Eye Flashcards
What is the purpose of the eyelids?
Protection of the eye
Blink reflex
Ensures front of the eye is lubricated with tear fluid
Where is tear fluid produced from?
Lacreal glands above the eye, the tear fluid then drains through canals
Describe the part of the eye we can see?
Sclera - white of the eye
Pupil - hole in the middle, controls the amount of light received by the eye
Iris - coloured portion of the eye
What is the conjunctiva?
Membrane that covers, protecting the eye. Tear fluid then sits at the front allowing outer surface protection
What is the purpose of the cornea?
Circular transparent window at the front of the eye
Ensures the refraction of light as it enters
What is the purpose of the anterior chamber?
The anterior chamber is the front part of the eye between the cornea and the iris and provides the aqueous humour.
What is the purpose of the aqueous humour?
It provides oxygen and nutrients to the transparent organs within the eye as well as maintaining the intraocular pressure which keeps the eye inflated.
Describe the production and supply of the aqueous humour.
To ensure constant intraocular pressure within the eye, turnover is required. It is produced from the ciliary body, it flows across the front of the lens, out through the pupil and the drains into the canal of Schlemm (circular production pathway).
Which muscle is the iris composed of?
Smooth muscle which aids pupil response to light.
What is the main purpose of the lens?
It is a transparent organ with its primary focus to fine focus the light on to the retina. The lens is held in place by suspensory ligaments which attaches it to the ciliary body which contains smooth muscle which aids fine focusing.
Where is vitreous humour found?
In the chamber of the eye and is responsible for absorbing energy, important for protecting the neural retina.
What are the two parts of the retina?
The retina is found at the back of the eye and is divided into two parts:
Neural retina which is light sensitive
Retinal pigment epithelium
When light enters the eye where is it focused on?
The fovea at the back of the eye due to having the highest visual acuity.
Where is the choroid found and what is its function?
It is found behind the retina. It is pigmented ensuring light absorption and highly vascular. Provides a blood supply for the outer layers of the retina.
What is the purpose of the optic disk?
This is the point where all the axons meet to form the optic nerve to transmit information to the brain. This is also the point that blood come in supplying the inner retina.
Where is the blind spot located?
At the optic disk as they are no light sensitive cells here.
What are the two types of smooth muscle controlling pupil size?
Circular smooth muscle (fibres run in a circular fashion) and radial smooth muscle
Explain in what circumstances circular muscle contraction occurs.
In response to light, circular smooth muscle contraction occurs stimulated by the parasympathetic nervous system. This causes pupil constriction - restricting the amount of light entering the pupil.
Explain in what circumstances radial smooth muscle contraction occurs.
In darkness, radial smooth muscle contraction occurs stimulated by the sympathetic nervous system. This results in the radial fibres becoming shorter leading to pupillary dilation.
What is the first point of focusing light?
The cornea which enables the greatest degree of refraction. To ensure the refraction occurs evenly the cornea surface must be very smooth - astigmatism results due to unsmooth cornea.
What occurs in laser corrective surgery?
The curvature of the cornea is altered which in turn then changes the plane of focus - how light is focused on to the retina.
Explain what is meant by the concept of accommodation.
Accommodation of the eye refers to the eye’s ability to adjust its focus to see objects clearly at different distances. This is achieved by changing the shape and curvature of the eye’s natural lens to bring images into sharp focus on the retina, remember unable to change the curvature of the cornea.
Explain the concept of fine focusing in relation to far vision.
For far vision the circular ciliary muscle is relaxed, the intraocular pressure pulls the lens taut (thin and flat) resulting in less curvature and therefore is able to focus parallel light coming from a distant point.
Explain the concept of fine focusing in relation to near vision.
For near vision, more curvature is required so the circular ciliary muscle contracts, the suspensory ligaments become slack which enables the lens to become more rounded more refraction to ensure light is focused from a close source. This is mediated under the control of the parasympathetic nervous system which innovates the ciliary muscle of contraction.
Is accommodation mediated by the sympathetic or parasympathetic nervous system?
Parasympathetic nervous system innovates ciliary muscle contraction for fine focusing on near vision. Whilst the sympathetic nervous system activation does enable some ciliary muscle relaxation enabling fine focusing on far vision this is mainly mediated by the switching off of the parasympathetic nervous system.
How is light reflected on to the fovea?
It presents as upside down and back to front which is then sorted by the brain.
What occurs in the condition presbyopia?
The lens becomes less flexible with age, can’t focus on near vision as the lens is unable to become as rounded as it once was.
What causes the condition myopia?
