eye and retina physiology Flashcards
what is visible light?
electromagnetic radiation between the wavelengths of about 400 nanometers (blue) and 750 nm (red)
what length of EM radiation can we absorb?
shorter wavelengths
what is colour blindness?
inability to detect some wavelengths
how many people are affected by colour blindness in the UK?
affects 8% of UK men and 0.5% of UK women
what is the lacrimal apparatus?
lacrimal glands and associated ducts
what do lacrimal glands do?
secrete tears which provide an optically smooth refracting surface
what is the function of tears?
provide an optically smooth refracting surface
contain antibodies and lysozymes to prevent bacterial growth
what is lacrimation?
secretion of tears
what stimulates lacrimation?
stimulated by parasympathetic stimulation via the facial nerve
describe the steps of lacrimation stimulation
o Preganglionic parasympathetic efferents travel in the facial nerve (VII)
o Branch off in the greater petrosal nerve which goes to the pterygopalatine ganglion (PPG).
o Post ganglionic fibres from the PPG innervate the lacrimal glands and also stimulate nasal secretions over the olfactory mucosa
what are the layers of the eye and what do they contain?
outer - sclera, surrounds eyeball. continuous with the dura mater around optic nerve
middle - blood vessels, connective tissue, includes the iris, ciliary body and the choroid
inner - retina
what does the sclera become on the front of the eye?
the cornea
what is the sclera visible as?
the white of the eye
describe the structure of the sclera
continuous with the dura mater around the optic nerve
exterior is smooth and white
interior is brown and grooved
tendons attach to it
describe the properties of the sclera
tendons attach to it
flexibility adds strength
what controls the size of the pupil?
sphincter muscles
what is the pupil? what is its average size?
the hole through which light passes – diameter between 3-7mm
what does eye colour depend on?
the amount and distribution of the pigment melanin
what is the cornea?
the clear bulging surface in the front of the eye
the main refractive surface on the eye
what is the index of refraction of the cornea?
1.37
describe the structure of the cornea
o Normally transparent, uniformly thick
o Nearly avascular
o Richly supplied with sensory unmyelinated nerve fibres from trigeminal nerve
o Sensitive to foreign bodies, cold air and chemical irritation
where does the cornea get its nutrition from?
the aqueous humour
how do tears help the cornea?
maintain oxygen exchange and water content – prevent scattering and improve optical quality
what is the conjunctiva?
Layer of stratified columnar epithelium, goblet cells and capillaries that covers the sclera and the inside of the eyelids
what are the layers of the conjunctiva?
- Palpebral conjunctiva – on the inner eyelid
* Bulbar conjunctiva – on the eyeball
what is the function of goblet cells in the conjunctiva?
Layer of stratified columnar epithelium, goblet cells and capillaries that covers the sclera and the inside of the eyelids
where does light go after entering the cornea?
anterior chamber –> pupil –> lens –> posterior chamber
what is the anterior chamber filled with?
aqueous humour
what is the posterior chamber filled with?
vitreous humour
what produces the vitreous humour?
glial cells in the retina
what is the function of the vitreous humour?
o Holds the shape of the eye to make sure its constant – maintains focusing accuracy
what is the aqueous humour?
like CSF – a nearly protein free filtrate of blood
what makes the aqueous humour?
the ciliary body
describe the passage of the aqueous humour in the eye
• Formed by the ciliary body –> circulates behind the iris –> through the pupil –> into the anterior chamber –> drains into the canal of Schlemm
what is the canal of Schlemm?
a venous sinus
what keeps the cornea as a sphere?
intraocular pressure
what is the normal intraocular pressure of the cornea?
between 10 and 21 mmHg
where is visual acuity highest?
fovea
what is the fovea?
a small central point composed of closely packed cones (centre of the macula)
what is the blind spot?
where visual axons leave the eye to form the optic nerve. Slightly lateral to fovea - no photoreceptors here
what produces the intraocular pressure?
the difference between the formation and drainage of aqueous humour
how can you measure the intraocular pressure?
by pressing on the cornea and measuring the indentation
what happens if aqueous humour drainage is blocked?
pressure in the anterior chamber rises
Increases pressure on the vitreous humour –> presses on the retina
Prolonged pressure damages the retina and causes glaucoma
what are the 2 main sub-groups of glaucoma?
open angle glaucoma (chronic simple)
closed angle glaucoma (acute)
what is open angle glaucoma? what is the angle between the cornea and the iris?
slowly progressive condition which occurs when Sclemm’s canal gradually becomes blocked.
