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Head and Neck Physiology > Visual System > Flashcards

Visual System Flashcards

1
Q

Describe the iris and it’s function?

A

• Contains blood vessels, pigment cells (melanocytes), two layers of smooth muscle
• Contraction of these muscles changes diameter of pupil

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2
Q

Describe the pupil and it’s functions?

A

• Central opening of the iris, appears black in colour
Regulates the amount of light entering the eye during:
• Close vision and bright light - pupils constrict
• Distant vision and dim light - pupils dilate

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3
Q

Describe the cornea?

A

• Thin transparent epithelium devoid of blood of vessels, covers the coloured iris

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4
Q

Describe the choroid and it’s functions?

A
  • Vascular
  • accounts for 85% of the total blood flow in the eye
  • Relatively low O2 content
  • Covered by sclera and attached to the outermost layer of retina
    Functions
    1. Supplies the outer retina with nutrients
    2. maintains the temperature and volume of the eye
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5
Q

Describe the sclera and it’s functions?

A

• White of the eye, structural support, attachment of extrinsic muscles
Note: Posteriorly, the sclera fuses with the sheath of the optic nerve.

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6
Q

What is scleral/conjunctival icterus?

A

yellowing of the sclera due to build-up of bilirubin

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7
Q

What are the causes of scleral conjunctival/icterus in adults?

A
  1. Bile duct obstruction
  2. Liver dysfunction
  3. Cholecystitis
    Note: bilirubin concentrations are at >3mg/dl in blood
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8
Q

What are the causes of scleral/conjunctival icterus in newborns?

A
  1. Physiologic jaundice
    - liver is immature, cannot take up enough bilirubin
  2. Breast milk jaundice
    - increase absorption of bilirubin from intestines
  3. Breastfeeding failure jaundice
    - decrease in bowel movements allows intestines to absorb more bilirubin
    Note: bilirubin concentrations >2mg/dl in blood
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9
Q

Describe the ciliary epithelium and it’s functions?

A

Ciliary epithelium secretes aqueous humor to anterior chamber
Functions
1. Keeps anterior chamber of eye
pressurised = 20 mmHg, to maintain eye shape and relative positions of the components
2. Provides 02, nutrients, and metabolic waste removal to the lens and the cornea
3. Refraction

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10
Q

Describe drainage of anterior chamber of the eye?

A

The anterior chamber is drained by Canals of Schlemm that empty into extraocular veins
- replaced every 90 minutes

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11
Q

What is glaucoma and state it’s cause?

A
  • a group of diseases that damage the optic nerve and can result in vision loss and blindness
  • Caused by fluid buildup in the anterior chamber mainly due to clogging of Canals of Schlemm
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12
Q

What is the function of vitreous humor?

A
  1. Maintain Intraocular pressure
  2. refraction
  3. phagocytosis
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13
Q

Describe the lens?

A
  • Onion-like structure with closely packed columnar cells arranged in concentric shells and encased by thin and transparent capsule
  • Cells have high concentration of a-crystallins, that increase lens density and enhance its focussing
    > Cataracts (anything that obstructs light in the lens)
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14
Q

Describe the ciliary muscle?

A
  • Smooth muscle oriented in three different directions;
    longitudinal, radial and circular
  • Have the greatest density of mitochondria than any other muscle
  • Parasympathetic innervation (ACh)
    through oculomotor nerve
  • Attached to lens through inelastic zonules and provides accommodation
    > contraction loosens the zonular fibers increasing the convexity of the lens
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15
Q

What are tears?

A

Plasma ultrafiltrate produced from lacrimal gland (pH 7.4)

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16
Q

What are the functions of tears?

A
  1. Bathe the cornea in a thin layer (1 ml/day) 10 um thick, reduce friction, remove debris
  2. Allow oxygen to diffuse from air to corneal cells
  3. Contain lysozyme and antibodies to counter infection
  4. Superficial oil layer slows evaporation for good optical surface
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17
Q

Describe the flow of tears?

A

Lacrimal gland > Lacrimal ducts > Superior or inferior lacrimal canal > Lacrimal sac > Nasolacrimal duct > Nasal cavity

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18
Q

Describe the production of tears in babies?

A

Newborns typically do not shed tears because their lacrimal glands are still developing
• By 2 weeks, they produce just enough tears to keep their eyes moist
• Full tears are produced by 2 months

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19
Q

Describe the optic disc?

