nasal cavity (nasolacrimal duct into inferior meatus)
What is it and what is the purpose of tear film?
Thin layer of fluid that covers the cornea
Maintains smooth cornea air surface
Maintains clear vision and removes surface debris during blinking
Source of oxygen and nutrient supply to anterior segment
It is a bactericide ( kills bacteria)
What are the three layers of the tear film?
Superficial oily layer : 0.1uM, reduces tear film evaporation, produced by a row of Melbomian Glands along the lid margins
Aqueous tear film: main bulk of tear film, 9uM, delivers oxygen and nutrients and includes the bactericide
Mucinous layer: 0.3uM, maintains surface wetting, ensure tear film sticks to they eye surface, mucin binds water molecules to hydrophobic corneal epithelial cell surface
Conjunctiva is a transparent layer on top of the cornea that is very vascular - it has goblet cells that produce mucin
What does conjunctiva cover?
Outer surface of the eye
Describe the sclera
The white of the eye
tough opaque tissue that is protective
high water content
What is a blowout fracture?
Fracture of the floor of the orbit and so eyeball can fall into the cheek a bit, maxilla fracture
Describe the cornea
Front most anterior part of ant. seg. continuous with sclera transparent 2/3 of refractive power convex curvature and higher refractive index than air physical barrier from opportunistic inf.
What are the 5 layers of the cornea?
Epithelium
Bowman’s membrane
Stroma - regularity contributes to transparency (corneal nerve endings, no blood vessels)
Descemet’s membrane
Endothelium - pumps fluid out of the corneal and prevent corneal oedema ( only 1 cell layer, no regeneration power, density down with age, dysfunction = oedema/cloudy)
IF YOU HYDRATE CORNEA GOES WHITE BECAUSE CORNEA H20 IS DOWN AND SCLERA H20 UP
What is the uvea?
Composed of iris, ciliary body and choroid
Disease of one part of uvea can affect other parts
Where does vascular coat of eyeball lie?
Between the sclera and retina
What is the choroid?
Between retina and sclera
Layers of blood vessels
Nourish the back of the eye
Retina requires 2 ways of feeding some vessels for outer and inner retina
What is the iris?
The coloured part of the eye
Controls light levels inside the eye
Round opening = pupil
Iris is embedded with tiny muscles that dilate and constrict the pupil size SPHINCTER PUPILLAE
Anterior – stromal layer containing muscle fibres
Posterior – epithelium
What is 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 Increases refractive power by CN3, can vary thickness and shape working as biconvex lens controlled by ciliary body
How is the lens suspended?
Lens is suspended by lens zonules consisting of passive connective tissue
These fibres don’t stretch at all, they merely transmit force from the contraction of the ciliary muscles
Summarise focusing light
Ciliary muscle contraction is pushing towards themselves making lens thicker
When ciliary relaxed the choroid acts like a spring pulling on lens and zonulae fibres causing the lens to flatten
THICKER LENS = increased refractive and closer viewing
THINNER LENS = decreased refractive power and further viewing
What is the retina?
Thin layer of tissue that lines the inner part of the eye
Capturing light rays that enter the eye
These light impulses then sent to brain via the optic nerve
Where is the highest concentration of photoreceptors?
Macula ( red circle)
Optic nerve head (yellow circle)
Describe the blood supply of retina
Retinal arteries and veins provide circulation to inner 2/3 and outer 1/3 is by choroid vasculature
4 branches of vessel arcades:
sup and inf temporal
sup and inf nasal
Where is macula located?
Roughly in centre of the retina
temporal to the optic nerve
Small highly sensitive part of retina reported for detailed central vision
Fovea is the very centre of the macula, allows us to appreciate detail and perform tasks that require central vision - reading
What can macula degeneration cause?
1st cause of irreversible blinding disease, silent and asymptomatic
Cataract = highest cause of reversible blindness
glaucoma linked to degeneration of retina and optic nerve
What is the ciliary body?
Ring shaped tissue surrounding the lens
Between anterior and posterior segments and is located behind the iris
What does retina and choroid give rise to?
Retina gives rise anteriorly to ciliary body epithelium and posterior layer of iris (epithelial)
Choroid gives rise to ciliary body stroma and the stromal Ant. layer of iris
Function of ciliary body?
Ciliary body secretes aq. fluid
Aq. fluid then travels anteriorly into ant. chamber
Trabecular network drains the fluid out of the eye into canals of schlemm
10-20% = uveal scleral outflow not a distinctive pathway
What is normal intraocular pressure?
