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Neurology > Motor units and muscle spindles > Flashcards

Motor units and muscle spindles Flashcards

1
Q

Where can upper motor neurones be found?

A

Brain

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2
Q

Where can lower motor neurones be found?

A

Soma of brain stem and ventral horn of spinal cord

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3
Q

What is the relationship between upper and lower motor neurones?

A

UMN supply input to LMN to modulate their activity

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4
Q

From which sources do LMNs receive input?

A

UMN
Proprioception
Interneurons

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5
Q

What are the different types of LMNs?

A

Alpha motor neurones that innervate the bulk of fibres within a muscle that generate force
Gamma motor neurones that innervate a sensory organ within the muscle known as the muscle spindle

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6
Q

Where are the spinal cord enlargements?

A

Cervical (C3-T1)

Lumbar (L1-S3)

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7
Q

What is a motor unit?

A

An alpha motor unit and all of the skeletal muscle it innervates

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8
Q

What is a motor neurone pool?

A

The collection of alpha motor neurones that innervate a single named muscle

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9
Q

What determines the force of muscle contraction?

A

Frequency of action potential discharge of the alpha motor neurone
Recruitment of additional, synergistic motor units

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10
Q

Where can the cell bodies of LMNs be found?

A

Ventral horn

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11
Q

Where can LMNs that innervate axial muscles be found?

A

Medial to those innervating distal muscles

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12
Q

Where can MNs supplying flexors be found?

A

Dorsal to those supplying extensors

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13
Q

What are the sources of input to an alpha motor neurone?

A

Central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
UMNs in motor cortex and brain stem
Spinal interneurones

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14
Q

What does muscle strength depend on?

A

Activation of muscle fibres

Force production by innervated muscle fibres

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15
Q

What does activation of muscle fibres depend on?

A

Firing rate of LMNs (force of motor unit increases to a max as a function of LMN firing frequency)
Number of LMNs that are simultaneously active
Co-ordination of movement

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16
Q

What does the force production of innervated fibres depend on?

A

Fibre size - hypertrophy

Fibre phenotype - fast or slow contracting muscle

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17
Q

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

A

Twitch in muscle fibre

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18
Q

What does a sustained twitching of muscles result in?

A

Tetany

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19
Q

Describe the variable sizes of motor units

A

Small - extraocular eye muscles

Large - postural antigravity muscles

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20
Q

Describe the relationship between the size of a cell body of an alpha motor neurone and excitability

A

Smaller cell bodies are more excitable

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21
Q

What is each muscle fibre innervated by?

A

Single motor axon at endplate (neuromuscular junction) which is usually at the centre of the fibre

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22
Q

What is the difference between fast and slow muscle fibres?

A

Alpha motor neurones innervating fast type tend to be larger and have faster conductive axons than those of slow units

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23
Q

What determines a slow and fast twitch muscle?

A

Differs in how quickly myosin ATPase splits ATP to provide energy for the cross bridge formation
Expression of different myosin heavy chains

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24
Q

Describe a slow oxidative type 1 muscle fibre

A

ATP derived from oxidative phosphorylation
Slow contraction and relaxation
Fatigue resistant
Red fibres due to high myoglobin content

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25
Q

Describe a fast type 2a muscle fibre

A

ATP derived from oxidative phosphorylation
Fast contraction and relaxation
Fatigue resistant
Red and well vascularised

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26
Q

Describe a fast type 2b muscle fibre

A

ATP derived from glycolysis
Fast contraction but not fatigue resistant
Pale in colour and poorly vascularised
White

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27
Q

What are the 3 types of motor units?

A

Fast fatiguing
Fatigue resistant
Slow

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28
Q

Describe a fast fatiguing motor unit

A 
High tension 
Large alpha motor neurone 
High threshold 
Type 2b fibres
Utilised in burst power
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29
Q

Describe a fatigue resistant motor unit

A 
High tension 
Slow fatiguing 
Intermediate alpha motor neurone 
Intermediate threshold 
Type 2a fibres 
Utilised in sustained locomotion
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30
Q

Describe a slow motor unit

A 
Low tension 
Fatigue resistant
Small alpha motor neurone 
Low threshold 
Type 1 fibres 
Utilised in antigravity and sustained movement
31
Q

What is the henneman size principle?

A

Smaller alpha motor neurones (slow motor units) have a lower threshold than larger ones
Slow motor units are more easily activated and trained by any training that activates the muscle

32
Q

Describe what occurs with the activation of an UMN

A

LMN excited

33
Q

Describe muscle fibre recruitment

A

Motor units (LMN and muscle fibres) are recruited in order of size i.e. progressively increasing - small LMNs are more easily excited than large ones

34
Q

What does this activation in terms of increasing size of LMNs allow for?

