Structure and Function of the Eye Flashcards
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 - protective outer coat
- Maintains the shape of the eye
- High water content
Choroid
- Pigmented and vascular
- Provides circulation to the eye
- Shields out unwanted scattered light
Retina
- Neurosensory
- Converts light into neurological impulses
What are the two segments of the eye separated by?
Lens separates anterior and posterior segments
Which humours are found in the two segments of the eye?
Anterior = aqueous humour
Posterior = vitreous humour
What name is given to the fibrous strands that suspend the lens from the ciliary bodies?
Zonules
Describe the production and drainage of aqueous humour.
Aqueous humour is produced by the ciliary body.
It is drained via the trabecular meshwork into the canals of Schlemm
What is the role of aqueous humour?
Provides nutrients to the cornea and other tissues in the anterior chamber
Describe vitreous humour.
It is 99% water trapped inside a jelly matrix
What is the function of vitreous humour?
Mechanical support for the eye
Describe how the vitreous humour changes with age.
It loses its jelly consistency, liquefies and can become detached from the retina
Vitreous detachment in seen as floaters
What are the potentially disastrous consequences of vitreous humour detachment?
Detaching from the retina could cause a small tear in the peripheral retina.
If there is a small tear, liquid vitreous could seep into the sub-retinal space and lead to retinal detachment If untreated, it can lead to blindness
What are the two layers of the iris?
Anterior) – stromal layer containing muscle fibres (dialtor pupillae .
Posterior – epithelium
Describe how the retina and choroid contribute to the different parts of the iris and ciliary body.
Retina gives rise anteriorly to the ciliary body epithelium and the posterior (epithelial) layer of the iris.
Choroid gives rise anteriorly to the ciliary body stroma and the anterior layer of the iris (stromal layer)
What is the collective term for the choroid, iris and ciliary body?
Explain it’s properties
Uvea
Vascular coat of eyeball and lies between the sclera and retina.
The 3 parts are Intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree.
What is the normal range for intraocular pressure?
11-12 mm Hg
What is glaucoma?
Condition of sustained raised intraocular pressure
What changes can be seen in the retina in glaucoma?
Retinal ganglion cell death.
Enlarged optic disc cupping
What are the consequences of untreated glaucoma?
Progressive loss of peripheral vision
Blindness
What is the most common type of glaucoma and what is it causedby?
Primary open angle glaucoma- It is caused by a functional blockage of the trabecular meshwork
State another relatively common type of glaucoma. What is it caused by?
Closed angle glaucoma- This can be acute or chronic.
It is caused by the forward displacement of the iris-lens complex –narrowing the trabecular meshwork
In what type of patients does closed angle glaucoma tend to occur and what is the treatment?
Small eyes (hypermetropic)
Treatment: peripheral laser iridotomy
Describe the structure of the lens. give me it’s function
It has an outer acellular capsule.
There are regular inner elongated fibres, which give the lens its transparency
NOTE: may lose transparency with age; becomes opaque (cataracts).
Functions are:
- 1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous
- Accommodation
- Elasticity
Which two structures provide the majority of the refractive power of the eye?
Cornea = 2/3
Lens = 1/3
What layer of the eye is the cornea continuous with?
also describe the water content of the cornea
Sclera
LOW water content.
Other than its role in refracting light, what else is the cornea necessary for?
Physical barrier – protects the eye from opportunistic infection
What are the consequences of prolonged contact lens wear? explain
Reduce the oxygen supply to the cornea. This is because the cornea gets it’s oxygen supply from the air
What are the 5 layers of the cornea?
1 Epithelium
2 Bowman’s membrane
3 Stroma – its regularity contributes towards transparency (collagen fibres)
4 Descemet’s membrane
5 Endothelium – pumps fluid out of corneal (stroma) and prevents corneal oedema
What is the role of the endothelial layer of the cornea?
It pumps out fluid from the stroma and prevents stromal oedema
What is the role of tear film?
Maintains clear vision (smooth cornea-air surface)
Removes surface debris
Bactericide
Oxygen supply to cornea.
What are the three layers of the tear film?
Draw it and state the function of each layer.
Superficial oily layer
- reduces tear film evaporations
- Produced by row of Meibomian glands along the eyelid margins
Aqueous tear film- Main bulk of it
- It delivers oxygen and nutrient to the surrounding tissue.
