7.1 Structure and Function of Eye Flashcards
The eye is a spherical structure which sits in the orbit (enclosed by bony walls medially, laterally, superiorly and inferiorly) → anteroposterior diameter of 24mm in adults:
Sclera: Hard fibrous, opaque (continuous with cornea)
- Maintains the physical shape of the eye
- Protects the eye
Choroid –pigmented and vascular (continuous with _______________)
- Provides blood circulation
- Shields unwanted light from entering the eye (pigmentation)
Retina: Transparent and neurosensory (continuous with _______________ anteriorly):
- Converts light signals into neurological signals transmitted via CN II
ciliary bodies and iris;
ora serrata
Anterior compartment: ocular structure anterior to the lens, space between the _______________
- Filled with aqueous humour (clear aqueous fluid synthesised from the ______________ from blood plasma)
- Supplies nutrients to surrounding tissue (especially avascular cornea)
cornea and lens;
ciliary body
Vitreous cavity (at posterior segment) – between ___________________, filled with a jelly gel-like substance (vitreous body)
- 98 to 99% water trapped inside a jelly-like collagen and glycosaminoglycan gel, providing mechanical support
- Ageing vitreous: the vitreous humour liquefies and collapses with age → _______________ detachment
- Complication: contracting vitreous humour may pull the retina on the side → retinal tear
- Symptoms: patients may see the detached vitreous debris (detached from retina) as floaters → patients experiencing this are advised to see an ophthalmologist for a retinal exam to exclude an ____________________
lens and retina;
posterior vitreous ;
early retinal tear
The cornea is the transparent window at the front of the eye continuous with the ____________
- Important optical properties – transparent and contributes to 2/3 of the refractive power of the eye (ability to focus light at a point)
- Role in refracting light – _____________ curvature with a higher refractive index than air
5 layers of cornea
1) Epithelium: Consists of several layers of ___________________
• High capacity for regeneration → corneal abrasions heal quickly
2) Bowman’s membrane: Smooth acellular non-regenerating layer composed of __________________
3) Stroma: Thickest layer → corneal nerve endings provide sensation and nutrients for healthy tissue:
• Transparency of cornea: regular arrangement of collagen fibres, no blood vessels
4) Descemet’s membrane: Basement membrane for the endothelial layer composed of different types of collagens (types ______& _______)
• Site of copper deposition in patients with Wilson’s disease → formation of Kayser-Fleischer rings
5) Endothelium: Single layer of endothelial cells:
• Pumps fluid out to the corneal stroma (prevents corneal oedema → dehydrates the cornea)
• Cannot regenerate → cell density decreases with age
• Dysfunction causes corneal oedema and cloudiness
sclera;
convex;
epithelial cells;
strong randomly-oriented collagen fibres;
IV & VIII
[Uvea: refers to the choroid, ciliary body and the iris]
Ciliary body is composed of tissues derived from the retina and choroid, and lies _____________ with the iris
- Ring of tissue encircling the lens
- Composed of the epithelium and stroma (ciliary muscle fibres)
- Important for the production and secretion of aqueous humour (intraocular fluid)
Iris – Lies anterior to the ciliary body and the lens; composed of two layers:
- Anterior layer: ____________ layer (smooth muscle fibres, stromal tissue, sphincter pupillae* and dilator pupillae)
- Posterior layer: ____________ layer (pigmented)
- Aka pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers
posterior and continuous;
stromal ;
epithelial
Lens – The lens is located immediately posterior to the _________ and is responsible for the remaining one-third of the refractive/focusing power of the eye:
- Lens is suspended by a ring of fibrous strands known as lens zonules
- Lens zonules connect the __________________
- Composed of an outer acellular capsule encasing a core of regular inner elongated cell fibres (maintains clarity and transparency of lens)
- May lose transparency with age → _____________
- Functions: transparency (regular structure), refractive power (one-third; higher refractive index than the aqueous and vitreous humours), accommodation (elasticity)
lens and ciliary body;
iris;
cataracts
Optical function of the eye: The human eye focuses incoming light rays from distant objects onto the _________ to form a clear image, which is converted into nerve impulses transmitted to the brain via __________:
Refraction – cornea (2/3 of power) and lens (1/3 power)
- Cornea has a ________ refractive index than air, with a convex surface
- Cornea-air interface contributes 2/3 of the refractive power
Regulation of amount of incoming light: Pupil, pigmented uvea (choroid, iris)
- The iris regulates the amount of light entering into the eye by __________________.
- The pigment on the iris and ciliary body and choroid absorbs stray light and prevents light scattering within the eye.