Myopia also known as short-sightedness occurs as a result of the eyeball being too long so when light becomes in from a distant object it is focused in front of the retina meaning it becomes blurred.
Can see close images but not far away.
What causes the condition hyperopia?
Hyperopia is when the eyeballs are too short, unable to see near objects and as light is reflected behind the retina.
Can’t see close images but able to see far away.
What is the purpose of the retina?
It is the light sensitive part of the eye which is responsible for visual transduction, converting light energy to electrical signals within neurons.
What is the purpose of photoreceptors?
They are responsible for converting light energy to electrical signals.
Describe the two different types of photoreceptors.
Rods which are responsible for black and white (grayscale vision) and cones which are responsible for colour.
Describe the visual pathway that occurs within the retina.
When light hits the back of the retina photoreceptors both cones and rods are responsible for converting light energy into an electrical synapse. These then synapse with bipolar cells which then synapse with ganglion cells and it is the extended axons of these ganglion cells which then form the fibres of the optic nerve and transmit the electrical signals to the brain.
Light enters at the front of the retina and travels to the back which is the opposite direction of visual processing which starts at the back of the retina.
Which wavelengths can photoreceptors respond to?
Between 400-700 nM
Which other cells are also found in the retina?
Horizontal cells and amacrine cell which are across the retina and are involved with lateral processing.
What are the three sections of photoreceptors.
Each contain a outer segment, an inner segment and a synapse.
Describe the structure and the function of outer segment of photoreceptors.
Rod photoreceptors have a rod shaped outer segment and cone photoreceptors have an cone shaped outer segment. They consist of flattened, stacked membranous discs. We are unable to replace our photoreceptors and therefore constant turnover is undertaken by the cells of the retinal pigment epithelium which phagocytose discs at one end and new ones form at the other to prevent accumulated damage. These discs contain light absorbing photopigment.
What is present in the inner segment of photoreceptors?
Contains the nucleus, mitochondria, ribosomes
What occurs at the synapse?
Neurotransmitter is released, photoreceptors synapse with bipolar cells.
How many rods and cones do we have in our retina?
In the retina there are approximately 100 million rods and 6 million cones
Describe the difference in sensitivity between rods and cones.
Cones have a low sensitivity to light (therefore need bright light for colour vision) therefore day vision is dominated by cones; rods have a high sensitivity to light (required for night vision) night vision is dominated by cones.
Describe the difference in location between rods and cones.
Cones are most abundant in the fovea (colour vision); rods are most abundant in the periphery (black and white vision) this is reflected in our vision - colour in our central vision, black and white in our peripheral vision.
Describe the difference in the degree of convergence between rods and cones.
Cones in the fovea have little or no convergence – 1:1 ratio of cone to ganglion cell; whereas in the periphery rods are highly convergent – many rods
(up to 100) feed into one ganglion cell
What is the result of differences in convergence between cones and rods?
In the central region within the fovea due to the absence of convergence of cones, this provides a high resolution image as the electrical signalling of one photoreceptor then transmits to one bipolar cell and one ganglion cell. But in the periphery hundreds of rods can feed into one ganglion cell meaning there is lower visual acuity (peripheral vision is more blurred) but has a higher sensitivity as there is a wider receptive field.
Define visual acuity.
Visual acuity (VA) is a measure of the ability of the eye to distinguish shapes and the details of objects at a given distance
Where is visual acuity the highest and why?
Visual acuity in the fovea is highest in this area giving fine
resolution of the image. This is due to:
– More cones – 1:1 coupling
– Lateral inhibition so when one photoreceptor is stimulated it switch the next one off allowing the distinguishment of edges
– Other areas of the retinal are moved aside here, so light doesn’t travel through other layers of the retina
What is the peripheral vision most sensitive to?
Movement
Flashes of light
Which part of the photoreceptor is responsible for signal transduction?
Photoreceptors contain photopigments in the outer segment membranes, which are able to capture the light which is the first step in signal transduction. The photopigment consists of a protein known as opsin and retinal which is a chromophore which is the molecule that actually absorbs the light.
What type of protein is opsin?
A GPCR with the ligand being retinal which is derived from Vitamin A with deficiency of Vitamin A leading to night blindness.
What are the four types of photopigment?
There are 4 photopigments – one in rods (rhodopsin, which is sensitive to all wavelengths of light) and 3 in cones, sensitive to red, green and blue light
How does visual transduction begin?
It starts with the absorption of light from the retinal. This induces a shape change within retinal from 11-cis-retinal with a bend to all-trans-retinal. This is the ligand for opsonin so this change in retinal shape induces a conformation change within the GPCR causing a signalling cascade.