A

Lacks photoreceptors
Any image that falls on this region will NOT be seen
It is in this region that the optic nerves come together and exit the eye on their way to the brain
Neural part of retina
> blind spot

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20
Q

Describe the retina and it’s functions?

A

consists of single-cell thick pigmented epithelium, photoreceptors and neural layers
Functions
1. Pigmented layer contains melanin, which helps to absorb stray light rays
2. provide essential nutrition and waste removal for the photoreceptor cells

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21
Q

Describe the visual acuity of people with albinism and why?

A

• Visual acuity of people with albinism is reduced due to light scattering
> no melanin in pigmented layer to absorb excess light
> Light is reflected onto retina, degrading the visual image

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22
Q

How many photoreceptors are in the receptor layer of the retina?

A

120 million rods and 6 million cones

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23
Q

What is the macula lutea?

A

The macula lutea is the centre of the posterior portion of retina

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24
Q

What is the fovea centralis?

A

The fovea centralis is the depression in the centre of macula lutea and forms the centre of visual axis (1 square mm)

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25
Q

Describe the retina in the fovea?

A

This retina in the fovea has lateral displacement of the cells above the photoreceptors, allowing light to strike the photoreceptors without passing through the other retinal cell layers

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26
Q

Describe the neuronal layer of the retina?

A
  • Forward extension of the brain
  • Consists of ganglion, bipolar, horizontal and amacrine cells
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27
Q

Describe the function of bipolar cells of the neuronal layer of the retina?

A

Bipolar cells take the electrical information from the photoreceptor cells and pass it along to other retinal cells

28
Q

Describe the function of the horizontal cells of hen neuronal layer of the retina?

A

Horizontal cells receive input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors
- help integrate and regulate the input from multiple photoreceptor cells increasing visual acuity

29
Q

Describe the function of amacrine cells of the neuronal layer of the retina?

A

Amacrine cells generally receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells

30
Q

Ganglion layer + functions?

A

the outer layer of neurons that contribute axons to the optic nerve
- Receive synaptic input from the bipolar neurons

31
Q

What is an inverted retina?

A

An inverted retina is where the photoreceptors face away from the light, forcing the incoming light to travel through the front of the retina to reach the photoreceptors

32
Q

What are the functions of the inverted retina?

A
  1. Allows absorption of excess light by the pigment epithelium, preventing reflection that may distort the image
  2. Allows photoreceptors to be in close contact with the choroid which supplies the photoreceptors with blood - steady stream of retinal
33
Q

Describe the cooling system of the retina?

A

Because the light is converted largely into heat, the retina must have a very effective cooling system
- provided by the choroidal blood supply directly behind the pigment epithelium

34
Q

What are the light sensitive cells in the retina?

A

Rods and cones
- some types of ganglion cells are also light sensitive but do not play a major role in visual perception

35
Q

Which cells are the only source of output from the retina?

A

Ganglion cells

36
Q

Which retinal neurons are the only ones that fire action potentials?

A

Ganglion cells
- essential for transmitting information outside the eye
Note: All other retinal cells depolarize or hyperpolarize, with a rate of neurotransmitter release that is proportional to the membrane potential, but they do not fire action potentials.

37
Q

What are the 4 refractive interfaces of the lens system?

A
  1. the interface between air and the anterior surface of the cornea
    - why things look blurry when you open your eyes underwater; the water-cornea interface has very little focusing power
  2. the interface between the posterior surface of the cornea and the aqueous humor
  3. the interface between the aqueous humor and the anterior surface of the lens of the eye
  4. the interface between the posterior surface of the lens and the vitreous humor
38
Q

How do you increase the refractive power?

A

Accommodation/focusing
- Contraction of either set of smooth muscle fibers in the ciliary muscle (parasympathetic signals) relaxes the ligaments to the lens capsule, and the lens assumes a more spherical shape, increasing its refractive power

39
Q

What is presbyopia?

A

The lens becomes non accommodating
- the lens grows larger and thicker and becomes far less elastic
Note: seen as a person gets older

40
Q

How do you correct presbyopia?

A

bifocal glasses needed with the upper segment focused for far-seeing and the lower segment focused for near-seeing

41
Q

What is emmetropia?

A

Normal vision

42
Q

What is hyperopia?