A

Fine control of muscle force across a wide range of tensions developed

35
Q

Which will be recruited first, type 1 or type 2 fibres?

A

Slow type 1 before fast fatigue resistant type 2a which comes before type 2b
This results in increasing increments towards the maximal force the muscle exerts

36
Q

What detects changes in length and rate of change of a muscle?

A

Muscle spindles

37
Q

What is contained within the sensory organ the muscle spindle?

A

Fibrous capsule
Intrafusal muscle fibres
Sensory afferents; 1a class
Gamma motor neurone efferents which innervate intrafusal fibres

38
Q

Describe a 1a sensory afferent

A

Myelinated

Very fast conduction

39
Q

What do extrafusal fibres generate?

A

Muscle force

40
Q

Which neurotransmitter mediated the myotatic reflex?

41
Q

Which muscles is the monosynaptic myotatic reflex most prominent in?

A

Extensor muscles

42
Q

Which spinal levels mediates the biceps reflex?

43
Q

Which spinal level mediates the supinator reflex?

44
Q

Which spinal level mediates the triceps reflex?

45
Q

Which spinal levels mediate the quads (knee) reflex?

46
Q

Which spinal levels mediate the gastrocnemius (ankle) reflex?

47
Q

What are intrafusal fibres?

A

Non contractile equatorial regions innervated by 1a sensory afferents
Contractile polar ends that receive efferents from gamma motor neurones

48
Q

Where are gamma motor neurones cell bodies?

A

Ventral horn of spinal cord

49
Q

What will cause the intrafusal fibres of the muscle spindles to contract?

A

Stimulation of gamma motor neurones

50
Q

What are the different types of nuclear bag fibres?

A

Bag 1/dynamic: very sensitive to the rate of change of muscle length
Bag 2 / static: very sensitive to the absolute length of muscles

51
Q

What innervates bag 1 nuclear fibres?

A

Dynamic gamma motor neurones

52
Q

What innervates bag 2 nuclear fibres?

A

Static gamma motor neurones

53
Q

What are chain fibres?

A

Sensitive to absolute length

54
Q

What innervates chain fibres?

A

Static gamma motor neurones

55
Q

What are the 2 types of afferent fibres that innervate the intrafusal fibres?

56
Q

Which nerve ending will 1a afferents form?

A

Annulospiral nerve ending that winds around the centre of all intrafusal fibres

57
Q

Which nerve ending will 2 fibres form?

A

Flowerspray endings on all intrafusal fibres EXCEPT bag 1 dynamic type

58
Q

What will 1a fibres respond to?

A

Rate of change of muscle length (dynamic) and absolute length (steady state)

59
Q

What will stimulation of the static gamma fibre result in?

A

Steady state response

60
Q

What will stimulation of the dynamic amma fibre result in?

A

Dynamic response to stretch

61
Q

When will static gamma motor neurones be active?

A

In activities in which muscle length changes slowly and predictably

62
Q

When will dynamic gamma motor neurones be active?

A

In activities in which muscle length changes rapidly and unpredictably

63
Q

Where are golgi tendon organs found?

A

Junction of muscle and tendon

64
Q

What will golgi tendon organs monitor?

A

Changes in muscle tension

65
Q

Are golgi tendon organs in series or parallel to extrafusal fibres?

A

In series

Muscle spindles are parallel

66
Q

What innervates golgi tendon organs?

A

Group 1b sensory afferents

67
Q

What will golgi tendon organs protect the muscles from?

A

Overload e.g. weight lifting

Regulates muscle tension to an optimal range

68
Q

Where will group 1b afferents enter the spinal cord and synapse?

A

Upon inhibitory interneurons which will synapse on alpha motor neurones of homonymous muscle forming the reverse myotatic reflex

69
Q

Which reflex does the golgi tendon organ mediate?

A

Inverse myotatic reflex; tells a contracting muscle to relax

70
Q

Where can proprioceptive axons be found?

A

Connective tissue of joints (joint capsules and ligaments)

71
Q

What will proprioceptive axons respond to?

A

Changes in angle, direction and velocity of movement of a joint
Prevents excessive flexion and extension

72
Q

Where will proprioceptive information arise from?

A

Muscle spindles
Golgi tendon organs
Joint receptors

73
Q

Describe the golgi tendon pathway

A

Activation of 1b afferents from tendon organ
Excitation of inhibitory spinal interneuron
Inhibition of alpha motor neurone supplying homonymous muscle
Relaxation of muscle

74
Q

What does activation of gamma motor neurones along with alpha motor neurones during voluntary movement prevent?

A

Slackening as the body of the muscle contract due to alpha motor neurones activation - allowing continued signalling by the spindle

Neurology (48 decks)

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