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Neurology > Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors > Flashcards

Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors Flashcards

1
Q

What does the somatic motor system consist of?

A

The skeletal muscles and the elements of the nervous system that control them

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2
Q

What do the neural elements of the somatic motor system comprise of?

A

The UPPER MOTOR NEURONES (UMN’S)  within the brain.
+
The LOWER MOTOR NEURONES (LMN’s)  with soma within brainstem and ventral horn of the spinal cord.

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3
Q

What do UMN’s do?

A

Supply input to LMN’s to modulate their activity

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4
Q

Aside from UMN’s, what else do LMN’s receive input fro,?

A

Proprioceptors
AND
Interneurones

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5
Q

What do LMN’s do?

A

Command muscle contraction and form the ‘final common pathway’

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6
Q

Name 2 components of LMN’s and state what they do.

A

Alpha Motor Neurones – innervate the bulk of fibres within a muscle that generate force.

Gamma Motor Neurones – innervate a sensory organ within the muscle known as a MUSCLE SPINDLE.

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7
Q

What innervates the muscle spindle?

A

Gamma Motor Neurones

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8
Q

Biceps brachia and brachiallis work together as ……….

A

SYNERGISTS

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9
Q

What do axial muscles control?

A

Movements of the trunk (maintain posture)

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10
Q

Where are proximal muscles found?

A

In the shoulder, elbow, pelvis and knee

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11
Q

What do distal muscles do?

A

Move the hands, feet and digits

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12
Q

What do axons on LMN’s exit the spinal cord in?

A

Ventral roots of via cranial nerves

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13
Q

What do each ventral (anterior) and dorsal (posterior) root join together to form? What types of fibre do these contain?

A

A MIXED spinal nerve

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14
Q

What does a mixed spinal nerve contain?

A

Sensory AND motor fibres

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15
Q

Name the 2 enlargements of the spinal cord.

A

Cervical

Lumbar

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16
Q

What is the level of the cervical enlargement?

A

C3-T1

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17
Q

What is the level of the lumbar enlargement?

A

L1-S3

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18
Q

What is contained within each enlargement?

A

Large numbers of motor neurones

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19
Q

What does the cervical enlargement supply?

A

ARM

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20
Q

What does the lumbar enlargement supply?

A

LEG

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21
Q

Where are the motor neurones that innervate distal and proximal musculature mainly found?

A

In the cervical and lumbar-sacral segments of the spinal cord

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22
Q

Where do the motor neurones innervating the axial musculature occur?

A

AT ALL LEVELS

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23
Q

What is a ‘motor unit’?

A

An α-MN and all of the skeletal muscle fibres that it innervates

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24
Q

A motor unit is the SMALLEST functional component of the motor system

A

TRUE

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25
Q

In terms of motor units, what does muscle contraction result from?

A

The individual and combined actions of the motor units, which must be co-ordinated

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26
Q

What is the motor neurone pool?

A

The collection of alpha motor neurones that innervate a single muscle

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27
Q

The force of muscle contraction is graded by alpha motor neurones by two principle mechanisms. What are these?

A
  1. Frequency of action potential discharge of the α-MN ie. the faster the AP freq, the greater the force of contraction.(note: each action potential causes a muscle ‘twitch’ (rapid sequence of contraction followed by relaxation)
  2. The recruitment of additional, synergistic, motor units.
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28
Q

The cell bodies of LMN’s show a distinct (somatotopic) distribution in the ventral (anterior) horn. Describe this.

A
  • LMN’s innervating axial muscles MEDIAL to those innervating distal muscles.
  • LMN’s innervating flexors DORSAL to those supplying extensors.
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29
Q

LMN’s innervating axial muscles are LATERAL to those innervating distal muscles

A

FALSE - medial

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30
Q

There are only ___ sources of input to alpha motor neurones to regulate its activity

A

3

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31
Q

Name the 3 sources of input to alpha motor neurones.

A
  • Central terminals of DORSAL ROOT GANGLION CELLS whose axons innervate the muscle spindles.
  • UMN’s in the motor cortex and brain stem.
  • SPINAL INTERNEURONES.
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32
Q

What 2 factors does muscle strength depend on?

A
  1. Neuromuscular Activation.

