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Ocular Pathophysiology > EOMs > Flashcards

EOMs Flashcards

1
Q

What muscle type are EOMs?

A

Skeletal muscle

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2
Q

What are thick myofibrils composed of?

A

Hundreds of myosin subunits.

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3
Q

Describe the structure of a myosin fibril.

A

Composed of two-headed filaments, with heads arranged in a spiral.

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4
Q

Describe the structure of an actin myofibril.

A

Composed of double-helix filament to which troponin-tropomyosin complex is attached.

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5
Q

Thin myofibrils are formed by what?

A

By a protein called actin

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6
Q

What is a sarcomere?

A

The contractile unit of striated muscle

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7
Q

Name the regions of the sarcomere, and what they contain.

A

I band

  • Only actin (thin) myofibrils
  • Light band

A band

  • Contains both myosin AND acting
  • Runs the entire length of myosin (thick) myofibrils

H zone
- Contains only myosin (thick) myofibrils

Z line
- Dark stripe bisecting the I band

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8
Q

During muscle contraction, ___ filaments slide past the ____ filaments.

A

Actin, myosin

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9
Q

During muscle contraction, which part(s) of the sarcomere decrease in size?

A

The H zone, the sarcomere itself, and the I band.

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10
Q

During muscle contraction, what part(s) of the sarcomere stay the same in size?

A

The A band and the lengths of the myosin/actin.

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11
Q

Select the incorrect statement. EOMs:

A. Have a denser blood supply than other skeletal muscle
B. Have a denser innervation than other skeletal muscle
C. Have a range of fiber sizes
D. Have slow and fast twitch fibers
E. Are among the fastest but fatigue prone muscles of striated muscle

A

E is incorrect.

EOMs are among the fastest and most fatigue-resistant of striated muscle.

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12
Q

Describe the innervation of EOMs

A

EOMs contain fewer muscle fibers in a motor unit.

Whereas in a the striated muscle of the leg which has the innervation of 1 axon per several hundred muscle fibers, in EOMs each axon innervates 3-10 fibers.

EOMs get innervation a lot quicker.

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13
Q

What advantages does the dense innervation afford?

A
  • Precise fine motor control
  • High velocity ocular movements, necessary in saccades
  • Very accurate pursuits
  • Fixations
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14
Q

Slow twitch fibers allow which type of movement?

A

Slow fibers produce slower pursuit movements and provide muscle tone.

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15
Q

Fast twitch fibers produce which type of movement?

A

Quick saccadic movements.

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16
Q

Describe Fick’s Axes

A

x-axis: horizontal or transverse axis; nasal to temporal

y-axis: sagittal axis; anterior to posterior pole

z-axis: vertical axis; superior to inferior

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17
Q

Describe the primary position of gaze

A

Eyes are focused for infinity.

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18
Q

Describe the secondary positions of gaze

A

Rotations around either the vertical axis or horizontal axis

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19
Q

Describe the tertiary positions of gase

A

Rotations around BOTH the vertical AND horizontal axes

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20
Q

What are ductions?

A

Movements of ONE eye

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21
Q

Define adduction

A

The rotation of the eye medially

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22
Q

Define abduction

A

The rotation of the eye laterally

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23
Q

Define elevation (supraduction)

A

The rotation of the eye up

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24
Q

Define depression (infraduction)

A

The rotation of the eye down

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25
Q

What are torsions/cyclorotations?

A

Rotations around the sagittal axis (y-axis) in relation to a point at the 12-o’clock position on the superior limbus

Intorsion (incyclorotation): Rotation of that point nasally

Extortion (excyclorotation): Rotation of that point temporally

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26
Q

Why do torsional movements occur?

A

In an attempt to keep the horizontal retinal raphe parallel to the horizon

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27
Q

What are vergence movements?

A

When the eyes move in OPPOSITE left-right directions (DISCONJUGATE) to maintain fixation

  • Convergence: Each eye is adducted
  • Divergence: Each eye is abducted
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28
Q

What are version movements?

A

CONJUGATE movements that occur when the eyes move in the SAME direction.

  • Dextroversion
  • Levoversion
  • Supraversion
  • Infraversion
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29
Q

Dextroversion

A

A version movement.

Right gaze.

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30
Q

Levoversion

A

A version movement.

Left gaze.

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31
Q

Supraversion

A

A version movement.

Both eyes are elevated.

32
Q

Infraversion

A

A version movement.

Both eyes are depressed.

33
Q

List the EOMs from longest to shortest.

A

Superior rectus, medial rectus, lateral rectus, inferior rectus

34
Q

What is the common tendinous ring?

A

An oval band of connective tissue located at the apex of the orbit. It is origin of recti muscles (attachment site).

Forms a muscle cone!

35
Q

Describe the parts that make up the common tendinous ring.

A
  • Upper tendon (of Lockwood)
    > Arises from the body of the sphenoid
    > Origin to MR, LR, and SR
  • Lower tendon (of Zinn)
    > Attached to the inferior root of the lesser wing of the sphenoid
    > Origin to MR, LR, IR
36
Q

Recti muscles attach where in relation to the equator?

