Anatomy: Eyeball Flashcards
How many layers does the eyeball have?
3
What is the outer layer or the eyeball comprised of?
Sclera (white part) which continues anteriorly as the cornea
Is the cornea transparent?
Yes
Describe the outer layer
Tough and fibrous
Provides attachment for extra ocular muscles
What does the middle layer comprise of?
Choroid - vascular layer
Ciliary body
Iris
What does the ciliary body connect?
The choroid with the iris
What is the inner layer comprised of?
Retina
What are the 2 chambers of the eyeball
Anterior chamber
Posterior chamber
The anterior chamber is the space between…
Cornea and iris
Posterior chamber is the space between…
Iris and ciliary body and the lens
What structure secretes the aqueous humour that fills the chambers of the eye?
Ciliary processes within the ciliary body
What is the outer layer of the eyeball continuous with?
Dura mater covering optic nerve
Describe the iris
A thin contractile diaphragm with a central aperture for transmitting light
The central aperture of the iris is also called…
The pupil
What two muscles control the size of the pupil?
Sphincter and dilator pupillae
Is the lens anterior or posterior to the iris?
Posterior
Is the lens biconcave or biconvex?
Biconvex
The lens is enclosed in a capsule and attached to the ciliary body by…
The suspensory ligaments
The cavity behind the lens is filled with what type of humour?
Vitreous
Describe the vitreous humour
A transparent jelly like substance that supports the lens and holds the retina in place
What route does the aqueous humour take?
Flows from posterior chamber, through pupil into the anterior chamber. Drains through the iridocorneal angle. Then via trabecular meshwork into the canal of Schlemm
What structures does the aqueous humour nourish?
Lens and cornea - they are avascular (need to be transparent to let light pass through)
What does the canal of Schlemm drain into?
Venous circulation
The retina consists of what 2 layers?
Neural layer (photosensitive) Pigmented layer
Is the pigmented layer or photoreceptive layer closer to the choroid?
Pigmented layer
What is the role of the pigmented layer?
Contains melanin
Absorbs scattered light to help focus images
What does the photosensitive layer of the retina generate in response to light?
Action potentials
The photosensitive layer is made up of photoreceptors called…
Rods and cones
What are some characteristics of rod photoreceptors?
Responsible for vision in low intensity light
Do not discern colour
More abundant in peripheral retina
What are some characteristics of cone photoreceptors?
Responsible to high visual acuity and colour vision
Concentrated in the macula
The centre of the macula (fovea) is only cones
What is the macula?
Dark area lateral to the optic disc
Centre of vision
Centre of macula contains fovea = only rod cells
Where is the area of most acute vision?
Fovea centralis (depression in centre of macula)
What is the 3 main roles of the cornea?
Maintain transparency
Ocular protection - including corneal reflex
Refraction of incoming light (along with tear film)
What are the 5 layers of the cornea?
Epithelium Bowman’s membrane Stroma Descemet’s membrane Endothelium
What type of cells make up the epithelial layer or the cornea?
Non keratinised stratified squamous cells (approx 5-7 layers thick)
When epithelial cells of cornea are damaged, what happens?
Cells migrate from the basal layer to surface (increase in the mitotic activity)
Where are corneal stem cells located?
At the periphery of the cornea (limbus)
Central corneal wounds heal slower that more peripheral
In the cornea, below what level will result in corneal scarring if trauma occurs?
Below Bowman’s layer (only the epithelium is regenerated)
What layer forms the bulk of the cornea?
The stroma layer - connective tissue (regularly organised collagen fibres)
What does Descemet’s membrane act as in the cornea?
The basement membrane to the endothelium
What is the main role of the corneal endothelium?
Maintain hydration in order for cornea to stay transparent
What type of cells make up the endothelial layer of cornea?
Simple squamous - does not regenerate
Number of cells fall as get older - less than 800 cells/m2 leads to corneal oedema and poor vision
Cellular loss accelerated by trauma, surgery and exposure to UV radiation
What is responsible for maintaining an adequate pressure in the eye?
Aqueous humour
Intra ocular pressure is a balance between…
Rate of production and drainage of aqueous humour
The majority of aqueous humour drains via canal of Schlemm, but 10-20% drains via a different route. What is this?
Uveoscleral route
Passes into the root of the iris and or ciliary muscle before draining into the scleral vascular system
How is intra ocular pressure measured?
Measuring the force needed to flatten the corneal surface using a tonometer - greater force needed, the higher the pressure
What is the normal range for intra ocular pressure?
11-21mmHg
Above what IOP is considered to be ocular hypertension?
Above 21
Autonomic control of IOP is via adrenergic or cholinergic mechanisms?
Adrenergic
What receptor stimulation reduces IOP by reducing aqueous production (and may increase uveoscleral outflow)?
Alpha 2 receptors
What receptor stimulation increases IOP by increasing aqueous production?
Beta 2 receptors
What is the role of the lens?
Refract light
The lens can vary the amount it refracts light by changing its shape = accommodation
Is increased accommodation required for near or far objects?
Near - light rays are more divergent so require greater refraction to bring them into focus on retina
Light refracts as it passes through a number of structures and ‘fluids’. What are they?
From air to liquid tear film
Through cornea
Through lens
Where does most light refraction occur?
At air- cornea interface
During accommodation reflex, what happens?
