11 - Structure & function of the eye Flashcards
What is the name of the cavity that the eye sits within?
the orbit
“Orbit” can refer to the bony socket, or it can also be used to imply the contents.
What name is given to the white part of the eye?
sclera
What are the names for the 2 corners of the eye (inner and outer)?
medial and lateral canthus
What is the palpebral fissure?
the elliptic space between the medial and lateral canthi of the two open lids.
In simple terms, it refers to the opening between the eye lids.
What is the caruncle?
the red prominence at the inner corner of the eye
What is the (corneal) limbus?
the border of the cornea and the sclera (the white of the eye)
stem cells are located in the basal epithelial layer of the corneal limbus
What produce tears?
the lacrimal gland
Give the order from tear production to drainage
- tears produced by the lacrimal gland
- drain through the two Puncta
- flow through the superior and inferior canaliculi
- gather in the tear sac
- exit the tear sac through the tear duct into the nasal cavity
What are the punctum?
opening of the tear ducts
What is the function of the tear film?
protects and lubricates the eye
contributes towards the optical function of the eye - maintains clear vision
maintains a smooth cornea-air surface
is the oxygen supply to the cornea (has no vasculature)
removal of debris
bactericide
What are the three layers of the tear film?
Superficial oily layer
Aqueous tear film
Mucinous layer
What produces the superficial oily layer of the tear film?
Meibomian glands
What are the roles of each of these three layers?
superficial oily
aqueous tear film
mutinous layer
Superficial Oily Layer
- Reduces tear film evaporation
Aqueous Tear Film
- Oxygen and nutrients
- Bactericide
Mucinous Layer
- Ensures that tear film sticks to the eye
What is the conjunctiva?
- the mucous membrane that covers the front of the eye and lines the inside of the eyelid
- a transparent layer above the cornea that is very vascular
- The conjunctiva has goblet cells that produce mucin
How is the conjunctiva nourished?
there are tiny blood vessels that are nearly invisible to the naked eye
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)
What are the three layers of the coat 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 average anterior-posterior diameter of the orbit?
24 mm
Describe the properties of the sclera
‘white of the eye’
has a high water content
very tough, opaque tissue
continuous with the cornea
Describe the properties of the cornea
- front most part of the anterior segment
- continuous with the scleral layer
- powerful refracting surface, providing 2/3 of the eye’s focusing power
- acts as a physical and infection barrier
- for nourishment, relies on tear film and aqueous fluid
- LOW WATER CONTENT
What layer of the eye is the cornea continuous with?
Sclera
Other than its role in refracting light, what else is the cornea necessary for?
Physical barrier – protects the eye from opportunistic infection
Which two structures provide the majority of the refractive power of the eye?
Cornea = 2/3 Lens = 1/3
What drains the water from the cornea?
corneal endothelium - pumps excess fluid out of the cornea
What are the consequences of prolonged contact lens wear (for the cornea)?
Reduce the oxygen supply to the cornea
What are the 5 layers of the cornea?
Epithelium Bowman's membrane Stroma Descemet's membrane Endothelium
What happens to the cornea with age?
the endothelial cell count declines with age (the endothelial layer has no capacity to regenerate)
less excess fluid is pumped out
becomes opaque —–> blurred vision
eventually turns white
What is the uvea?
the vascular coat of the eyeball
lies between the sclera and the retina
What are the 3 components of the uvea?
iris
ciliary body
choroid
What are the properties of the choroid and what does It nourish?
composed of a layer of blood vessels that nourish the outer retina
How do the sphincter papillae muscles affect the pupil size?
When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil.
When they relax, the iris dilates
Describe the structure of the lens.
It has an outer acellular capsule
There are regular inner elongated fibres, which give the lens its transparency
What is the name for when the lens loses it transparency with age, resulting in an opaque lens?
cataract
What name is given to the fibrous strands that suspend the lens from the ciliary bodies?
Lens Zonules
What is the cause of shortsightedness in elderly patients?
the lens loses its elastic properties, so when the muscle constricts it remains wide and thin
What does the optic nerve carry?
axons of the redial ganglion cells
What is the visible portion of the optic nerve called?
the optic disc
Where is the macula located?
roughly in the centre of the retina, lateral/temporal to the optic nerve
What is the macula?
a small, highly sensitive part of the retina responsible for detailed central vision
has the fovea at the centre
What is glaucoma?
a disease of the retina, where the neurones die
increased intraocular pressure is a major risk factor
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
How can you tell if there is infection or inflammation of the eye when looking at the anterior chamber?
will see debris and cells (is normally completely transparent)
What is the ciliary body? What is its function?
a ring shaped tissue that surrounds the lens
it secretes aqueous fluid into the anterior chamber
What are the two layers of the iris?
