The Visual system Flashcards
1
Q
how large is the human eye
A
24mm in diameter
2
Q
what are the 3 layers of the human eye
A
- outer layer is the corneoscleara
- uvea
- the innermost layer is the retina
3
Q
describe the corneosclear
A
- outermost layer
- When it reaches the eye, the dura mater covering the optic nerve becomes the sclera
- the sclera covers the back of the eyeball.
- At the front of the eye the sclera becomes the conjuctiva(white of the eye) and in front of the pupil of the eye the conjunctiva merges with the transparent cornea.
4
Q
what is the urea inside of
A
it is inside of the sclera
5
Q
what are the 3 parts of the uvea
A
- the choroid
- ciliary body
- iris - projects inwards from the ciliary body
6
Q
what is the choroid
A
this is a dense capillary network that is all around the back of the eye
7
Q
what is the ciliary body
A
- this is where the chord fuses with the ciliary body
- the ciliary body contains the muscles that control the shape of the lens
8
Q
what is inflammation of the uvea called
A
- uveititis
9
Q
describe the retina
A
- it is the light sentivie layer
- the retina is thin and about 0.2mm
10
Q
what does the corona do
A
- The cornea provides a transparent protective coating for the optical structures of the eye.
11
Q
what does the cornea cover
A
It covers the pupil (black central part) and the iris (coloured margin) of the central part of the eye
12
Q
what does the lateral margin of the cornea fuse with
A
the conjuctiva
13
Q
what is the conjuctiva
A
- it is a specialised epithelium that covers the white sclera of the eye
14
Q
is the cornea vascularised or not
A
- the conjunctiva and sclera have a blood supply, but the cornea is normally not vascularized.
- In pathological conditions it may become vascularised and this effects vision.
15
Q
what is behind the cornea
A
- the fluid filled anterior chamber, this is bounded posteriorly by the iris and the pupil
16
Q
what varies the size of the pupil
A
- muscles in the iris
17
Q
what does the storma of the iris contain
A
- melanocytes, these reflect or absorb light in order to give the rise its colour
- circumferentially organised sphincter muscles of the iris
- radially arranged dilator muscles
18
Q
what is innervated by the iris sphincter
A
- cholinergic parasympathetic axons
19
Q
if the sphincter muscles contract what happens
A
- if the sphincter muscle contract then this decreases the diameter of the pupil (Miosis)
20
Q
what opposes the iris sphincter
A
- the dilator muscle
21
Q
what do sympathetic nerve endings do in the iris
A
release norepinephrine onto the radially arranged muscles, and their contraction results in pupil enlargement (mydriasis)
22
Q
what is miosis
A
pupil decreasing in size
23
Q
what is mydriasis
A
pupil enlargement
24
Q
what is the pupillary light reflex
A
this is when an increase in light level produces a decrease in pupil diameter and a decrease in light causes an increase in pupil diameter
25
what can also cause pupil enlargement
- excitement
- pain
- fear
26
by which circumference of the pupillary margin does the pupil enlarge by
- The circumference of the pupillary margin changes by a factor of six
27
how does the pupil change diameter
- acetylcholine is released onto the sphincter and dilator muscles
- this activates muscarinic receptors that depolarise the sphincter muscles and cause contraction
- whereas on the dilator muscle is causes the inhibition of noradrenaline release and blocks dilator contraction
- therefore the sphincter contracts and dilator relaxes and this strengths the response to light
28
describe the lens
- transparent
| - made out of collagen fibres
29
what does the lens do
- focuses the light on the retina
30
what is the process of focusing called
accommodation
31
what is the majority of focusing of light in the eye done by
cornea
32
why is the majority of light focusing done by the cornea and not by the lens
- because the bending of light at an interface depends on the change in refractive index at the interface
- The difference in refractive index at the air/corneal junction is much greater than the difference at the lens/ aqueous humour interface
33
what are cataracts
Opacities in the lens
34
what do cataracts cause
cloudiness of the lens
35
what causes cataracts
- bycongenital defects
- persistent exposure to ultraviolet light
- aging proces
36
what is the treatment of cataracts
- replace the lens with an inert plastic prosthesis and restore site but there could by a loss of accommodation
37
what is the second fluid filled space (posterior chamber) bound by
- bounded anteriorly by the iris and posteriorly by the lens
- its encircling suspensory ligament
38
what do the anterior and posterior chambers have in them
- they are filled with a clear fluid - aqueous humour
39
what produces the aqueous humour
epithelium over the ciliary body
40
describe the production of the aqueous humour
- produced by the epithelium over the ciliary body
- it flows through the pupillary opening into the anterior chamber
- then drains into the canals of Schlemm which is a set of modified veins
41
where are the canals of Schlemm located
round the rim of the anterior chamber in the angle of the eye where the iris meets the cornea
42
how often is the aqueous humour replaced
2-3 hours
43
what determines the intraocular pressure
- regulation of the production and reabsorption of the aqueous humour determines the intraocular pressure
44
what is the intraocular pressure range
normally between 15-20mmhg
45
what does the intraolcular pressure do
- maintains the shape of the eye
46
what causes the intraolcular pressure to increase
- if the reabsorption is compromised
47
How is a glaucoma caused
because the suspensory ligament encircling the lens consists of discrete strands, the fluid in the posterior chamber is in contact with the vitreous body,
- .If untreated, increased intraocular pressure can cause blindness by restricting blood flow to the retina, a condition known as glaucoma.
