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Brain and Behaviour > Retina physiology > Flashcards

Retina physiology Flashcards

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1
Q

what part of the electromagnetic spectrum can we see

A
  • The part we can see runs from 400nm to 750nm
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2
Q

why can we only see this part of the electromagnetic spectrum

A
  • Can only detect a small bit due to a limitation of biological tissue
  • Cannot focus longer infrared wavelengths and out tissues absorb shorter wavelengths
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3
Q

what is colour blindness

A
  • Colour blindness is an inability to detect some wavelengths, this affect 8% f men and 0.5% of women
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4
Q

what does the lacrimal apparatus consist of

A
  • This consists of lacrimal gland and associated ducts
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5
Q

what do lacrimal glands do

A
  • these secrete tears which provide an optically smooth refracting surface
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6
Q

what do tears contain

A
  • They also contain antibodies and lysozyme to prevent bacterial growth
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7
Q

how do tears enter and exit the eyes

A
  • Tears enter the eye via excretory ducts they exit the eye medially via the lateral punctum and drain into the nasolacrimal duct
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8
Q

what is the innervation of the lacrimal glands

A
  • this is stimulated by parasympathetic efferents
  • Preganglionic parasympathetic efferents travel in the facial nerve (VII) and then branch off in the greater petrosal nerve which goes to the pterygopalatine ganglion (PPG).
  • Post ganglionic fibres from the PPG innervate the lacrimal glands and also stimulate nasal secretions over the olfactory mucosa
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9
Q

what are the 3 layers of the eye

A

1) The outer layer is the sclera which surrounds the eyeball. This is continuous with the dura covering the optic nerve. At the front of the eye the sclera thins and becomes transparent and is known as the cornea
2) the middle layer consists of blood vessels & connective tissue including the iris, the ciliary body and the choroid
3) The inner layer is the retina (the light sensing organ)

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10
Q

what is the eye

A

The eye is a fluid-filled sphere enclosed by three layers of tissue

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11
Q

what are the extra ocular muscles attached to

A

the sclera

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12
Q

what is the white of the eye

A
  • conductive - this is where the blood vessels of the eye
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13
Q

what is the iris

A
  • sphincter muscle that constricts in order to reduce the amount of light that gets into the eye
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14
Q

describe the iris

A
    • Iris is heavily pigmented
    • Sphincter muscle to constrict or dilate the pupil
    • Eye color (brown, green, blue, etc.) dependent on amount and distribution of the pigment melanin
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15
Q

describe the pupil

A
  • Pupil is the hole through which light passes

* Pupil diameter ranges from about 3-7 mm

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16
Q

describe the sclera characteristics

A
  • Extremely durable

* Flexibility adds strength

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17
Q

describe the structure of the sclera

A

The sclera is visible as the white of the eye.

* Exterior is smooth and white
* Interior is brown and grooved
* Continuous with sheath of optic nerve
* Tendons attached to it
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18
Q

what is the cornea

A

The cornea is the clear bulging surface in front of the eye.

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19
Q

what is the main refractive surface of the eye

A

The cornea

• Index of refraction: n = 1.37

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20
Q

describe the structure of the cornea

A
  • Normally transparent and uniformly thick
  • Nearly avascular
  • Richly supplied with sensory unmyelinated C fibres(these are too small for you too see as they don’t detract the light) nerve fibers from trigeminal nerve
  • Sensitive to foreign bodies, cold air, chemical irritation
  • Nutrition from aqueous humor - gets glucose from the aqueous humour
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21
Q

what do tears do in the cornea

A
  • Tears maintain oxygen exchange and water content

* Tears prevent scattering and improve optical quality

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22
Q

How are the coronea alive if it is avascular

A
  • they get there oxygen directly from the air dissolving in the lacrimal fluid outside or from the aqueous humous
  • glucose is in the aquemsous humour and that’s how they get there glucose
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23
Q

what does the conjunctiva do

A
  • Enables the eyelid to slide backwards and forwards

- it is richly vascular

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24
Q

describe the structure of the conjunctiva

A
  • Conjunctiva is a layer of stratified columnar epithelium, goblet cells & capillaries that covers the sclera and the inside of the eyelids.
  • It contains goblet cells produce mucus that mixes with the tears (this may stop the tears draining too quickly
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25
Q

where is the anterior chamber

A

between the cornea and sclera

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26
Q

describe what light passes through

A

Light passes through the cornea into the anterior chamber of the eye which is filled with a watery fluid the aqueous humour.
- It then passes through the pupil, the lens and into the posterior chamber of the eye which is filled with vitreous humour

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27
Q

What is the vitreous humour

A

Vitreous humour is a watery gel produced by a form of glia cell in the retina.

