Structure & Function of the Eye Flashcards
what are the 3 types of tear production?
1) basal tears (constantly produced)
2) reflex tears (response to irritation)
- afferent CnV1
- efferent PNS fibres of CnVII
3) crying/emotional tears
pathway of tears in lacrimal system
- lacrimal glands
- drainage through two puncta
- via superior and inferior canaliculi
- collect in tear sac
- drain through tear duct in nose (via inferior nasal meatus)
afferent and efferent reflex tear
afferent: CN V1
efferent: PNS
4 functions of the tear film?
1) maintains smooth corneal-air surface
2) facilitate oxygen supply to cornea
3) remove debris
4) bactericide
what are the 3 layers of the tear film?
1) superficial oily layer
- reduce tear film evaporation
2) aqueous layer
- contains bactericides
3) mucin layer
- maintains wet corneal layer for diffusion
what produces the superficial oily layer?
Meibomian glands
superficial layer made of lipids to prevent evaporation
what produces the aqueous layer of tear film?
tear glands
what is the conjunctiva?
a thin transparent tissue that covers the outside surface of the eye.
position of the conjunctiva?
begins at outer edge of cornea
covers visible surface of eye
lines eyelids
nourished by near-invisible blood vessels
what are the 3 layers of the eye?
1) sclera (superficial)
2) choroid
3) retina (deep)
sclera
tough and opaque
white tissue, continuous with the cornea
high water content
choroid
- component of the uvea
(uvea= choroid+iris+ciliary body) - composed of vascular layer
- pigmented
what are the components of the uvea?
iris
ciliary body
choroid
choroid and sclera water content
choroid: low water content
sclera: high water content
how much of the focusing power does the cornea provide?
66%
refractive index of cornea
provides 2/3 of the refractive power
(low water content compared to sclera)
- higher than air due convex structure
5 layers of the cornea (from superficial to deep)
1) epithelium
2) Bowman’s membrane
3) stroma (thickest, has nerve endings)
4) Descemet’s membrane
5) Endothelium
stroma is the thickest and avascular so receives nutrients from corneal nerves
function of the corneal endothelium
(last layer) pumps fluid out of the cornea, so prevent corneal oedema
pathology related to corneal endothelial
- only 1 cell thick, has no capacity to regenerate
- cell density decreases with age–> corneal oedema and corneal cloudiness
- responsible for pumping out fluid to prevent corneal oedema
what is the uvea?
where does it lie?
iris+choroid+ ciliary body
vascular coat of the eye ball
lies between the sclera and the retina
what is the lens suspended by?
lens zonules (fibrous ring) made of passive connective tissue they push and bulge the lens
what is the visible part of the optic nerve called?
optic disk
what is located near the optic nerve connection to the eye?
macula, temporal to the optic nerve
what is the macula?
what is it involved in?
what is located in the macula?
- small and highly sensitive part of the retina
- involved in detailed central vision
- the fovea is in the centre of the macula
what are the boundaries of the anterior segment of the eye?
what is this area filled with?
between the cornea and the lens
filled with clear fluid for nutrition
what is the posterior segment? what is filled in it?
behind the lens (collagenous)
filled with vitreous humour
what is normal intraocular pressure?
12-21/24 mmHg
what is the effect of hydration of the cornea?
whiteness/cloudiness of cornea due to high water content of sclera
what produces the aqueous humour?
ciliary body
active process
no correlation to BP
2 methods of aqueous humour absorption?
1) uveal-scleral outflow
2) Canal of Schlemm and trabecular meshwork
where does the aqueous leave in uveal-scleral outflow?
between sclera and choroid
Prostaglandins can target this
where does aqueous humour go into via the CoS?
into the blood stream
proportion of drainage in U-S outflow and CoS?
U-S outflow 20%
CoS into blood stream- 80%
what are the 2 characteristics of glaucoma?
retinal ganglionic cell death
enlarged optic disk cupping
main consequences of glaucoma
increased IOP
visual field loss and blindness
2 types of glaucoma?
1) primary open angle (most common, trabecular dysfunction)
2) closed angle (acute or chronic, lens/iris bulge restricting outflow)
what is the cause of primary open angle glaucoma?
trabecular meshwork dysfunction
what is the mechanism of closed angle glaucoma?
increased IOP causes lens/iris to bulge out
this restricts the access to the TM and therefore outflow of the aqueous
acute or chronic issue
risk factors of closed angle glaucoma
having a small eye and naturally small angle
how can closed angle glaucoma be treated?
peripheral laser iridotomy to create drainage holes
concentrations of cones and rods in macula?
highest conc of cones
lowest conc of rods
how can the fovea be clinically assessed?
Optical Coherence Tomography
what are the uses of central vision?
- detailed day vision
- reading and facial recognition
pathology and assessment of central vision
macular degeneration affects acuity
assessed by visual acuity assessment
what are the uses of peripheral vision?
for shape, movement and night vision
assessment of peripheral vision
visual field assessment
the 3 layers of the retina?
outer- 1st order neurones
middle- 2nd order neurones
inner layer- 3rd order neurones
1st order neurones (outer)
(photoreceptors detect light)
2nd order neurones (middle)
(bipolar cells regulate/improve sensitivity and process light)
3rd order neurones (inner)
(retinal ganglionic cells for signal transmission to the brain)
receptor position in retina
proximal to pupil
light hits the retinal pigment epithelium first, at the back, then reflects onto the receptors in proximity
outer segment of cones vs rods
rods has a longer outer segments that is 100x more sensitive to light
light sensitivity of cones vs rods
rods more sensitive than cones
response speed of cones vs rods
rods are slower in response than cones
what is the rod (scotopic vision) responsible for?
peripheral
night vision
recognise motion
what is the cone (photopic vision) responsible for?
central colour
day vision
detail
relative populations of cones and rods in the eye
120 million rods
6 million cones
fovea photoreceptors
no rods found
what is the commonest form of colour vision deficiency?
red-green confusion (deuteranomaly)
M and L cone peaks are very close to each other
at what wavelength is the rods peak sensitivity?
500nm
what is the test for red-green deficiencies?
Ishihara test (numbers in dots)
adaptation of cones and rods in the dark
increase in light sensitivity
cones initially most sensitive (7mins) than rods adapt and become more sensitive (30mins)
happens as a biphasic process
what happens in light adaptation?
neuro-adaptation
bleaching of photopigments
inhibition of rod and cone function
