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Neurology > Motor Units and Control of Muscle Force > Flashcards

Motor Units and Control of Muscle Force Flashcards

1
Q

What does the somatic motor system consist of?

A

Skeletal muscles and the elements of the nervous system that control them

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2
Q

What does the neural element of the somatic motor system comprise of?

A

Upper motor neurons within the brain and lower motor neurons within the brainstem of the ventral horn of the spinal cord

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3
Q

What is the function of upper motor neurons?

A

Supply input to LMNs to modulate their activity

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4
Q

What do lower motor neurons receive input from, other than UMN?

A

Proprioceptors and interneurons

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5
Q

What is the function of lower motor neurons?

A

Command muscle contraction and form the final common pathway

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6
Q

What do lower motor neurons consist of?

A

An alpha motor neuron that innervates the bulk of the fibres within a muscle that generates force

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7
Q

What do gamma motor neurons innervate?

A

A sensory organ within the muscle known as a muscle spindle

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8
Q

Where do axons of LMN exit the spinal cord?

A

In ventral roots or via cranial nerves

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9
Q

What does the joining of each ventral root to a dorsal root form?

A

A mixed spinal nerve = contains both sensory and motor fibres

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10
Q

What do motor neurons belong to?

A

A spinal segment (e.g C8)

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11
Q

What areas of the spine have a greater number of motor neurons?

A

Cervical enlargement = C3-T1, to supply upper limb

Lumbar enlargement = L1-S3, to supply lower limb

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12
Q

Where are motor neurons that innervate the distal and proximal musculature located?

A

The cervical and lumbo-sacral segments

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13
Q

Where are motor neurons that innervate axial musculature located?

A

Occur at all levels

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14
Q

What does a motor unit consist of?

A

An alpha motor neuron and all of the skeletal muscle fibres that it innervates

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15
Q

What does muscle contraction result from?

A

Individual and combined actions of motor units which must be co-ordinated

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16
Q

What is a motor neuron pool?

A

The collection of alpha motor neurons that innervate a single muscle

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17
Q

How is force of muscle contraction graded by alpha motor neurons?

A

Frequency of AP discharge of the alpha motor neuron

The recruitment of additional synergistic motor units

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18
Q

How are the cell bodies of LMNs innervating axial muscles distributed in the ventral horn?

A

They are medial to those innervating distal muscles

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19
Q

How are the cell bodies of LMNs supplying flexors arranged in the ventral horn?

A

Dorsal to those supplying extensors

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20
Q

What are the three sources of input to an alpha motor neuron that regulate its activity?

A

Central terminal of dorsal root ganglion cells whose axons innervate the muscle spindles
UMNs in the motor cortex and brainstem
Spinal interneurons

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21
Q

What does muscle strength depend on?

A

Activation of muscle fibres and force production by innervated muscle fibres (fibre size and phenotype)

22
Q

What does the activation of muscle fibres depend?

A

The firing rates of the LMNs involved

The co-ordination of the movement

23
Q

What does a single AP in an alpha motor neuron cause?

A

Makes a muscle fibre twitch

24
Q

What does the summation of twitches cause?

A

A sustained contraction as the number of incoming APs increases

25
Q

How does the size of motor units impact the function of the muscle?

A

Small size for fine movements

Large size for large postural muscles

26
Q

Does the size of the motor unit impact the size of the alpha motor unit which innervates it?

A

Yes = small units innervated by small alpha motor neurons

27
Q

What does force of contraction that each motor unit produces depend on?

A

The size of the motor unit

28
Q

What are the features of alpha motor neurons innervating fast type motor units?

A

Tend to be larger and have faster conducting axons than those of slow units

29
Q

What is each muscle fibre innervated by at the endplate?

A

A single motor neuron = usually at the centre of the fibre

30
Q

How do skeletal muscle fibres differ?

A

Differ in how quickly myosin ATPase splits ATP provide energy for cross bridge cycling and express different myosin heavy chains

31
Q

What are the features of slow oxidative (type 1) fibres?

A

ATP largely derived from oxidative phosphorylation
Slow contraction and relaxation, and fatigue resistant
Red fibres because of high myoglobin content
Used for antigravity sustained movement

32
Q

What are the features of fast (type IIa) fibres?

A

ATP largely derived from oxidative phosphorylation
Fast contraction and relaxation, and fatigue resistant
Red and reasonably well vascularised
Used for sustained locomotion

33
Q

What are the features of fast (type IIb) fibres?

A

ATP mainly derived from glycolysis
Fast contraction but not fatigue resistant
Pale in colour and poorly vascularised
Used for burst power

34
Q

What is the Henneman Size Principle?

A

The susceptibility of an alpha motor neuron to discharge APs is a function of its size

35
Q

How does the size of the alpha motor neuron affect its threshold?

A

Smaller alpha motor neurons have a lower threshold than larger ones

36
Q

Are slow motor units more easily activated?

A

Yes = more easily activated and trained by any activity which activates the muscle

37
Q

What does recruitment of alpha motor neurons by size allow?

A

Fine and graded development of muscle force = allows for fine control over a wide range of tensions

38
Q

What does the activation of an UMN cause?

A

Causes the LMN it supplies to be activated

39
Q

How are motor units recruited?

A

In order of their size = small LMNs are more easily excited than large LMNs

40
Q

How are LMNs recruited?

A

In an order appropriate to the physical task being performed

41
Q

What does recruiting motor neurons in order of slow before fast allow?

A

Results in increasing increments towards the maximal force the muscle exerts

42
Q

What is the myotatic reflex?

A

When a skeletal muscle is pulled, it pulls back

43
Q

What registers non-conscious proprioception?

A

Muscle spindle = registers change in length

44
Q

What do spindles consist of?

A

Fibrous capsule, intrafusal muscle fibres, sensory afferents, gamma motor neurons

45
Q

What supplies intrafusal fibres?

A

Sensory afferents and gamma motor neurons

46
Q

What is the only monosynaptic reflex in humans?

A

The myotatic reflex = most prominent in extensor muscles

47
Q

What does stimulation of intrafusal fibres by gamma motor neurons cause?

A

Spindles contract

48
Q

What do intrafusal fibres consist of?

A

Contractile polar ends = receive efferent input from gamma motor neurons with cell bodies in the ventral horn
Non-contractile equatorial region innervated by Ia sensory neurons

49
Q

What occurs during voluntary movement?

A

Alpha and gamma motor neurons normally co-activated so that intrafusal fibres contract in parallel with extrafusal fibres

50
Q

What is the purpose of co-activation of gamma and alpha motor fibres during voluntary movement?

A

Maintains sensitivity of the spindle to stop it going slack when extrafusal fibres contract

Neurology (48 decks)

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