48
what is the vitreous body
the gelatinous mass that fills the main space of the eyeball between the lens and the retina
- jelly like substance that preserves the shape of the eyeball and therefore maintains its focusing precisions
49
what contains the vitreous body
posterior aspect of the eyeball
50
what is between the vitreous body and the choroid
retina
51
what is the retina composed of
- the neural retina
| - the retinal pigment epithelium
52
what is tenns inner and outer
- tenns inner - this refers to structured that are located towards the vitreous and centre of the eyeball
- tenns outer - these are structures that are located toward the pigment and choroid
53
what is the retinal pigment epithelium
The retinal pigment epithelium is a continuous sheet of pigmented cuboidal cells bound together by tight junctions that block the flow of plasma or ions
54
what does the retinal pigment epithelium do
a) supplies the neural retina with nutrition in the form of glucose and essential ions,
b) protects retinal photoreceptors from potentially damaging levels of light,
c) plays a key role in the maintenance of photoreceptor anatomy.
55
what does the neural retina contain
- photoreceptors and associated neurones of the eye
56
what is the neural retina specialised for
- sensing light
| - processing the resultant information
57
what do the photoreceptors do
The photoreceptors absorb quanta of light (photons) and convert this input to an electrical signal.
- The signal is then processed by retinal neurons
58
what do the retinal neurones do
- process the photon of light
| - they then send the processed signals to the brain via axons that travel into the optic nerve
59
is the contact between the neural retina and pigment epithelium stable
unstable
| - shown in a detached retina when the neural retina tears away from the pigment epithelium.
60
describe what happens with a detached retina
- the neural retina tears away from the pigment epithelium
- Because photoreceptors are metabolically dependent on their contact with pigment epithelial cells, a detached retina must be repaired to avoid further damage
- The detached part of the neural retina is welded to the pigment epithelium
- Although this repair prevents an increase in the area of detachment, the detached portion of the retina does not regain function.
61
the photoreceptor outer segments interlock with...
the melanin filled processes of pigment epithelial cells
62
describe what happens when the photoreceptor outer segments interlock with the leaning willed processes of ligament epithelial cells
- they are mobile
- they elongated into the pigmented layer with the light entering the eye is bright (photooptic conditions)
- and retract with the light is dim (sctopiconditions)
- this is in order to protect the retina form light conditions that could damage the photoreceptors
63
what does acuity mean
- ability to see fine detail in an image
64
what part of the retina has the best acute
the fovea centralis, which is in the centre of the macula lutea (yellow spot
65
what is special about the retinal surface
- it is the only place in the Body where blood vessels can be viewed direclty for pathological changes
66
describe the location of the macula
- at the exact centre of the eye in the visual axis
| - located temporally to the topic disc
67
what is the blood supply to the eye
- the ophthalmic artery
| - this is a branch of the internal carotid
68
how does the ophthalmic artery enter the eye
- enters the orbit via the optic foramen along with the optic nerve
69
how do you see the blood supply to the eye
ophthalmoscope
70
describe the branches of the ophthalmic artery
- a branch enters the optic nerve and forms the central artery of the retina, this supplies the inner layers of the retina
- the ophthalmic artery also forms ciliary arteries these enter the back of the eye and branch into the choroid layer of the eye which is between the sclera and pigment epithelium and forms a dense capillary plexus behind the receptors
- anteriorly the choroid merges into the ciliary body which contains the ciliary muscle
71
describe the structrue of rod cells
- outer segment is a membrane-hound cylinder containing hundreds of tightly stacked membranous discs
- there are molecules of rhodopsin within these membranes
72
what does the outer segment of the rod cell do
- site of transduction
- Photons travel through cells of the neural retina before striking the membranous discs of the rod outer segment.