A

Farsightedness
- usually due to either an eyeball that is too short
- or occasionally, a lens system that is too weak
• nearby objects look blurry

43
Q

What is myopia?

A

Nearsightedness
- due to too long an eyeball
- but it can result from too much refractive power in the lens system of the eye
• far-away objects look blurry

44
Q

What is astigmatism?

A

often results from too great a curvature of the cornea in one plane of the eye
• can make far-away and nearby objects look blurry or distorted

45
Q

What are the corrective lenses required for the different errors of refraction?

A
  1. Concave for myopia
  2. convex for hyperopia
  3. cylindrical for astigmatism
46
Q

What is the pupillary light reflex?

A
  • The pupil contributes to the optical functioning of the eye by
    continuously adjusting for different ambient light levels
  • The pupillary light reflex constricts the pupil in response to light
  • It involves connections between the retina and neurons in the brain stem that control the sphincter muscles that constrict the pupils
47
Q

How is the pupillary reflex consensual?

A

shining a light into only one eye causes the constriction of the pupils of both eyes.
Note: The lack of a consensual pupillary light reflex is often taken as a sign of a serious neurological disorder involving the brain stem

48
Q

What is the visual field?

A

• The structure of the eyes, and where they are positioned in the head, limits how much of the world we can see at any one time
• Monocular vs binocular vision

49
Q

What is visual acuity?

A

The ability of the eye to distinguish two points near each other
- Depends on several factors but especially on the spacing of photoreceptors in the retina and the precision of the eye’s refraction

50
Q

What is the purpose of eye charts?

A
  • Distance across the retina can be described in terms of degrees of visual angle
  • Eye charts test the eye’s ability to resolve points that are separated by a certain number of degrees of visual angle
    e.g. The Snellen eye chart tests our ability to discriminate letters and numbers at a viewing distance of 20 feet.
51
Q

How do you qualify legal blindness?

A
  • Macuity no greater than 20/200 in the better eye, with best correction
  • a visual field of 20 degrees or less (tunnel vision)
52
Q

Describe the structure of photoreceptors?

A

• Photoreceptors are elongated cells with synaptic terminals, an inner segment and outer segment
1. Outer segment is a highly modified cilium and is the site of signal transduction site
2. Inner segment has mitochondria

53
Q

What is the importance of disk membrane in photoreceptors?

A
  • Disk membrane contains photopigments rhodopsin in rods and related molecules (colour pigments) in cones
  • Both rods and cones decompose on exposure to light and, in the process, excite the nerve fibers leading from the eye
  • The greater number of disks and higher photopigment concentration in rods makes them over 1000 times more sensitive to light than cones
54
Q

Functions of rods?

A

• More sensitive to light, used for scotopic vision (low light)
• Provide black and white vision in low light conditions (no colour)
• Cannot signal colour differences because they contain single pigment (rhodopsin)
• Loss of rod function results in night blindness
• Rods predominate in the extrafoveal retinal portions - absent from fovea centralis

55
Q

What is rhodopsin?
How is it made?

A
  • A purple pigment that consists of the protein opsin covalently bound to a yellow photosensitive pigment called retinal (derived from vitamin A)
    Note: Vitamin A deficiency results in b elow-normal amount of rhodopsin, leading to night blindness
  • Rhodopsin synthesized from inner segments moves across through stalk and into outer segments in vesicles and is incorporated into discs
56
Q

How does rhodopsin work?

A
  1. Retinal absorbs photons, changes from 11-cis retinal to 11-trans retinal
  2. This isomerisation triggers conformational changes in the opsin leading to activation of transducin (a G protein)
57
Q

Functions of cones?

A

• For photopic vision (bright light).
• Contain the pigment iodopsin which consists of retinal combined with a photopigment opsin protein
• The central fovea is populated exclusively by cones
• Humans (and other primates) are trichromats

58
Q

What are the 3 types of cones?

A
  1. S-cones, sensitive to blue light (short wavelength)
  2. M-cones, sensitive to green light (medium wavelength)
  3. L-cones, sensitive to red light (long wavelength)
    • Provide colour vision and greater visual acuity
    • People with colour blindness lack or have a deficiency of one of the cones.(red/green traffic lights)
59
Q

Describe sensory adaptation of cones?