- It contains factors against potentially harmful bacteria.
Mucinous layer.
- This renders the surface of the eye “wettable”.
- The mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface.
What produces the superficial oily layer?
Meibomian glands
What are the roles of each of these three layers of tear film?
Superficial Oily Layer
- Reduces tear film evaporation
Aqueous Tear Film
- Oxygen and nutrients
- Bactericide
Mucinous Layer
- Ensures that tear film sticks to the eye
- Conjunctiva is a transparent layer above the cornea that is very vascular
- The conjunctiva has goblet cells that produce mucin
Where is the lacrimal gland located?
Superio-laterally to the orbit
What are the three types of tears?
Basal Tears – produced at a constant level in the absence of irritation.
Reflex Tears – increased tear production in response to irritation.
Emotional Tears – crying
Describe the innervation of the cornea.
The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)
Describe the drainage of tears.
Tears are drained into two puncta via two tiny holes in the upper and lower medial lid margins.
It then drains into superior and inferior canaliculi –> common canaliculus –> tear sac.
Tear sac then drains through the nasolacrimal duct, which opens up in the inferior meatus
What two things regulate how much light reaches the retina?
Pupil
Pigmented Uvea
What is the term given to perfect focusing ability?
Emmetropia
What is the technical term for long-sightedness?
Hypermetropia
How can long-sightedness be corrected and what is it caused by?
Convex lenses It is usually caused by having short eyeballs It is occasionally caused by a flat corneal surface
What is the technical term for short-sightedness?
Myopia
How can short-sightedness be corrected and what is it caused by?
Concave lenses It is usually caused by having a long eyeball .
It is occasionally caused by having a highly curved cornea
What is astigmatism?
The cornea is oval rather than round.
This means that the refractive power varies in different planes (in some planes you will be hypermetropic, and in others you would be myopic)
Which nerve is responsible for accommodation?
Oculomotor Nerve (CN III)
What is the near response triad?
Pupillary Miosis (constriction of sphincter pupillae) – increases the depth of field.
Convergence – the medial recti of both eyes contract.
Accommodation – ciliary muscles contract to thicken the lens and increase its refractive power
What is the term given to naturally occurring loss of accommodation with age?
Presbyopia
What are the four branches of vessel arcades radiating from the optic disc?
Superior Temporal
Inferior Temporal
Superior Nasal
Inferior Nasal
Describe the difference in perfusion between the outer and inner parts of the retina.
Inner 2/3 of the retina = retinal arteries .
Outer 1/3 of the retina = choroidal vasculature
What part of the retina is responsible for central vision?
Fovea (it has the highest concentration of cones)
What is peripheral vision responsible for?
Shape, movement, night vision
Describe the structure of the retina. give function of each
Just inside the choroid you have the retinal pigment epithelium.
Then you have the neuroretina, which consists of:
- Outer Layer – photoreceptors (rods and cones)- detects light
- Middle Layer - bipolar cells (2nd order neurone): local signal processing to improve contrast sensitivity, regulate sensitivity
- Inner Layer – retinal ganglion cells- transfer from eye to brain
What is the function of the retinal pigment epithelium?
Transports nutrients from the choroid to the photo-receptor cells and removes metabolic waste from the retina
Describe how the fovea appears on a cross-section of the macula on histology
It appears as a foveal pit due to the absence of overlying retinal ganglion cells
State the two classes of photo-receptor and their properties.
Rods
- Longer outer segment with photo-sensitive pigments
- 100 times more sensitive to light than cones
- Slow response to light
- Responsible for night vision (scotopic vision)
- 120 million rods
Cones
- Less sensitive to light
- Faster response to light
- Responsible for daylight vision and vision and colour vision (photopic vision)
- 6 million cones
Describe the recycling of photopigments.
Photopigments are synthesised in the inner photo-receptor segment and then are transported to the outer segment discs.
The distal discs with deactivated photo-pigments are shredded from the tips and phagocytosed by retinal pigment epithelial cells.
The deactivated photo-pigments are regenerated inside the retinal pigment epithelial cells and are then transported back to the photo-receptors
What is the rod photopigment?
Rhodopsin
It is a G-protein coupled receptor system
What is the co-factor for this photopigment? (rhodopsin)
11 cis retinal (vitamin A derived)
To what wavelength does rhodopsin react maximally in humans?
