Motor Units and Control of Muscle Force

Question 1

What does the somatic motor system consist of?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

What is the function of upper motor neurons?

Answer 3

Supply input to LMNs to modulate their activity

Question 4

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

Answer 4

Proprioceptors and interneurons

Question 5

What is the function of lower motor neurons?

Answer 5

Command muscle contraction and form the final common pathway

Question 6

What do lower motor neurons consist of?

Answer 6

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

Question 7

What do gamma motor neurons innervate?

Answer 7

A sensory organ within the muscle known as a muscle spindle

Question 8

Where do axons of LMN exit the spinal cord?

Answer 8

In ventral roots or via cranial nerves

Question 9

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

Answer 9

A mixed spinal nerve = contains both sensory and motor fibres

Question 10

What do motor neurons belong to?

Answer 10

A spinal segment (e.g C8)

Question 11

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

Answer 11

Cervical enlargement = C3-T1, to supply upper limb

Lumbar enlargement = L1-S3, to supply lower limb

Question 12

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

Answer 12

The cervical and lumbo-sacral segments

Question 13

Where are motor neurons that innervate axial musculature located?

Answer 13

Occur at all levels

Question 14

What does a motor unit consist of?

Answer 14

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

Question 15

What does muscle contraction result from?

Answer 15

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

Question 16

What is a motor neuron pool?

Answer 16

The collection of alpha motor neurons that innervate a single muscle

Question 17

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

Answer 17

Frequency of AP discharge of the alpha motor neuron

The recruitment of additional synergistic motor units

Question 18

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

Answer 18

They are medial to those innervating distal muscles

Question 19

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

Answer 19

Dorsal to those supplying extensors

Question 20

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

Answer 20

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

Question 21

What does muscle strength depend on?

Answer 21

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

Question 22

What does the activation of muscle fibres depend?

Answer 22

The firing rates of the LMNs involved

The co-ordination of the movement

Question 23

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

Answer 23

Makes a muscle fibre twitch

Question 24

What does the summation of twitches cause?

Answer 24

A sustained contraction as the number of incoming APs increases

Question 25

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

Answer 25

Small size for fine movements | Large size for large postural muscles

Question 26

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

Answer 26

Yes = small units innervated by small alpha motor neurons

Question 27

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

Answer 27

The size of the motor unit

Question 28

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

Answer 28

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

Question 29

What is each muscle fibre innervated by at the endplate?

Answer 29

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

Question 30

How do skeletal muscle fibres differ?

Answer 30

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

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What is the Henneman Size Principle?

Answer 34

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

Question 35

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

Answer 35

Smaller alpha motor neurons have a lower threshold than larger ones

Question 36

Are slow motor units more easily activated?

Answer 36

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

Question 37

What does recruitment of alpha motor neurons by size allow?

Answer 37

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

Question 38

What does the activation of an UMN cause?

Answer 38

Causes the LMN it supplies to be activated

Question 39

How are motor units recruited?

Answer 39

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

Question 40

How are LMNs recruited?

Answer 40

In an order appropriate to the physical task being performed

Question 41

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

Answer 41

Results in increasing increments towards the maximal force the muscle exerts

Question 42

What is the myotatic reflex?

Answer 42

When a skeletal muscle is pulled, it pulls back

Question 43

What registers non-conscious proprioception?

Answer 43

Muscle spindle = registers change in length

Question 44

What do spindles consist of?

Answer 44

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

Question 45

What supplies intrafusal fibres?

Answer 45

Sensory afferents and gamma motor neurons

Question 46

What is the only monosynaptic reflex in humans?

Answer 46

The myotatic reflex = most prominent in extensor muscles

Question 47

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

Answer 47

Spindles contract

Question 48

What do intrafusal fibres consist of?

Answer 48

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

Question 49

What occurs during voluntary movement?

Answer 49

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

Question 50

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

Answer 50

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