12-21 mmHg
What are the two types of glaucoma?
Primary open angle glaucoma
Closed angle glaucoma
What is primary open angle glaucoma?
Commonest
Functional blockage of trabecular meshwork
What is Closed angle glaucoma ?
relatively common
acute or chronic
forward displacement of iris/lens complex narrowing trabecular meshwork
occurs in patients with small eyes (hypermetropic)
Sudden painful red eye with acute drop in vision
Treatment - peripheral laser iridotomy
How can you differentiate venule and arteriole?
Veins are thicker and darker
What is glaucoma?
Sustained raised increased intraocular pressure
gradual and accumulative damage to optic nerve
Retinal ganglion cell death and ENLARGED optic disc cupping
Untreated = loss of peripheral vision GRADUAL, eventual blindness
Where is the blind spot?
Where the optic nerve meets the retina there are no light sensitive cells
Describe the cone and rod cell concentration of fovea
Fovea has highest concentration of cones but low conc. of rods (more sensitive to light hence everything in periphery brighter)
Fovea is most central macula and most sensitive part of the retina
What are the responsibilities of central and peripheral vision>
Central:
detail day vision (fovea has highest cone)
Reading, facial recognition
Assessed by visual acuity assessment
loss of foveal vision = poor visual acuity
Peripheral: shape, movement, night vision navigation vision assessed by visual field assessment With loss become unable to navigate in environment, patient may need white stick even with perfect visual acuity.
Describe retinal structure?
Outer layer: photoreceptors first order (detect light, point away from pupil and light bounces pack of pigmented epithelium of retina and is then detected)
Light hitting different parts of retina gives spatial awareness
Middle layer: bipolar cells = 2nd order = local signal processing to improve contrast sensitivity, regulates sensitivity
Inner layer: retinal ganglion cells - signal from eye to brain
What is the foveal pit?
Fovea forms pit at centre of macula due to absence of overlying ganglion cell layer, can be assessed with OCT
Describe the visual processing photoreceptors
Rod - longer outer segment with photosensitive pigment 100 times more sensitive than cones to light
Slow response to light
Night vision, 120 mill rods
Cone - less sensitive to light, faster response, fine vision during day, 6 mill
Photopigments are synthesised in inner photoreceptor segments and are transported to outer segment discs
Distal discs with de-activated pigment are shredded from tips by phagacotytic retinal pigment epithelial cells and regenerated inner and bulk transfer
What is light dark adaption?
Dark - Increase in light sensitivity on dark. cone adaption in 7 mins and rod in 30 mins after regeneration of rhodopsin
Light: dark to light, 5 mins, bleaching of photopigments, inhibition of rod/cone functoin
pupil adaption - constriction of pupil with light
What is emmetropia?
Perfect focusing
Parallel rays converge exactly on fovea forming a clear image on the retina
No accommodation needed
Cornea - 2/3
lens - 1/3
What is ametropia?
Mismatch between axial length and refractive power
Dont fall on retina - myopia(near sight), hyperopia(far), presbyopia, astigmatism
What is myopia?
SHORT SIGHTED
Parallel rays converge at focal point ant. to the retina, etiology is not clear maybe genetic
Cause: excessive long globe more common and excessive refractive power
symptoms: blurred distance vision, headache, squinting
CONCAVE LENSE
What is hyperopia?
LONG SIGHTED
Parallel rays focus behind the retina
blurred vision
Not enough focusing power or short globe
CONVEX LENS
symptoms: visual acuity at near tends to be blurry, eyepain, headache in frontal region, burning sensation, blepharoconjunctivitis
What is astigmatism?
cornea is OVAL rather than spherical so you get different refraction along one meridian
parallel rays come to focus in 2 focal lines rather than a single focal point
hereditary
symptoms: astenopic symptoms (headache and eye pain)
blurred vision
distorted vision
head tilting and turning
treatment: cylindrical lenses, if irregular you need surgery
Summarise accommodation
-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
What is near response triad?
Adaptation for Near Vision • Near Response Triad – Pupillary Miosis (Sphincter Pupillae) to increase depth of field, maintain clear focus across range of distances without accommodation If shallow lose focus easily with movement of an object. – 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, lens becomes thicker
What is presbyopia?
Naturally occurring loss of accommodation
onset from 40
distant vision intact
corrected by reading glasses to increase refractive power of the eye