Eyes converge - image brought into focus on same point of retina in both eyes Pupils constrict Lens bicovex(fatter)
To make the lens more biconvex, what happens?
Ciliary body muscle contracts and move inwards - release tension on suspensory ligaments
As we age the lens becomes stiffer, so degree to which it can accommodate becomes less. What is this called?
Presbyopia - age related inability to focus near objects
Most notable from 40-45 onwards
What type of lenses can be prescribed for presbyopia?
Convex lenses
When measuring ocular pressure, there is a correction factor. What is it?
The corneal thickness
Does having a high IOP equate to a diagnosis of glaucoma?
Not necessarily
A high IOP is a risk factor for what?
Glaucoma
Beta blockers can be used to reduce IOP. What are some examples of these?
Timolol, carteolol
Reduce aqueous production
Alpha agonists can be used to reduce IOP. What are some examples?
Apraclonidine, brimonidine
Reduce production and small increase in drainage
How do prostaglandin analogues work in terms of IOP?
Increase uveoscleral outflow
How do carbonic anhydrase inhibitors work with regards to IOP?
Reduce production of aqueous
How do parasympathomimetics work with regards to IOP?
Increase outflow of aqueous by ciliary muscle contraction opening trabecular meshwork
What is another name for the suspensory ligaments?
Zonules
Do we require increased accommodation for near or far tasks?
Near
Describe the lens capsule
A thickened basement membrane that envelopes the lens.
Responsible for changing lens shape during accommodation
Does the lens continue to grow throughout life?
Yes
Lens covered in epithelium - undergo mitosis and form lens fibres
Newest layers on surface and older progressively deeper, so outer lens soft, whereas centre is hard
What is a cataract?
Any opacity (loss of transparency) of the lens
What makes up the uveal tract?
Iris
Ciliary body
Choroid
The iris attaches to ciliary body at the …angle
Iridocorneal angle
What does the iris control?
Amount of light that enters the eye by varying the size of the aperture (pupil)
In conditions of low light or sympathetic activation the pupil dilates. This is called..
Mydriasis
Contraction of dilator pupillae pulls the pupil outwards to enlarge pupil diameter
In conditions of bright light the pupil constricts. This is called…
Miosis
Sphincter pupillae is responsible for constriction - stimulated by parasympathetic activation (fibres carried in inferior division of CN 3)
Does the sympathetic or parasympathetic NS innervate the sphincter pupillae?
Parasympathetic
Pre and post ganglionic ACh (so antimuscarinics/ muscarinics will act here)
The dilator pupillae is under sympathetic control. NA activates what receptor on the muscle?
Alpha 1 receptor
Tropicamide is an antimuscarinic, what effect will it have on the pupil?
Mydriasis
Phenylephine is a sympathomimetic . What effect will it have on the pupil?
Mydriasis
Pilocarpine is a muscarinic antagonist. What effect will it have on the pupil?
Miosis
With age does miosis or mydriasis occur to pupil?
Miosis - dilator pupillae becomes atrophic and sphincter more fibrotic
What are the primary roles of the ciliary body?
Accommodation
Aqueous humour production
Attachment for lens suspensory ligaments
When the ciliary muscle contracts, what happens to the lens?
When muscle contracts it moves inwards, so less tension on suspensory ligaments and increasing the power of accommodation (lens more spherical)
The choroid extends from the ciliary body to the…
Optic nerve
What is the function of the choroid?
Allows nerves and vessels to reach anterior eye by passing through it
Removes waste products from outer retina
Supplies nutrients to outer half retina
Absorbs any light passing through retina
What is the name of the basement membrane that attaches choroid to retina?
Bruch’s membrane
What supplies the outer half of the retina?
Rich capillary bed in the choroid - choriocapillaris
The vitreous cavity fills what fraction of the volume of the eye?
2/3
What is the vitreous attached to?
Posterior lens capsule and retina
What percentage of the vitreous is water?
99%
And it is 3x more viscous than water
During what surgery can the anterior vitreous membrane be disrupted?
Cataract surgery (posterior lens capsule disruption -> anterior membrane of vitreous disruption)
Describe vitreous detachment
With age fluid fills between potential space between retina and vitreous - detachment where it is weakly bound
What can a vitreous detachment predispose to?
Retinal detachment
What are the two layers of the retina?
The inner neural layer and the outer retinal pigment epithelium (RPE)
What is the potential space called between the neural layer and RPE?
Subretinal space
When does the neural layer and RPE fuse?
Early foetal life
The inner 2/3 of the retina is supplied by what?
Central retinal artery
How many branches does the central retinal artery divide into?
4 and each supplies a segment of the retina
Superior nasal and superior temporal
Inferior nasal and inferior temporal
They are end arteries
Retinal arteries overly…
Retina veins
In certain situations e.g HTN characteristic changes can be seen where the vessels cross
Which branches of the central retinal artery arch over the posterior pole?
Superior and inferior temporal retinal arteries
Does the macula have a dense capillary network?
Yes BUT the fovea is absent from capillaries - dependent on underlying choriocapillaris
What is the posterior pole?
Usually refers to the retina between the optic disc and the macula
Is the macula located temporal or nasal to the optic disc?
Temporal
And bounded by the temporal superior and inferior vascular arcades
The macula accounts for what percentage of the visual field?
Almost 10%