A

• If you look at a bright red circle for a time and then at a gray or white background, you see a blue/green circle or afterimage
• Prolonged viewing of the red stimulus causes adaptation in the red-selective cones:
• like other primary sensory cells, they stop firing if bombarded for too long with a stimulus.
• When the retina is subsequently exposed to a white light, the red cones are unable to fire for a short time, and the ganglion cells connected to these cones decrease their firing.
• The green cones are not fatigued, so the ganglion cells receiving green light send messages through the red-green channel, and the brain receives only a green signal.
• For the red-green channel to signal “white,” it must receive signals from both red and green, which then “cancel” each other.
• Other pairs of colours will also give colour afterimages: blue and yellow, and black and white.

60
Q

What is red green color blindness?

A

most common in genetic males
• Difficulty in distinguishing red and green
• genes encoding the red and green pigments are on the X chromosome, whereas the gene that encodes the blue pigment is on chromosome 7.
• Men have abnormal red-green vision if there is a defect on the single X chromosome they inherit from their mother.
Women have abnormal red-green vision only if both parents contribute abnormal X chromosomes
- protanopia

61
Q

What is blue yellow colour blindness?

A

Difficulty in distinguishing blue and green and yellow and red
- tritanopia

62
Q

Describe photo transduction in the dark?

A
  1. In the dark when the rod is not excited, receptor potential is approx. -30 mV (depolarized)
  2. Guanyly| cyclase catalyses synthesis of cGMP from GTP
  3. The high [CGMP] causes cGMP-gated cation channels to spend much of their time open > influx of Na* (dark current)
  4. Glutamate is released when receptors are depolarised, and release is reduced when receptors are hyperpolarised
  5. Glutamate causes inhibitory post-synaptic potential (IPSP) in bipolar cells of the retina.
63
Q

Describe phototransduction during bleaching?

A
  1. Receptor potential of rods is hyperpolarising (when exposed to light/when excited).
  2. Confórmation change of retinal from cis- to trans- activates transducin which activates cyclic GMP phosphodiesterase.
  3. cGMP phosphodiesterase catalyses conversion of cGMP to GMP.
  4. cGMP diffuses away from Na+ channels, resulting in closure of the channels and hyperpolarization of the cell (-70mV )
  5. Glutamate release is turned off which excites bipolar cell
64
Q

What is light adaptation?

A
  1. If a person has been in bright light for hours, large portions of the photochemicals in both the rods and the cones will have been reduced to retinal and opsins.
    - Much of the retinal of both the rods and the cones will have been converted into vitamin A
  2. The concentrations of the photosensitive chemicals remaining in the rods and cones are considerably reduced, and the sensitivity of the eye to light is correspondingly reduced
    • E.g. feeling “blind” inside the house after being out in the sun
65
Q

What is dark adaptation?

A

• A gradual increase in photoreceptor sensitivity due to rhodospin
regeneration in the dark
• The bleaching reaction during light results in low rhodopsin levels in rods
• A light-adapted person will have lowered sensitivity when they first enter a dark room

66
Q

Describe the processing of visual input in the retina?

A
  1. Neurotransmitter molecules released by rods and cones induce local graded potentials in both bipolar and horizontal cells
  2. Between 6 to 600 rods synapse with a single bipolar cell
    - Light sensitivity of rod vision increases but image perceived is slightly blurred
  3. A cone often synapses with a single bipolar cell
    - Sharper vision
  4. Horizontal cells transmit inhibitory signals in areas lateral to excited rods and cones.
    - Enhances visual contrast between areas in the retina that are strongly stimulated and adjacent areas with weak stimulation.
    - Differentiation of colours
  5. Bipolar cells excite amacrine cells that synapse with ganglion cells
  6. When bipolar or amacrine cells transmit excitatory signals to ganglion cells, the ganglion cells become depolarized and initiate nerve impulses.
67
Q

Describe the neural pathways?

A
  1. The left and right optic nerve come together at the optic chiasm.
  2. Some fibres from the nasal half of each retina cross over to the opposite side
  3. They synapse in the lateral geniculate body of the thalamus
  4. Fibres then continue to visual cortex (area VI) in occipital cortex where visual images are perceived
    - association area for vision where interpretation of visual perception takes place
  5. The frontal cortex (frontal eye field) is responsible for eye movement
  6. Other axons from the optic chiasm pass directly to suprachiasmatic nuclei of hypothalamus where they form connections that synchronize a variety of endocrine and other circadian rhythms associated with light-dark cycle

Head and Neck Physiology (18 decks)

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