2. Force production by innervated muscle fibres.

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33
Q

What does neuromuscular activation depend on?

A
  • Firing rates of LMN’s involved
  • The number of LMN’s that innervate a muscle
  • The co-ordination of the movement
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34
Q

What does force production by innervated muscle spindles depend on?

A
  • Fibre size (hypertrophy)

* Fibre phenotype (fast or slow contracting)

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35
Q

What does a single action potential in an alpha motor neurone cause?

A

A muscle fibre to twitch

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36
Q

What does the summation of muscle fibre twitches cause?

A

A sustained contraction as the number of incoming action potentials increases

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37
Q

Motor units are of variable size

A

TRUE

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38
Q

What are small motor units of few fibres used for?

A

Fine movements ie. extraocular eye muscles

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39
Q

What are large motor units of hundreds of fibres used for?

A

Large postural (antigravity) muscles e.g leg muscles

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40
Q

Small motor units are innervated by small alpha-motor neurones (soma diameter), whereas the converse is true for large motor units

A

U got that right gurlllll

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41
Q

What is the force of contraction that each motor unit produces dependant on?

42
Q

Compare the alpha-motor neurones innervating the fast type to those innervating the slow units.

A

They tend to be larger and have faster conducting axons.

43
Q

Fast and slow type fibres contain both types of fibres

A

FALSE - only contain one type

44
Q

What is each muscle fibre innervated by? Where?

A

A single motor axon at the endplate (NMJ) which is usually at the centre of the fibre

45
Q

What are the 2 major skeletal muscle types?

A

Fast and Slow

46
Q

How do fast and slow muscle types differ?

A

Differ in how fast myosin ATPase splits ATP (different myosin ATPase isoforms and activities) to provide energy for cross-bridge cycling.

47
Q

Describe slow oxidative (type I fibres)

A
  • ATP derived from oxidative phosphorylation
  • Contraction and relaxation is SLOW
  • RESISTANT TO FATIGUE
  • DARK red - due to high myoglobin content
48
Q

Describe fast type IIa fibres.

A
  • ATP derived from oxidative phosphorylation
  • Contraction and relaxation is FAST
  • Red - well vascularised
49
Q

Describe slow oxidative (type I fibres)

A
  • ATP derived from oxidative phosphorylation
  • Contraction and relaxation is SLOW
  • RESISTANT TO FATIGUE
  • DARK red - due to high myoglobin content
  • Low tension
  • Slow and low aMN threshold
50
Q

Describe fast type IIa fibres.

A
  • ATP derived from oxidative phosphorylation
  • Contraction and relaxation is FAST
  • Red - well vascularised
  • Fatigue resistant
  • High tension
  • Intermediate aMN threshold
51
Q

Describe fast type IIb/x fibres.

A
  • ATP derived from glycolysis
  • FAST contraction
  • NOT fatigue resistant
  • Pale - not well vascularised
  • VERY high tension
  • Large aMN, high threshold
52
Q

Slower, smaller motor units are recruited last

A

FALSE - first

53
Q

Do smaller alpha-motor neurones have a higher or lower threshold than larger ones?

54
Q

What type of motor unit is more easily activated and ‘trained’?

A

SLOW motor units

55
Q

What are smaller aMN’s part of?

A

SLOW motor units

56
Q

What are large aMN’s part of?

A

Fatigue-resistant, or fast fatiguing, motor units

57
Q

aMN’s are recruited by size, what does this allow for?

A

The fine and graded development of muscle force

58
Q

What does ‘myotatic reflex’ mean?

A

When a skeletal muscle is pulled, it pulls back

59
Q

What is the change in length (+ rate of change) of a muscle registered by?

A

The muscle spindle

60
Q

What do spindles consist of?

A
  • A fibrous capsule
  • Intrafusal muscle fibres
  • Sensory afferents
  • Gamma motor neurone efferents
61
Q

What do EXTRAfusal fibres do?

A

Generate force

62
Q

The striking of the patellar tendon with a tendon hammer causes a ‘knee jerk.’ What is this knee jerk underpinned by?

A

Myotatic reflex

63
Q

What can the myotatic reflex be used to assess?

A

The functional integrity of the spinal cord at specific levels

64
Q

Supinator/wrist?