Processing of visual information: Retina, optic nerve (CN II)
retina;
CN II;
higher ;
varying the size of the pupil acting like the aperture of the camera
Hypermetropia – long-sightedness
• Caused by ___________________
• Cornea and lens do not have sufficient ___________ to focus image near the retina
• Parallel rays (from distant objects) are brought to a point of focus __________ the retinal surface
• Blurred vision without optical correction (glasses) for distance and near → can be corrected with _____________ to provide additional converging power
short globe or flat corneal surface (less convex);
refractive power;
behind;
convex lens
Myopia – Short Sightedness
• Globe too ________ or very _________ corneal curvature
• Parallel rays (from distant objects) converge in front of the retinal surface
• Divergent rays (from near objects) converge on the retinal surface
• Patient able to see near objects clearly but requires ___________ lenses to see distant objects clearly
long;
convex;
concave
Astigmatism – blurred vision
• Different ____________ along different orientations e.g. mild myopia in the horizontal meridian and mild hyperopia in the vertical meridian as shown in the diagram resulting in astigmatism
• Can be optically corrected with astigmatic glasses – ____________
light refraction power;
cylindrical
Accommodation – lens is elastic and can alter shape to change the refractive power when focusing on objects of different distances, mediated by _________________
Lens is suspended by a ring of zonules (passive elastic bands with no active contractile muscles) anchored to the _______________
When changing point of focus from a distant to a near object:
- Contraction of the ciliary muscle inside the ciliary body
- Zonules that are normally stretched between the ciliary body attachment and the lens capsule relax
- In the absence of zonular tension, the lens returns to its _______________ shape due to its innate elasticity (lens thickens centrally)
- This increases the _____________ of the lens
- Focuses ________________ from near objects on the retina
oculomotor nerve (CN III);
ciliary body;
natural convex ;
refractive power;
divergent rays
Near response triad: adaptation for near vision
- Pupillary contraction: increasing depth of focus directing light through the ______________, mediated by _________________
- Convergence: ______________ of both eyes to align both eyes towards a near object (adduction)
- Accommodation: ________________ increases refractive power of the lens
thicker central part of the lens;
circular sphincter pupillae;
medial recti;
ciliary muscle
Presbyopia – naturally occurring loss of accommodation, onset from age 40 years
- Distance vision remains intact, worsened ability to _________________
- Corrected by reading glasses (_________ lenses) to provide additional refractive power to converge divergent rays from near objects
focus on near objects;
convex
The ciliary body secretes intraocular fluid (aqueous fluid) into the eye, which supplies vital nutrients to the surrounding tissue (especially the anterior segment):
- Intraocular fluid flows anteriorly into the anterior chamber (via the pupil) → drains into the _______________
- Normal intraocular pressure (IOP): 12 – 21 mmHg (maintained by fine balance between the production and drainage of intraocular fluid)
trabecular network (at junction between ciliary body and cornea)
Primary open angle glaucoma – commonest
- Commonest type ; caused by functional blockage of the _____________________
trabecular meshwork (IOF cannot drain from the anterior
Primary angle closure glaucoma – can be acute or chronic
• May be acute or chronic (perpetuated by a vicious cycle):
• Risk factors: ___________, _______________
• Contact between the ____________ also known as pupil block is the precipitating event. Aqueous is unable to flow into the _____________, and accumulates behind the iris. This results in the peripheral iris bulging forward, physically closing the already narrow drainage angle and blocking the trabecular network. This obstructs the outflow of aqueous and results in an acute increase in intraocular pressure.