- rhodopsin within the membranes undergo a conformational change and along with transducin and phosphodiesterase (PDE) induce biochemical changes in the rod outer segment, which reduce levels of cyclic GMP (cGMP)
73
what are the cGMP levels in the dark like
- In the dark, cGMP levels in the rod outer segment are high
74
what does cGMP do
- keeps sodium channels in the membrane open and allows a constant sodium current to flow into the membrane, this is called the dark current
- high level of sodium permeability means that the resting potential for rods to depolarise is -40mv
75
what happens to cGMP in the light
- int he light the cGMP is reduced
- sodium segments close
and the rod cells is hyperpolarised
- resting potential is -70mC
76
why does the dark current not fill the cells with sodium
The dark current does not fill the cells with sodium as this ion is constantly pumped out of the cell in the region of the cell body
77
describe what happens if the rod segment is hyperplarised
- it sends messages through the perikayrn to the rod presynaptic ending
78
what happens in the absence of light and with light at the rod presynaptic ending
in the absence of light, the photoreceptor terminals constantly release the transmitter glutamate
- a light-induced wave of hyperpolarization causes a transient reduction in this tonic release of glutamate
79
what do the cones cell contain instead of rhodopsin
one of three isotopes of the cone receptor protein iodopsin
80
describe how rhodopsin molecules are replaced
- the distal 1/10th of the outer segment is broken of a phagocytsed but he pigment epithelium
- therefore the outer segment is continouslaly renewed
- new discs are formed at the base of the outer segment and move outwards so that the shed discs are replaced
- this means that the rod remains a constant length
81
how often is the outer segment renewed
every 10 days
82
why do photoreceptors have one of the highest metabolic rates of any cell in the body
- The constant need for manufacture of new receptor protein membrane
- the constant demand for sodium pumping to sustain the dark current
83
why are receptors outside the the retina
- receptors need oxygen so they have to be close to the dense capillary plexus which is the choroid
- light cannot get through this plexus due to the blood
- therefore the blood supply has to be on the opposite side to the receptors due to the light input therefore the receptors are outside the retina so they can be close to the choroid
84
what does light have to pass through in order to reach the receptors
- has to pass through the ganglion and bipolar cells of the retina to reach the receptors
- the bipolar and ganglion cell layer is very thin , and these cells have much the same refractive index as the vitreous humour, so the light passes through them without distortion
- the ganglion cells need less oxygen than the receptors therefore the vessels that supply them are small and can be kept away from the maximum acuity at the fovea
85
what do the pigment epithelial cells do
- it removes the membrane debris that are constantly being sheered from the outer parts of the rods and cones
86
what happens if the pigment epithelial cells do not function
- membrane debris build up
- pushes the receptors away from the choroid
- this reduces the diffusion of oxygen dn receptors gradually die due to lack of oxygen
- this is called retinitis pigments as the retina can be seen to have coloured clumps fo pigment
87
describe the structure of the presynaptic ending of the photoreceptor
- large
- the synapse between a receptor and bipolar cell is called a ribbon synthase
- horizontal cells integrate between the presynaptic receptor terminal and bipolar cell dendritic
- this enables the horizontal cells to modulate transmission from the receptor to bipolar cell
88
what is the synapse between a receptor and bipolar cell called
a ribbon synthase
89
what are the two different effects that glutamate produces in bipolar cell s
- depolarises the cells
- hyperpolairses them - potassium channels may be opened by glutamate
- they produce two different effects as there are two kinds of bipolar cell by gating different ion channels
90
describe how glutamate and light work in bipolar cells
- in the dark some bipolar cells are tonically depolarised by the steady efflux of glutamate and some are hyper polarised
- when light touches the receptor membrane the receptor becomes more polarised
- this stops the efflux of glutamate and the bipolar cells that were depolarised become less so and the hypoerlaisrsed cells become less so
91
what are the three types of cones
```
L cones (red cones) - sensitive to long wavelengths
M cones(green cones) - sensitive to medium wavelengths
S cones (blue cones)- sensitive toshrot wave lengths
```
92
what happens if a cone is missing
- the individual confuses certain colours and is colour blind
93
why is colour blind more common in men
cause the genes for the L-cone (red-absorbing) and M-cone (green-absorbing) opsins are located on the X chromosome, color blindness is more common in men
94
where is S cone located
- on an autosome therefore alteration of it is much rarer
95
what is the inability to detect pure red known as
protanopia
96
what is the inability to detect green known as
deuternopia