Describe the signalling cascade for photoreceptors in the dark.
The outer segment of the photoreceptor contains a cGMP-gated cation channel. In the dark cGMP levels are high due to the action of guanylate cyclase converting GTP to cGMP. When cGMP binds this opens the cation channel, allowing sodium influx causing depolarisation to around -40mV.
This then induces the opening of the voltage gated calcium channels within the synaptic terminal enabling the influx of calcium into the cell and then calcium mediated exocytosis of neurotransmitters.
Therefore in the dark, the photoreceptor is depolarized and is releasing neurotransmitter.
Describe the signalling cascade for photoreceptors in the light.
Light comes in and is absorbed by retinal causing a change in shape from 11-cis-retinal to all-trans-retinal this then induces a conformation change within the GPCR activating its G-protein Transducin. This then activates phosphodiesterase which in turn degrades cGMP causing a close of the cGMP-cation channel. This prevents sodium influx causing a hyperpolarisation of the membrane potential, calcium channels are closed, decreasing neurotransmitter release.
Therefore in the light, the photoreceptor is hyperpolarized and neurotransmitter release decreases.
What is the neurotransmitter released in response to phototransduction?
Glutamate
How are we able to detect different responses to light?
The brighter the light the less neurotransmitter is released and hence this response is graded accordingly.
How does signal termination occur regarding photoreceptors?
Once activated the photoreceptors become bleached and they remain unresponsive until the recycling of retinal has occurred. This then terminates the signal and is mediated by enzymes converting all-trans retinal back into 11-cis retinal.
Does depolarisation/hyperpolarisation of retinal cells mimic that of photoreceptors?
No whilst in response to light this causes hyperpolarisation in photoreceptors, in retinal ganglion cells they become activated causing transmission of information to the brain. Whereas in the dark although photoreceptors are depolarised, there is hyperpolarisation in the retinal cells inhibiting neurotransmitter release.
What are the three types of opsonins?
Blue, red and green that are all sensitive to different wavelengths of light including their colour which crucially enables us to see a range of colour - for example seeing green will stimulate all three cones. Seeing yellow light for example stimulates both green and red cones.
The colours relate to the photopigment which they contain.
How does colour blindness occur?
When we are lacking one of the cones and hence this then changes our perception of light.
Describe the visual pathways.
Essentially a cross over with both eyes. Light from the right visual field goes to the left side of the eye and light from the left visual field goes to the right side of the eye.
Left side of the retina in both eyes goes to the left side of the brain. Right side of the retina in both eyes goes to the right side of the brain.
Retinal ganglion cells travel by the optic nerve to the thalamus called the lateral geniculate nucleus which is where synapse occurs, then travels to the occipital lobe where processing occurs.
What are the drug classes used to treat glaucoma?
- Prostaglandin analogues
– β-blockers
– Carbonic anhydrase inhibitors
– α2
-agonists
– Muscarinic agonists
What is the main aim of treatment for glaucoma?
All drugs used in the treatment of glaucoma work by decreasing the intraocular pressure hence slowing the progression of the disease and prevent the retinal ganglion cells from dying and to decrease the loss of visual field.
How is the ciliary body attached to the lens?
By the lens zonules
What secretes the aqueous humour?
Ciliary processors into the posterior chamber between the iris and the lens and then flows out through the pupil by the pupillary margin.
What is the purpose of the aqueous humour?
Aqueous humour supplies nutrients to and removes waste products from the avascular organs of the anterior eye (lens and cornea)
It is produced at a rate of 2-3µl / min
How does the aqueous humour leave?
70-90% leaves by the conventional pathway which is via the trabecular mesh work and into the canal of schlemm.
10-30% leaves by the unconventional pathway in which the aqueous humour flows out via the intercellular spaces between the ciliary muscle fibres, the choroid and out via the sclera.
This component may be greater for patients with glaucoma.
What happens in primary intraocular glaucoma regarding the aqueous humour?
Usually the rate of production of aqueous humour matches the rate of outflow, however in primary intraocular glaucoma there is decreased outflow of aqueous humour and so there is increased intraocular pressure.
How can drugs used in the treatment of glaucoma reduce intraocular pressure?
Either by increasing outflow or reducing production of aqueous humour.
Describe the ciliary epithelia.
The ciliary epithelia consists of two layers of epithelial cells, one of which is pigmented and the other not. The ciliary processors are protrusions of the pigmented layer of the ciliary epithelia, increasing the surface area. Th pigmented layer contains melanin granules and the cells contain gap and tight junctions allowing cross talk and substance control between the two layers.
