Retinal Physiology Flashcards
What is the blood supply to the inner retina (ganglion and bipolar cells)?
Branches from the central ophthalmic artery
Why does the retina have a dual blood supply?
To cope with the demands of the photoreceptors for oxygen
What is the blood supply to the photoreceptors?
Supplied by the choroid, a network of capillaries that are supplied by the ciliary arteries (a branch of the ophthalmic artery) and penetrate the sclera at the back of the eye
Which blood vessels are visible when looking through an ophthalmoscope in the eye?
Branches of the central ophthalmic artery supplying the ganglion and bipolar cells
Describe the secretion and drainage of tears produced by the lacrimal glands
Enter the eye via the superolateral excretory ducts, exit the eye medially via the lacrimal punctum and drain into the nasolacrimal duct
What is the composition of tears?
Contain mucus, antibodies and lysozyme
What does the outer layer of the eye contain?
Sclera at the back and cornea at the front
What does the middle layer of the eye contain?
Iris, ciliary body and choroid
What does the inner layer of the eye contain?
The retina
Describe the cornea
Clear, bulging surface at the front of the eye, transparent and uniformly thick, nearly avascular (nutrition derived from aqueous humour), rich nerve supply, kept spherical by intraocular pressure, tears maintain oxygen exchange and water content
Describe the sclera
‘White’ of the eye, exterior surface is smooth and white, interior surface is brown and grooved, is continuous with the sheath of the optic nerve and the tendons attached to it
Describe the iris
Heavily pigmented, it is a sphincter muscle which works to constrict or dilate the pupil - iris is what displays eye colour and is dependent on melanin
Describe the aqueous humour
Present in the anterior chamber of the eye and is a watery fluid formed from the protein-free filtrate of produced in the posterior chamber by the ciliary body. After production by ciliary body it circulates over the anterior surface of the lens and into the anterior chamber
Where does the aqueous humour drain?
Into the canal of Schlemm
What is the vitreous humour?
Watery gel present in the posterior chamber of the eye
What is intraocular pressure?
Difference between the formation and drainage of aqueous humour
What is the cause of glaucoma?
Occurs if drainage of aqueous humour is blocked, causing the intraocular pressure to rise, damaging the retina
What are the two types of glaucoma?
Open angle glaucoma (chronic simple) or primary angle closure glaucoma (closed angle/acute glaucoma)
Describe open angle glaucoma
Slowly progressive condition, which occurs when the trabecular meshwork(around canal of Schlemm) gradually becomes blocked, preventing drainage of aqueous humour into the canal of Schlemm. The angle between the cornea and iris is normal (about 40 degrees)
Describe closed angle glaucoma
Occurs when the angle between the cornea and iris is reduced, meaning that the aqueous humour cannot pass through the canal of Schlemm at all, causing a rapid rise in intraocular pressure –> extreme pain and sudden vision loss
How may prostaglandin analogues be used in the treatment of glaucoma?
Increase outflow of aqueous humour through the canal of Schlemm
How may beta-adrenergic receptor antagonists be used in the treatment of glaucoma?
Decrease aqueous humour production
How may alpha 2 adrenergic receptor agonists be used in the treatment of glaucoma?
Decreases aqueous humour production and increases outflow
How may parasympathomimetic agonists and anticholinesterases be used in the treatment of glaucoma?
Contract the ciliary muscle, opening the canal of Schlemm
How may carbonic anhydrase inhibitors be used in the treatment of glaucoma?
Lessens aqueous humour secretion by inhibiting carbonic anhydrase in the ciliary body
How may surgery be used in the treatment of glaucoma?
An incision is made in the sclera at the cornea-sclera junction to allow the aqueous fluid to drain out onto the surface of the eye in order to lower the pressure
How does the aqueous humour usually prevent damage to the eye from UV light?
Ordinarily, the aqueous humour prevents damage from UV light by containing antioxidants, with a slightly alkaline pH which activates the vitamin C (antioxidant)
Describe the cause of cataracts
Biological tissue is easily damaged by UV light as the light catalyses the formation of free radicals which can attack the lens in particular as the light is focused within the lens. Therefore, if the level of antioxidants is reduced, or there is too much exposure to UV light –> the lens can gradually become opaque and the eye is unable to see, and this is called a cataract
What is the treatment for cataracts?