65
Q

Biceps/elbow?

66
Q

Triceps/elbow?

67
Q

Quads/knee?

68
Q

Gastrocnemius/ankle?

69
Q

What do intrafusal fibres consist of?

A
  • A non-contractile equatorial region

* Contractile polar ends

70
Q

What is the non-contractile equatorial region of intrafusal fibres innervated by

A

The Ia sensory neurones

71
Q

What do the contractile polar ends of intrafusal fibres receive efferent input from?

72
Q

Where are the cell bodies of these gamma neurones located? What are these driven by?

A

In the ventral horn of the spinal cord

These are driven by higher centres, NOT Ia afferents

73
Q

What causes the spindle to contract?

A

Stimulation of y MN’s

74
Q

What are normally co-activated by higher centres in voluntary movement?

A

a MN’s and y MN’s

75
Q

What is the effect of co-ordination of a MN’s and y MN’s?

A

Means that intrafusal muscle fibres contract in parallel with the extrafusal fibres

76
Q

What are type 1 dynamic nuclear bag fibres sensitive to?

A

The RATE OF CHANGE of muscle length

77
Q

What are type 1 dynamic nuclear bag fibres innervated by?

A

Dynamic, but not static, gamma motor neurones

78
Q

What are type 2 static nuclear bag fibres sensitive to?

A

The ABSOLUTE length of the muscle

79
Q

What are type 2 static nuclear bag fibres innervated by?

A

Static gamma motor neurones.

80
Q

What are chain fibres sensitive to?

A

The absolute length of the muscle

81
Q

What are chain fibres innervated by?

A

Static gamma motor neurones

82
Q

What 2 types of afferent fibre innervate the intrafusal fibre?

A

Ia afferents

II fibres

83
Q

What do the Ia afferents form?

A

A primary annulospiral nerve ending winding around the centre of all the intrafusal fibres

84
Q

What do II fibres, which are more slowly conducting, form?

A

Flowerspray endings on all intrafusal fibres, except the bag 1 dynamic type

85
Q

What do both Ia and II respond to?

86
Q

What do Ia fibres respond to?

A

Both the rate of change of muscle length (dynamic response) and absolute length (steady state, or static response) of the muscle

87
Q

What does the stimulation of static gamma fibres cause?

A

Steady state, or static response, to increase

88
Q

What does the stimulation of dynamic gamma fibre enhance?

A

The dynamic response to stretch

89
Q

In activities in which muscle length changes slowly and predictably, only what are active?

A

Static gamma motor neurones

90
Q

When are dynamic gamma motor neurones active?

A

During behaviours in which muscle length changes rapidly and unpredictably.

91
Q

Where are golgi tendon organs located?

A

At the junction of the muscle and the tendon

92
Q

What do golgi tendon organs monitor changes in?

A

Muscle tension

93
Q

Golgi tendon organs are IN SERIES with, rather than PARALLEL to, extrafusal fibres

94
Q

What are golgi tendon organs innervated by?

A

Group 1b sensory afferents (myelinated, slightly slower conducting than Ia fibres)

95
Q

Golgi tendon bodies….

A

Act to regulate muscle tension to:
protect muscle from overload (in extreme circumstances – e.g. weight lifting)
more generally regulate muscle tension to an optimal range

96
Q

Once group Ib efferents enter the spinal cord they synapse upon what?

A

Inhibitory interneurones

97
Q

What do inhibitory interneurones then synapse on?

A

The alpha motor neurones of the HOMONYMOUS muscle forming the basis of the REVERSE MYOTATIC REFLEX

98
Q

What happens in a POLYSYNAPTIC PATHWAY?

A

An inhibitory interneurone is interposed between the Ib afferent and alpha motor

99
Q

How do golgi tendon organs gauge the force generated by a muscle?

A

By measuring the tension in its tendon

100
Q

Where are proprioceptive axons present?

A

In the connective tissue of joints ie. joint capsules and ligaments

101
Q

What do proprioceptive axons do?

A
  • Respond to changes in angle, direction and velocity of movement of a joint.
  • Also prevent excessive flexion, or extension
102
Q

What does proprioceptive information arise from?

A
  • Muscle spindles
  • Golgi tendon organs
  • Joint receptors

Neurology (42 decks)

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