What are the main ions that move across the ciliary epithelia from the blood into the aqueous humour?
Sodium
Chloride
Bicarbonate
Water moves through aquaporins in the membrane
This results in the constant production of aqueous humour which is an active process.
Describe the ion channels present for aqueous humour secretion.
Na+/K+ ATPase is involved present, sodium ions and moving outwards and potassium move inwards.
Na+ movement is balanced by the movement of anions. Carbon dioxide and water is transferred to make bicarbonate together with hydrogen ions by the action of carbonic anhydrase. These ions are then efflux from the cell.
B2 adrenergic receptor is present, upon noradrenaline binding this stimulates the sodium/potassium ATPase - this will then increase the production of aqueous humour.
Describe the pharmacological control of aqueous humour secretion.
When stimulated by Noradrenaline on the B2 adrenergic receptors this causes an increase in aqueous humour secretion - this goes on to increase the activity of the sodium/potassium ATPase hence increasing the production of aqueous humour.
However stimulation of the a2 adrenergic receptors present on the pre-synaptic terminal then reduce cAMP activity, resulting in the closure of calcium ion channels and therefore calcium mediated exocytosis is reduced.
Inhibition of carbonic anhydrase then results in a decrease in aqueous humour production.
How do beta blockers work in decreasing the production of aqueous humour?
Antagonise B2 adrenergic receptors therefore causing a reduction in aqueous humour production as the sodium/potassium ATPase activity is then reduced.
What are the examples of B2 adrenergic drugs used in the treatment of glaucoma?
Timolol (non-selective β-blocker, inhibiting B1 and 2)
Carteolol (non-selective β-blocker; also has
intrinsic sympathomimetic activity therefore reducing side effects)
Betaxolol (more selective for β1
receptors, less efficient)
What are some of the side effects associated with beta blockers?
Ocular – include stinging, dry eye, itching, pain, erythema, corneal disorders although may be due to the preservatives
– Administered topically, but can cause systemic side effects
* Include bradycardia and breathlessness
* Contraindicated in bradycardia and heart block
Which a2 agonists are licensed for glaucoma?
Reduce the amount of noradrenaline released:
Brimonidine
Apraclonidine
What are some of the carbonic anhydrase inhibitors licensed for glaucoma?
Acetazolamide (not topical)
Dorzolamide
Brinzolamide
How does pupil size affect aqueous outflow?
Aqueous outflow via the trabecular meshwork is:
– Facilitated by constriction of the pupil, as the iris muscle will be stretched out keeping the angle open
– Inhibited by dilation of the pupil
– Facilitated by contraction of the ciliary muscle, circular muscle moves inwards pulling the area more taut, which helps with the flow of aqueous humour.
Describe how the iris and the ciliary muscle is innervated.
The parasympathetic nervous system stimulates the the circular muscle, this is by acetylcholine acting on muscarinic receptors causing pupillary constriction.
Whereas the sympathetic nervous system stimulates the radial muscle to contract, causing pupil dilation and this is mediated by noradrenaline acting on a1 adrenergic receptors.
The ciliary smooth muscle is innervated by parasympathetic nervous system causing the release of acetylcholine and contraction via muscarinic M3 receptors.
How do muscarinic agonists work?
Pilocarpine, the only licensed muscarinic agonist works by acting on the M3 receptors and is a parasympathomimetic causing contraction of the circular smooth muscle resulting in pupil constriction. This pulls the area more taut resulting in the facilitation of the flow of aqueous humour.
However there are numerous side effects when using muscarinic agonists – mainly ocular eg. Blurred vision in addition to systemic side effects. Used infrequently.
How do prostaglandin analogues work?
Analogues of PGF2α; act at PGF receptors (exception bimatoprost)
Decreases intraocular pressure by mainly by increasing (>2X) aqueous outflow via the uveoscleral pathway
Most recently introduced therapy for POAG –usually the initial therapy in POAG and currently the most frequently used drug to lower IOP
Increase extracellular matrix degradation within the sclera therefore increasing the outflow of aqueous humour through the intercellular pathways in order to decrease intraocular pressure.
What are some examples of prostaglandin analogues used in the treatment of glaucoma?
Latanoprost
Travoprost
Bimatoprost
These are usually the initial therapy for glaucoma
What are some of the side effects of prostaglandin analogues?
- Conjunctival hyperaemia (bloodshot eyes), foreign body sensation, ocular irritation
These are due to the drugs themselves and not the excipients - Eyelash lengthening, thickening, hyperpigmentation
- Iris hyperpigmentation (11 - 23% of patients) (most common with green-brown irises)
