Motor units and muscle spindles Flashcards Preview

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Flashcards in Motor units and muscle spindles Deck (74):
1

Where can upper motor neurones be found?

Brain

2

Where can lower motor neurones be found?

Soma of brain stem and ventral horn of spinal cord

3

What is the relationship between upper and lower motor neurones?

UMN supply input to LMN to modulate their activity

4

From which sources do LMNs receive input?

UMN
Proprioception
Interneurons

5

What are the different types of LMNs?

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

6

Where are the spinal cord enlargements?

Cervical (C3-T1)
Lumbar (L1-S3)

7

What is a motor unit?

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

8

What is a motor neurone pool?

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

9

What determines the force of muscle contraction?

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

10

Where can the cell bodies of LMNs be found?

Ventral horn

11

Where can LMNs that innervate axial muscles be found?

Medial to those innervating distal muscles

12

Where can MNs supplying flexors be found?

Dorsal to those supplying extensors

13

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

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

14

What does muscle strength depend on?

Activation of muscle fibres
Force production by innervated muscle fibres

15

What does activation of muscle fibres depend on?

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

16

What does the force production of innervated fibres depend on?

Fibre size - hypertrophy
Fibre phenotype - fast or slow contracting muscle

17

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

Twitch in muscle fibre

18

What does a sustained twitching of muscles result in?

Tetany

19

Describe the variable sizes of motor units

Small - extraocular eye muscles
Large - postural antigravity muscles

20

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

Smaller cell bodies are more excitable

21

What is each muscle fibre innervated by?

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

22

What is the difference between fast and slow muscle fibres?

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

23

What determines a slow and fast twitch muscle?

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

24

Describe a slow oxidative type 1 muscle fibre

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

25

Describe a fast type 2a muscle fibre

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

26

Describe a fast type 2b muscle fibre

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

27

What are the 3 types of motor units?

Fast fatiguing
Fatigue resistant
Slow

28

Describe a fast fatiguing motor unit

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

29

Describe a fatigue resistant motor unit

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

30

Describe a slow motor unit

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

31

What is the henneman size principle?

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

Describe what occurs with the activation of an UMN

LMN excited

33

Describe muscle fibre recruitment

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

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

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

35

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

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

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

Muscle spindles

37

What is contained within the sensory organ the muscle spindle?

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

38

Describe a 1a sensory afferent

Myelinated
Very fast conduction

39

What do extrafusal fibres generate?

Muscle force

40

Which neurotransmitter mediated the myotatic reflex?

Glutamate

41

Which muscles is the monosynaptic myotatic reflex most prominent in?

Extensor muscles

42

Which spinal levels mediates the biceps reflex?

C5-6

43

Which spinal level mediates the supinator reflex?

C5-6

44

Which spinal level mediates the triceps reflex?

C7

45

Which spinal levels mediate the quads (knee) reflex?

L3-4

46

Which spinal levels mediate the gastrocnemius (ankle) reflex?

S1

47

What are intrafusal fibres?

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

48

Where are gamma motor neurones cell bodies?

Ventral horn of spinal cord

49

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

Stimulation of gamma motor neurones

50

What are the different types of nuclear bag fibres?

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

What innervates bag 1 nuclear fibres?

Dynamic gamma motor neurones

52

What innervates bag 2 nuclear fibres?

Static gamma motor neurones

53

What are chain fibres?

Sensitive to absolute length

54

What innervates chain fibres?

Static gamma motor neurones

55

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

1a
2

56

Which nerve ending will 1a afferents form?

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

57

Which nerve ending will 2 fibres form?

Flowerspray endings on all intrafusal fibres EXCEPT bag 1 dynamic type

58

What will 1a fibres respond to?

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

59

What will stimulation of the static gamma fibre result in?

Steady state response

60

What will stimulation of the dynamic amma fibre result in?

Dynamic response to stretch

61

When will static gamma motor neurones be active?

In activities in which muscle length changes slowly and predictably

62

When will dynamic gamma motor neurones be active?

In activities in which muscle length changes rapidly and unpredictably

63

Where are golgi tendon organs found?

Junction of muscle and tendon

64

What will golgi tendon organs monitor?

Changes in muscle tension

65

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

In series
(Muscle spindles are parallel)

66

What innervates golgi tendon organs?

Group 1b sensory afferents

67

What will golgi tendon organs protect the muscles from?

Overload e.g. weight lifting
Regulates muscle tension to an optimal range

68

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

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

69

Which reflex does the golgi tendon organ mediate?

Inverse myotatic reflex; tells a contracting muscle to relax

70

Where can proprioceptive axons be found?

Connective tissue of joints (joint capsules and ligaments)

71

What will proprioceptive axons respond to?

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

72

Where will proprioceptive information arise from?

Muscle spindles
Golgi tendon organs
Joint receptors

73

Describe the golgi tendon pathway

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

74

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

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