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28
Q

what does the vitreous humour do

A

It holds the shape of the eye constant to maintain focussing accuracy

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29
Q

what is the most sensitive part of the retina

A

fovea

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30
Q

what is the point where the nerves from the retina leave the eye called

A

optic discs, they leave the eye through the optic nerve

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31
Q

what is the normal pressure of the intraocular pressure

A
  • Normally around 10 to 21 mmHg
32
Q

how is the cornea kept as a spherical surface

A

The cornea is kept as a spherical surface by the intraocular pressure

33
Q

where is visual acuity is at its highest

A

Visual acuity (sensitivity) is highest in the the centre of the retina, at the fovea

34
Q

what is the place where the visual axons leave the eye to form the optic nerve called

A

The place where the visual axons leave the eye to form the optic nerve is called the blind spot

35
Q

there are no…. in the blind spot

A

There are no photoreceptors in the blind spot

36
Q

describe how the aqueous humour is formed and leaves

A

Aqueous humour is formed by the ciliary body and circulates up behind the iris, through the pupil and into the anterior chamber. It drains into the canal of Schlemm

37
Q

how can intraocular pressure rise

A
  • either too much aqueous humour is produced

- or the drainage of aqueous humour decreases

38
Q

what produces the intraocular pressure

A

The difference between the formation and drainage of aqueous humour produces the intraocular pressure

39
Q

what happens if the intraocular pressure rises

A
  • If drainage of aqueous humour is blocked pressure in the anterior chamber rises.
  • This increases pressure on the vitreous humor which in turn presses on the retina.
  • Prolonged pressure damages the retina and causes GLAUCOMA
40
Q

what are the two types of glaucoma

A
  • Open angle and closed angle
41
Q

describe how open angle glaucoma works

A
  • is a slowly progressive condition which occurs when Schlemm’s canal gradually become blocked.
  • The angle between cornea and iris is open i.e. normal (about 40 degrees)
42
Q

describe how closed angle glaucoma works

A
  • occurs when the iris is pushed forward and the angle between cornea and iris is reduced.
  • This blocks the canal of Schlemm, causing a rapid rise in pressure inside the eye.
  • This causes extreme pain and sudden vision loss
43
Q

how do you treat glaucoma

A
  • can be treated by drugs or surgery
44
Q

How do you surgically treat glaucoma

A
  • In surgery an incision is made in the sclera at to the cornea-sclera junction. This allows the aqueous fluid to drain out on to the surface of the eye and thus lowers the pressure.
45
Q

what are the drugs that are used to treat glaucoma

A
  • Prostaglandin analogs increase outflow of aqueous humor through canal of Schlemm
  • Beta-adrenergic receptor antagonists decrease aqueous humor production
  • Alpha2-adrenergic agonists work by a dual mechanism, decreasing aqueous production and increasing outflow.
  • Parasympathomimetic agonists and anticholinesterases work by contraction of the ciliary muscle, which opens the canal.
  • Carbonic anhydrase inhibitors lower secretion of aqueous humor by inhibiting carbonic anhydrase in the ciliary body.
46
Q

what is the preferable treatment of glaucoma

A

Decreasing the production of aqueous humour can have adverse effects therefore surgery is more preferable

47
Q

How does UV light damage the eyes/ How does cataracts develop

A

light has to pass through the lens in your eye so it can’t be blocked or we would not be able to see.

  • If the level of antioxidants (in aqueous humour) is reduced or too much uv light is absorbed, or for many other reasons, the lens can gradually become opaque and the eye is unable to see.
  • This lens opacity is called a CATARACT
  • In a cataract the UV light cross-links proteins in the lens and makes them opaque
48
Q

How do antioxidants work in the eye

A

The aqueous humour contains antioxidants (vitamin C and others) to protect the lens and inner cornea against free radical damage. Its slightly alkaline nature activates the vitamin C and increases the antioxidant activity

49
Q

How do you treat cataracts

A

Treatment is by surgical replacement of lens

50
Q

what increases the risk of cataracts

A
  • diabetes or hypertension- these conditions can reduce the levels of antioxidants in the aqueous humour
  • There is a thought to be significantly decreased risk of cataract if there are high levels of vitamin C and E in the diet
  • exposure to uv light, infrared light or microwaves which are also risk factors
51
Q

Foveal damage is….

A

much more detrimental to vision than damage to the peripheral retina

52
Q

what does the fovea do

A

Our eyes scan an image so that the fovea moves repeatedly over the area of interest.

53
Q

describe the blind spot

A
  • The blind spot has no photoreceptors.