Thick anterior layer, composed of stromal tissue and smooth muscle
Thin posterior pigmented epithelial layer
Describe the production and drainage of aqueous humour.
Aqueous humour is produced by the ciliary body
It is drained (mostly) via the trabecular meshwork (situated at the junction between the ciliary body and the cornea) into the canals of Schlemm
What is the role of aqueous humour?
Provides nutrients to the cornea and other tissues in the anterior chamber
What are the two pathways by which aqueous humour leave the eye/is reabsorbed?
Trabecular Meshwork - active absorption (80%)
Drained via the trabecular meshwork into the canals of Schlemm. This canal is invisible and sits around the cornea, deep in the sclera. It is a modified vein that absorbs aqueous humour and pushes it into the venous system.
Uveoscleral Pathway - passive flow (20%)
The uveal-scleral outflow is a passive gradient flow, for the aqueous humour to be absorbed, between the choroid and sclera.
What is the treatment of glaucoma and which pathway of drainage of aqueous fluid does it target?
prostaglandins analogues
act on the uveal-scleral flow
What is glaucoma?
Condition of sustained raised intraocular pressure
results in gradual and accumulative damage to the optic nerve tissue
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
State some types of glaucoma
Primary open angle glaucoma - commonest
Closed angle glaucoma
What is the most common type of glaucoma and what is it caused by?
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/what are the risk factors and what is the treatment?
Small eyes (hypermetropic) or long sightedness
Treatment: peripheral laser iridotomy
What is the macula and where is it located?
What is it responsible for?
- located roughly in the centre of the retina, temporal to the optic nerve
- It is a small and highly sensitive part of the retina responsible for detailed central vision
- The fovea is the very centre of the macula – Your fovea is the most sensitive part of the retina
What part of the retina is responsible for central vision?
(macular 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.
(Just inside the choroid you have the retinal pigment epithelium)
Then you have the neuroretina, which consists of:
• Outer Layer – photoreceptors (rods and cones)
• Middle Layer - bipolar cells
• Inner Layer – retinal ganglion cells
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.
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
• present in the central part of the eye
Cones • Less sensitive to light • Faster response to light • Responsible for daylight vision and colour vision (photopic vision) • 6 million cones
What is Scotopic vision?
night and peripheral vision
What is Photopic vision?
central and day-tine vision
Describe the distribution of rods and cones across the retina.
- rods are distributed across the retina but 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 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
How is yellow colour perception stimulated?
Yellow light has a wavelength between the peak sensitivity wavelengths of M-Cones and L-Cones. Yellow light stimulates both M-cones and L-cones equally.
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 -> red-green confusion
What is the term given to shifted peaks?
Anomalous trichomatism
What test is used to diagnose colour blindness?
Ishihara Test
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 dark adaptation
This suppression of light sensitivity is caused by photopigment bleaching and neuro-adaptation inhibiting rod and cone function
Define Ametropia
vision disorders characterized by the eyes inability to correctly focus the images of objects on the retina. Its forms include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism.
refractive error
What is the technical term for long-sightedness?
Hypermetropia/hyperopia
What is the technical term for short-sightedness?
Myopia
Give the layers of the eye that light photons have to pass through
tear film -> cornea -> aqueous humour -> lens -> vitreous humour -> retina
What is Emmetropia?
when you have an eye with a refractive power of 0
i.e. perfect focusing ability
What are the causes and symptoms of myopia?
Parallel rays converge at a focal point anterior to the retina
Causes:
- Excessive long globe (axial myopia)
- Excessive refractive power (refractive myopia) - highly curved cornea
Symptoms:
- Blurred distance vision
- Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
- Headache
How can myopia be corrected?
Concave lenses
What are the causes and symptoms of hyperopia?
Parallel rays converge at a focal point posterior to the retina
Causes:
- Excessive short globe (axial hyperopia)
- Insufficient refractive power (refractive hyperopia)
Symptoms:
- Visual acuity at near tends to blur relatively early
- Asthenopic symptoms: eyepain, headache in frontal region, burning sensation in eyes, blepharoconjunctivitis
- Amblyopia – uncorrected hyperopia > 5D
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)
What are the symptoms for Astigmatism?
Asthenopic symptoms ( headache , eyepain)
Blurred vision
Distortion of vision
Head tilting and turning
What is the treatment for astigmatism?
Regular astigmatism: cylinder lenses with or without spherical lenses(convex or concave), Sx
Irregular astigmatism: rigid CL , surgery
What is the term given to naturally occurring loss of accommodation with age?
Presbyopia
How is Presbyopia treated?
- Corrected by reading glasses (convex lenses) to increase refractive power of the eye
- Treatment: convex lenses in near vision