Angle between the cornea and the iris is open – normal (abt 40 degrees)
what is closed angle glaucoma? what is the angle between the cornea and the iris?
occurs when the iris is pushed forward. Angle between cornea and iris is reduced (may decrease to near 0). Blocks canal of Schlemm and causes a rapid rise in pressure inside the eye –> extreme pain and vision loss
how can glaucoma be treated?
surgery
drugs
describe how the surgery for glaucoma works?
incision is made in the sclera at the cornea-sclera junction
o Allows aqueous fluid to drain out onto the surface of the eye –> lowers pressure
what is the aim of drug treatments for glaucoma?
to reduce intraocular pressure by reducing aqueous humour formation or increasing drainage
how are drugs for glaucoma applied?
topically to the eye
what do prostaglandin analogs do?
increase outflow of aqueous humour through Schlemm’s canal
what do • Beta-Adrenergic Receptor Antagonists do?
decrease aqueous humour production
what do Alpha 2 Adrenergic Agonists do?
decreasing aqueous production, increasing outflow
what do Parasympathomimetic Agonists do?
anticholinesterases, contraction of ciliary muscle (opens canal)
what do carbonic anhydrase inhibitors do?
inhibit carbonic anhydrase in ciliary body: lower secretion of humour
why is surgery the preferred treatment for glaucoma?
Decreasing production of aqueous humour can have adverse effects
why is UV light harmful to humans?
catalyses the formation of free radicals in the cells
what cells protect us against UV light and how?
Melanocytes produce melanin (darkens skin) which absorbs the UV light and stops it damaging other skin cells
how are the lens and inner cornea protected against free radial damage?
Aqueous humour contains antioxidants (Vitamin C etc)
Slightly alkaline nature activates vitamin C –> increases antioxidant activity
how do cataracts occur?
antioxidant level is too low/too much UV light absorbed –> lens becomes gradually opaque bc UV light cross links protein in the lens –> eye is unable to see
as by products of another condition
what conditions can cause cataracts?
diabetes
hypertension
what are risk factors of cataracts?
exposure to UV light, infrared light or microwaves
how can you decrease the risk of cataracts?
if the diet is high in Vitamin C and E
what is the treatment for cataracts?
surgical replacement of lens
what cells are found in the fovea?
cones
which is more detrimental to vision; foveal damage or peripheral retina damage? why?
foveal damage
cones are most closely packed in the fovea
where in the eye are photoreceptors not found?
blind spot
what are the photoreceptors?
rods and cones
what sight do cones allow for? when are they active?
Allow us to see in detail, active in daylight
what are the types of cones and how are they different?
3 types of cones with slightly different sensitivities to wavelength of light allow us to see different colours
which photoreceptor is most dense?
rods
where are rods found?
outside the fovea
how many types of rods are they?
1
what sight do rods allow for? when are they active?
o Active in the dark
o No colour differentiation – suppressed in daylight
why are we able to differentiate colour?
our brains compare the signal occurring in two or more cone populations
describe the structure of photoreceptors?
outer segment - photoreceptive part
inner segment - cell body and synaptic terminal
what do the outer segments of rods contain?
disks of membrane with rhodopsin
what is rhodopsin?
a protein complex that reacts to light
what is the cone equivalent of rhodopsin?
cone opsins
what is the advantage of rods having a stacking arrangement in their outer segment?
means that light passes through all plates in sequence, maximising chance of a photon interacting with a molecule of photopigment.
why do cones fold their membrane?
shorter outer segment –> enough light in daylight to guarantee photon will interact with a photopigment
why does the eye require so much oxygen?
bc photoreceptors are the most metabolically active cells in the body
what is dark current?
o In the dark, there’s a constant inward leak of sodium in the outermost keeps the cell depolarized and tonically releasing glutamate from its synaptic ending
how does light affect the dark current?
• Light hyperpolarizes the cell –> stops tonic glutamate release
how does light absorption affect rhodopsin?
- Absorption of light changes the shape of rhodopsin
- Changed rhodopsin acts via G-protein to reduce level of cyclic GMP in the rod
- Reduction of c-GMP closes the sodium channel cell hyperpolarizes and stops releasing glutamate synaptic transmission
what do bipolar cells do?
convert constant release of glutamate into varying depolarization
Transmitted to ganglion cells which project their axons into the optic nerve
what are the 3 main neurons in the path from photoreceptor to optic nerve?
- photoreceptors that connect to
- bipolar cells that connect to
- ganglion cells that send axons into the optic nerve
what are horizontal cells?
interneurons that modulate transmission at the synapse between photoreceptor and bipolar cell
why does the retina have a dual blood supply?
to cope with the demands of photoreceptors for oxygen
what forms the inner retina?
ganglion and bipolar cells
what blood vessel supplies the inner retina?
central retinal artery
what supplies the photoreceptors?
choroid
what is the choroid?
network of capillaries supplied by the ciliary arteries – branch off the ophthalmic artery and penetrate the sclera at the back of the eye
how does oxygen diffuse into photoreceptors?
from choroid capillaries through pigment epithelium
when photoreceptors grow new membranes, what happens to the old membrane?
new membrane at the cell body gradually gets transported outwards –> old, worn out membrane is being shed at the distal tip of the cell
Epithelial cells phagocytose the worn-out ends of the photoreceptors and transfer the debris into the capillaries of the choroid
what happens in retinitis pigmentosa?
pigment epithelial cells don’t carry away the cellular debris fast enough –> accumulates as black pigment in pigment epithelial layer
o Gradually thickens and decreases the diffusion of O2 to photoreceptors
o Photoreceptors become so hypoxic they die –> patient slowly goes blind