• Presentation: ____________________
• Treatment: ________________ (to create drainage hole on the iris → alternative pathway for drainage of IOF)
small eye (hypermetropia), narrow angle at trabecular meshwork;
pupil and the lens ;
anterior chamber;
sudden painful red eye with acute drop in vision;
peripheral laser iridotomy
Central vision – fovea has the highest concentration of _______________
- Detailed daytime vision, colour vision
- Reading, face recognition
- Loss of foveal vision results in ________________
Peripheral Vision – shape, movement, night vision, navigational vision, visual field assessment
- Extensive loss of visual field results in difficulty ________________
- Patient may need white stick even with perfect reading vision
cone photoreceptors;
poor visual acuity;
navigating in the environment
Retinal structure – The retina forms the innermost layer of the coat of the eye in the posterior segment:
- Consists of a single outer layer of retinal pigment epithelium in front of the __________, with the remaining being the ___________
- Choroid: Blood supply to the outer 1/3 of the retina (photoreceptors), pigment reduces light scattering
Retinal pigment epithelium: transports nutrients to retina, removes metabolic debris from photoreceptors, absorbs scattered light (diminishes photo-oxidative stress and improves quality of vision)
Neuroretina – anterior to retinal pigment epithelium
- Outer layer: ___________ → detect of Light
- Middle layer: _________________ → local signal processing
- Inner layer: _____________ → Transmission of signal from the eye to the brain
choroid;
neuroretina;
photoreceptors (1st order neurones) ;
bipolar cells (2nd order neurones);
retinal ganglion cells (3rd order neurons)
The macula lutea (yellow spot) is a 6mm-wide pigmented region at the centre of the retina:
- ______________ forms the pit at the centre of the macula (due to the absence of the ________________)
- Fovea contains the highest concentration of photoreceptors for fine vision
- Clinical assessment: _______________
Fovea centralis;
overlying ganglion cell layer;
optical coherence tomography (OCT) scan
There are two main classes of photoreceptors in the retina
Rod photoreceptor – 120 million rods
- Longer outer segment with more photo-sensitive pigment, 100 times more sensitive to light than cones
- Responsible for ___________, recognises motion
Cone photoreceptor – 6 million cones
- less sensitive to light but faster response
- Responsible for _________________
Photopigments are synthesized in the ____________ and transported to the ___________________
- Distal discs are shed from the tips and absorbed by retinal pigment epithelium cells by phagocytosis
- Deactivated photopigment are regenerated within retinal epithelial cells and transported back to photoreceptor cells
night vision (scotopic vision);
day light fine vision and colour vision (photopic vision), recognises colour and details;
inner segment;
outer segment discs
Photopigments – rhodopsin for rods and photopsin for cones
Rhodopsin (opsin) consist of ___________ with ___________ (vitamin A derived)
- Found in the ______________ → transmembrane protein in the outer segment of the photoreceptor cell:
- Light → conformational changes activate G-protein signalling → ________________ → Hyperpolarization → action potential
Photopsin – 3 subtypes reacting to 3 different light frequencies (______, __, _____)
transmembrane protein with cofactor 11 cis-retinal;
rod photoreceptors;
closure of ion channels (Na+ and Ca2+);
red, blue, green
what is the frequency of the blue green rod photopigment?
498 (nm)
what is the frequency of the blue s-cone photopigment?
420 to 440 nm
what is the frequency of the green m- cone photopigment?
534 to 545 nm
what is the frequency of the red l-cone photopigment?
564 to 580 nm
what is deuteranomaly?
(red green colour deficiency): most common form of colour vision deficiency due to shift of M cone sensitivity towards the L cone sensitivity peak → red green confusion
what is tritanomaly?
Malfunctioning S-cones
what is protanomaly?
Malfunctioning L-cones
what is tritanopia?
Missing S-cone (blue)
what is deuteranopia?
Missing M-cone (green)
what is protanopia?
Missing L-cone (red)
what is Rod monochromatism?
No cones, only rods
what is Blue cone monochromatism?
Blue cones only
Arterial supply: The human eye has 2 circulations: the retinal circulation and the choroidal circulation. Both are derived from the _____________ which is a branch of the _______.
From the opthalmic artery, the central retinal artery branches off and enters the eye to supply the inner 2/3 of the retina and is responsible for the retinal circulation. The outer 1/3 receives its supply from the uveal/ choroidal circulation.
The posterior ciliary arteries provide the choroidal circulation. They consist of long and short posterior ciliary arteries.
- _______________ travel forward and supply the choroid anteriorly
- __________ derived from ophthalmic artery as it crosses the optic nerve directly supply the choroid posteriorly
opthalmic artery; ICA;
Long posterior ciliary arteries;
Short posterior ciliary arteries
Central retinal artery occlusion – cherry red spot on fundoscopy
- Due to the difference in blood supply of the inner and outer retina
- Inner retina – pale due to ______________
- Outer retina continues to be perfused by _______________, circular perfused area of the retina can be seen
No overlying ganglion cells at the fovea – cherry red spot is seen
_____________ – present in up to 15% of cases, considered to be the commonest retinal vascular anomaly
- Anastomoses between ciliary circulation and central retinal circulation, helps to perfuse a small area of retina where papillomacular bundles run across
- Retina is less damaged in CRAO in this case
infarction of the retinal ganglion cells;
choroidal circulation;
Cilioretinal artery;
Venous drainage of the eye – via the superior and inferior ophthalmic veins
- Central retinal vein drains into the _________________
- Both ophthalmic veins drain to the ____________
superior ophthalmic vein;
cavernous sinus