A

Anterior

37
Q

Obliques attach where in relation to the equator?

A

Posterior

38
Q

Superior and inferior recti are located ___ degrees from the visual axis.

A

23

39
Q

Superior and inferior obliques are located ___ degrees from the visual axis.

A

54

40
Q

What is the clinical importance of recti insertions?

A

Insertion points are a landmark during strabismuc surgery. The sclera is the thinnest at the site of recti insertions (think blunt trauma).

41
Q

True or False:

Obliques attach laterally.

A

True.

42
Q

What is the origin of the inferior oblique?

A

The maxillary bone

43
Q

Which EOMs does the superior (lateral) branch of the ophthalmic artery supply?

A
  • SR
  • LR
  • SO
  • LP (levator palpebrae)
44
Q

Which EOMs does the inferior (medial) branch of the ophthalmic artery supply?

A
  • IR
  • MR
  • IO
45
Q

The lacrimal artery contributes to which EOMs?

A
  • SR

- LR

46
Q

The supraorbital artery contributes to which EOMs?

A
  • SR
  • SO
  • LP
47
Q

The infraorbital artery contributes to which EOMs?

A
  • IR

- IO

48
Q

The arteries to the four rectus muscles gives rise to _____.

A

The anterior ciliary arteries

49
Q

How many anterior ciliary arteries emerge from each tendon?

A

2, except for the lateral rectus muscle which only has 1

50
Q

What is the vestibulo-ocular reflex (VOR)?

A
  • Reflex eye movement
  • Stabilizes image on the retina during head movements
  • Makes eye movements in the direction opposite to head movement
51
Q

Does vestibulo-ocular reflex only work when there is light present?

A

No, it works in total darkness or when eyes are closed, too.

52
Q

If there is sustained head rotation (i.e., > 30 sec), does the vestibulo-ocular reflex (VOR) continue to be responsive?

A

No

53
Q

Which system is active when there has been sustained head motion (i.e., >30 sec)

A

The optokinetic system (OKN)

54
Q

What does the optokinetic system (OKN) do?

A

It stabilizes image if there is long term head rotation.

55
Q

The OKN drum tests for ___ then ____.

A

Pursuit, saccade

56
Q

If a person cannot follow the OKN drum, what does this indicate?

A

If the patient cannot track, they may have decreased VA or parietal lobe lesion.

57
Q

Head movements < 30 seconds. Which vestibular control system is in play?

A

Vestibulo-ocular reflex (VOR)

58
Q

Head movements > 30 seconds. Which vestibular control system is in play?

A

Optokinetic system (OKN)

59
Q

What structure is involved with the vestibulo-ocular reflex (VOR)?

A

The inner ear

60
Q

What role does the inner ear have with the VOR?

A

It converts mechanical vibrations into a neural signal

  • It is innervated by CN VII
  • The vestibulocochlear organs maintain balance, receive sound, and contribute to eye reflex actions
61
Q

What is the bony labyrinth of the inner ear comprised of? What do these structures do?

A
  1. Cochlea: controls hearing
  2. Vestibule: ultricle and saccule for balance
  3. Semi-circular canal: ampullae
62
Q

The utricle and saccule together are called _____.

A

Olithic organs

63
Q

Which structures are responsible for linear acceleration (side-to-side)/linear vestibulo-ocular reflex?

A

The vestibule (utricle and saccule)

64
Q

Which structure is responsible for angular acceleration (rotational movements)/angular vestibulo-ocular reflex?

A

The semi-circular canal

65
Q

What are saccades?

A

Rapid eye movements to maintain fixation

66
Q

What are saccades controlled by? (*)

A
  1. Contralateral frontal eye fields in the frontal lobe

2. Superior colliculus

67
Q

If a patient has a defect in saccades, what area(s) do we suspect to have a problem?

A
  1. Contralateral frontal eye fields

2. Superior colliculus

68
Q

What are pursuits?

A

Smooth tracking to follow slow moving objects.

69
Q

What are pursuits controlled by? (*)

A

The ipsilateral parietal lobe

70
Q

If a patient cannot pursue your finger or track the OKN drum, what do we suspect?

A

There is an issue with the parietal lobe.

71
Q

What is the vergence system controlled by?

A

The brainstem

72
Q

Describe Hering’s Law of Equal Innervation

A

Agonist-Antagonist Relationship

  • Muscles work in pairs to move the eye; synergistic
  • YOKED muscles receive equal innervation. For example: LR and MR, SR and IO, SO and IR
73
Q

Describe Sherrington’s Law

A

Agonist and antagonist EOMs of the SAME EYE are reciprocally innervated.

74
Q

What is Grave’s Disease?

A

An autoimmune disorder characterized by the immune cells attacking the thyroid gland. This results in excessive secretion of hormones, increasing metabolism.

75
Q

What effect does Grave’s disease have on the eyes?

A

IgG causes inflammation of the EOMs (pleomorphic cellular infiltration).

  • The muscles become enlarged
  • Subsequent degeneration of muscle fibers (fibrosis)
76
Q

What is the end result of Grave’s disease on the eyes?

A

Myopathy and diplopia

Ocular Pathophysiology (4 decks)

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