Surgical replacement of lens
Why does the eye require very high amounts of oxygen?
Photoreceptors are highly metabolically active as they have constant sodium pump activity, so the photoreceptors require a lot of oxygen in order to facilitate respiration to produce the ATP to power the pump
Describe optic metabolism in the dark (no light exposure)
In the dark there is a constant inward leak of sodium in the outermost part of the receptor and this keeps the cell depolarised and tonically releasing glutamate from it’s synaptic ending (informs the brain that there is no light present)
Describe optic metabolism on exposure to light
On exposure to light, the light hyperpolarises the cell and stops the tonic glutamate release; caused by sodium and potassium efflux from the photoreceptor
Why are photoreceptors known as ‘dark receptors’?
Because they tonically respond to the presence of darkness, and are interrupted by the presence of light
What happens to old photoreceptor membrane?
Photoreceptors are constantly growing new photoreceptor membrane at the cell body (gradually transported outwards) and the old membrane is shed at the distal cell tip and phagocytosed by pigment epithelial cells and transported into choroid capillaries
What is rhodopsin?
G-protein coupled receptor
Describe the role of rhodopsin in vision
On light exposure, there is a change in rhodopsin (GPCR) on the photoreceptor membrane which causes the formation of intracellular transducin which lowers the level of cyclic GMP in the cytoplasm –> decrease in cGMP closes sodium channels in outer segment and prevents sodium influx –> prevention of sodium influx causes the cell to hyperpolarise and stop producing glutamate (light is detected by the brain)
What is the role of transducin in vision?
It lowers the level of cycling GMP in the cytoplasm of photoreceptors and this decrease in cytoplasmic GMP closes the sodium channels and prevents sodium influx –> hyperpolarises the cell –> prevents glutamate release
Which type of photoreceptors distinguish between different colours?
Cone cells
Explain how we distinguish between different colours
The presence of the ‘red’, ‘blue’ and ‘green’ cone cell populations in the foveal area of the retina have slightly different sensitivities to different wavelengths with red cells detecting the longest wavelengths, and blue the shortest
Where are the cells located that distinguish between different colours?
Fovea of the eye
What does colour perception depend upon?
Depends on the difference in signals between two or more different cone populations that are simultaneously active
What is colour blindness?
Loss or reduction in the number of one of the cone cell populations at the fovea which means that they confuse wavelength and intensity at points in the visual spectrum where normally sighted people can discriminate on the basis of wavelength alone
Describe the passage of light in the eye
Light passes through cornea into anterior chamber of the eye (contains aqueous humour) –> light then passes through the pupil, the lens and then into the posterior chamber (filled with vitreous humour)
What does normal visual acuity largely depend upon?
The fovea; where the cone cells of visual perception are most dense
What are the two types of photoreceptor?
Cone cells and rod cells
Describe cone cells
Mediate the ability to see detail and are active in daylight, there are red, green and blue cells and all of the cells in the fovea are cone cells
What type of cell is present in the fovea of the eye?
Cone cells
What part of photoreceptors are sensitive to light?
Outer segment
Which part of photoreceptors contains the cell body and synaptic terminal?
Inner segment
Why do photoreceptors have a stacking arrangement (folding) of the outer cell membrane?
Means that light has to pass through all of the plates in sequence, maximising the chance of a photon interacting with a molecule of photopigment.
Why do cone cells have a shorter outer segment than rod cells?
There is enough light to guarantee a photo will react with photopigment
Describe rod cells
There is only one type of rod cell, and they are very sensitive to light, so are active in the dark, so there is no colour differentiation. These cells are suppressed in daylight.
What does the outer membrane of rod cells consist of?
Discs of membrane which contain the photopigment rhodopsin
Describe the transmission from photoreceptor to the optic nerve
Interneurones in the retina (horizontal cells) converts continuous release of glutamate (and inhibition due to light) into a varying frequency of action potentials in the optic nerve axons: photoreceptors –> horizontal cells (interneurons) –> bipolar cells –> ganglion cells –> optic nerve –> visual cortex (only ganglion cell isn’t an interneurone, rest are)