- It is where the optic nerve axons leave the retina. It is a circular to oval area measuring about 2 x 1.5 mm across

54
Q

what are the two different types of photoreceptors

A

rods or cones

55
Q

all cells in the fovea are…

A

cones

56
Q

what do the cones do

A
  • There are red blue and green cones- each cone type responds to a range of wavelengths
  • Mediate ability to see detail.
  • Active in daylight.
  • Three types with slightly different sensitivities to wavelength of light enable us to discriminate colours
57
Q

where are rods

A

most dense just outside fovea:

58
Q

describe rods

A
  • only one type;
  • very sensitive to light
  • Active in dark;
  • no colour differentiation;
  • suppressed in daylight
59
Q

what is perception of colour due to

A

Pereception of the colour is due to the difference between the different cones

60
Q

where is the inner and outer part of the rod

A

The rod is the inner part of the retina where the light comes in and the outer part is nearer the scleara

61
Q

describe the structure of the rod

A
  • Outer segment: in rods consists of ‘disks’ of membrane containing the photopigment rhodopsin ( a protein complex that reacts to light (cones contain similar proteins called cone opsins)
  • Stacking arrangement means that light passes through all plates in sequence, maximising chance of a photon interacting with a molecule of photopigment.
62
Q

describe why the structure of the rods and cones are like that

A

The photons are going upwards towards the sclera in order to make a response in the rod you have to absorb the photon, the photon might not hit the first disc, if you stack the plates up and up the probability of hitting up the plate you increase change of the photo hitting rhodopsin and triggering a response, this is why that mammalian eye can only see a few photos when they are fully in dark

63
Q

describe the outer and inner segment of both rods and cones

A

In both rods and cones the photoreceptive part is the outer segment. The inner segment contains the cell body and the synaptic terminal .

64
Q

describe the structure of cones

A

Cones have a shorter outer segment made by folding the membrane. There is enough light in daylight to guarantee a photon will react with photopigment

65
Q

describe how dark current works

A
  • there is constant inward leak of sodium in the dark into the outermost part of the cell
  • but it is constantly pumped out by a sodium pump
  • the constant influx eventually causes the cell membrane to be depolarised
  • this means that calcium channels at the synaptic end are open
  • and glutamate is therefore released from the presynaptic ending
  • therefore they need to have a high level of oxygen all the time in order to keep this working therefore it has to have a high metabolic rate

on the other hand

  • light hyper polarises the cell and this prevents the tonic release of glutamate
  • therefore photoreceptors are not light sectors but they are dark detectors as they release transmitter in the dark and stop it being released in the light
66
Q

why does the eye require high amounts of oxygen

A
  • this is because the photoreceptors are one of the most metabolically active cells in the body
67
Q

what happen when light is absorbed in the photoreceptors

A
  • Absorption of light changes the shape of the rhodopsin.
  • The changed rhodopsin then acts via a G-protein to reduce the level of cyclic GMP in the rod.
  • It is this reduction in c-GMP which closes the sodium channel and allows the cell to hyperpolarise and stop releasing glutamate.
68
Q

describe how glutamate works

A
  • it is continously released from the synaptic endings at the base of the cones in the dark and this release is suppressed by light
  • bipolar cells convert this continuous release of glutamate into a varying depolarisation
  • this is transmitted to ganglion cells which project axons into the optic nerve
69
Q

what are the three main neurones in the path from the photoreceptor to the optic nerve

A

1) photoreceptors that connect to
2) bipolar cells that connect to
3) ganglion cells that send axons into the optic nerve – has action potentials

70
Q

what is the blood supply to the eye

A

there is a dual blood supply

71
Q

what is the inner retina blood supply

A

the central retinal artery

72
Q

what is the blood supply of the photoreceptors

A

choroid

73
Q

what is the choroid

A

a network of capillaries supplied by the ciliary arteries, which branch off the ophthalmic artery and penetrate the sclera at the back of the eye.

74
Q

what cells are in the inner retina that are supplied by the central retinal artery

A

= ganglion and bipolar cells

75
Q

there is little tissue between…

A

There is little tissue between the choroid and photoreceptors so the diffusion of oxygen is very efficiency

76
Q

what do the pigment epithelium do

A
  • photoreceptors are always growing new membranes at the cell body
  • this is gradually removes the old and worn out membrane which is shed at the distal tip of the cell
  • the pigment epithelial cells phagocytose the worn out ends of the photoreceptors and transfer the debris into the capillaries of the choroid
77
Q

what is retinitis pigementosa

A
  • this is when the pigment epithelial cells do not carry away the cellular debris fast engouh
  • it accumulates as black pigment in the pigment epithelial layer
  • This gradually thickens and decreases the diffusion of oxygen to the photoreceptors.
  • Eventually the photoreceptors are so hypoxic they die.
  • The patient slowly goes blind.

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