498 nm
Black wave on graph
What is another term for night vision? describe what it allows you to do
Scotopic vision
More photoreceptors, more pigment, higher spatial and temporal (time) summation
Recognizes motion
What is another term for day-time vision? Describe what it allows you to do.
Draw the photocreceptor distribution
Photopic vision
- Central and day vision
- Recognizes colour and detail
Describe the distribution of rods and cones across the retina.
Rods have the highest density just outside the macula.
They decrease in density the further you move away from the macula.
There are NO rods in the macula.
Cones are ONLY found in the macula. The highest density of cones is in the fovea
What does a hill of vision represent?
Sensitivity of vision over a visual field
Where is the physiological blind spot located?
Optic disc: it is located 20 degress nasal to the macula
This is where the optic nerve meets the retina.
What are the three types of cone photopigment and which colours do they respond maximally to?
S-cone – short wavelength – BLUE
M-cone – medium wavelength – GREEN
L-cone – long wavelength – RED
What is the most common colour vision deficiency and what is it caused by?
Deuteranomaly -
It is caused by the shifting of the M-cone towards the L-cone; hence can’t tell between red and green.
What is the term given to shifted peaks?
Anomalous trichomatism
What test is used to diagnose colour blindness?
Ishihara Test.
the number 25 can be seen by everyone
Describe how light sensitivity changes in dark adaptation.
Light sensitivity increases in dark adaptation
How does retinal light change in light adaptation and what is responsible for this effect?
Light sensitivity decrease in light adaptation.
This suppression of light sensitivity is caused by photopigment bleaching and neuro-adaptation inhibiting rod and cone function
Describe the tear reflex pathway
Irritation of the conjunctiva and cornea stimulate receptors that initiate the lacrimation reflex.
The afferent limb of the reflex arc is via fibres in ciliary nerves to the nasociliary branch of the ophthalmic division of the trigeminal nerve (CN V). The cell bodies are located in the trigeminal ganglion.
The CNS component of the reflex arc involves the lacrimal nucleus (rostral part of the superior salivary nucleus) of the facial nerve (CN VII).
The efferent limb of the reflex involves preganglionic parasympathetic secretomotor fibres passing via the nervus intermedius to the facial nerve then the pterygopalatine ganglion. Postganglionic parasympathetic secretomotor fibres pass via the zygomatic branch of the maxillary nerve (to hitchhike along the lacrimal nerve).
The effector is the lacrimal gland.
what is the name for full colour blindness
Achromatopsia
What is dichromatism and monochromatism.
Explain the full consequences for this for both rods and cones
Colour Vision deficits can also be caused by the absence of one or more of the 3 cone photo-pigments.
In Dichromatism, only two cone photo-pigment sub-types are present.
In Monochromatism, there is complete absence of colour vision.
This can be caused by Blue Cone Monochromatism with the presence of only blue S-cones.
Or by Rod Monochromatism, in which there is a total absence of all cone photo-receptors.
Patients with Blue Cone Monochromatism have normal day light visual acuity,
whereas Patients with Rod Monochromatism have no functional day vision.
Contrast central and peripheral vision
Central
- Detail day vision, colour vision – fovea has the highest concentration of cone photoreceptors
- 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
- Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity
where can the macula be found and what does it do?
Located centre to the retine, temporal to optic nerve
A small and highly sensitive part of the retina responsible for detailed central vision.
The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading and facial recognition.
What is the visible portion of the optic nerve called?
optic disc/papilla
Describe the properties of conjunctiva.
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- unless inflammed in conjunctivitis
what are the functions of the iris
- Controls light levels inside the eye similar to the aperture on a camera.
- Round opening in the centre is the pupil.
- Embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
In what gender is colour blindness more prevalent in
MALE- 8%
female- 0.5%
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why do stars oout of the corner of your eyes appear brighter than when you look at them
Fovea is the most sensitive part of the retina.
It has the highest concentration of cones, but a low concentration of rods
Contrast deuteranomaly, protanomaly and trinotopia
Deuteranomaly - M cone isn’t working properly and it SHIFTS TO THE L cone side. Hence can’t see green well (you see red more). In opia ( M cone is not working at ALL)
Protanomaly- L cone SHIFTS towards M cone. Hence can’t see red but see GREEN more. In opia (L cone is not working AT ALL)
Trinotopia- S cone shifts towards M - can’t see blue well
