Week 14: Motor Units and Muscle Fibre

Question 1

What is a motor unit?

Answer 1

smallest functional unit of the voluntary motor system
one alpha-motor neurone can can innovate a variable number of skeletal muscle fibres
neurone + fibres = motor unit

Question 2

What comprises the neural element of the motor system?

Answer 2

upper motor neurone (brain)

low motor neurone (soma within brain & ventral horn of brain)

Question 3

How is LMNs activity monitored?

Answer 3

UMNs

propioreceptors & interneurones

Question 4

What are propioreceptors?

Answer 4

provide info about the body in space

Question 5

What are interneurons?

Answer 5

form part of a reflex arc

Question 6

What is the role of LMNs?

Answer 6

command muscle contraction & form the final command pathway, no intervening axon

Question 7

What are alpha-motor neurones?

Answer 7

innervate the bulk of fibres within a muscle that generate force.

Question 8

What are gamma motor neurones?

Answer 8

innervates a sensory organ within the muscle known as a muscle spindle

Question 9

pairs of muscle work together in the arm?

Answer 9

• Biceps brachii and brachialis work together as synergists.
• Triceps brachii and anconeus are synergistic muscles
• Oppose each other

Question 10

What pair of muscles are flexors?

Answer 10

Biceps brachii and brachialis

Question 11

What pair of muscles are extensors?

Answer 11

Triceps brachii and anconeus

Question 12

How do LMNs exit the spinal cord?

Answer 12

ventral roots (or via cranial nerves)

Question 13

What is the dorsal root axon?

Answer 13

axons of sensory nerve fibres, entering cord

Question 14

What is the ventral root axon?

Answer 14

axons of motor neurones

Question 15

What does the ventral root join with?

Answer 15

Dorsal-root ganglion

forming a mixed spinal nerve

Question 16

What does the mixed spinal nerve?

Answer 16

30 pairs containing sensory and motor fibres

Question 17

What is a motor neurone pool?

Answer 17

The collection of α-MNs that innervate a single muscle

Question 18

How is the force of muscle contraction graded? What 2 mechanism?

Answer 18

• Frequency of action potential discharge of the α-MN

- The recruitment of additional motor units

Question 19

Do motor units have to be coordinated?

Answer 19

YES

motor neurone pool for one bicep?

Question 20

What distinct horn do the cell bodies LMNs have a distinct distribution of?

Answer 20

ventral (anterior) horn

Question 21

What 3 sources of input regulate alpha-MN activity?

Answer 21

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

Question 22

What is the axial and medial muscles?

Answer 22

bicep= axial
tricep= distal

Question 23

What is relationship between LMNs innervating axial muscle and distal muscle?

Answer 23

LMNs innervating axial muscles medial to those innervating distal muscles

Question 24

What are the 4 types of muscle fibre contraction from least forceful to most?

Answer 24

Twitch 5Hz
Wave summation 10Hz
Unfused tetanus 20 Hz
fused tetanus 40Hz

Question 25

What 3 things does muscle strength activation depend on?

Answer 25

- The firing rates of LMNs involved - The number of a LMNs that are simultaneously active - The co-ordination of the movement

Question 26

what 2 things does the force production by innervation muscle fibres depend on?

Answer 26

- Fibre size (hypertrophy) | - Fibre phenotype (“fast” or “slow” contracting muscle)

Question 27

Can the number of fibres a motor unit stimulates be different eh between X and Y?

Answer 27

YES e.g. X motor neurone effects 5 fibres (smaller tension) Y motor neurone effects 9 fibres (tension greater)

Question 28

What is the different effects of small and large muscle fibres?

Answer 28

- Small (few fibres) for fine. movements e.g. eye muscles | - Large(hundreds to thousands of fibres) in large postural (antigravity) muscles e.g. leg muscles

Question 29

What is the difference in size between small alpha-MNs?

Answer 29

Small motor units are innervated by small α-MNs (soma diameter) whereas the converse is true for large motor units

Question 30

What is the features of a aplha-MNs innervating fast type muscle fibres?

Answer 30

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

Question 31

Can a muscle type be both fast and slow type?

Answer 31

NO

Question 32

How many motor axons innervate a single muscle fibre?

Answer 32

- Each muscle fibre is innervated by a single motor at the endplate - Some extraocular muscle fibres have multiple endplates)

Question 33

What myosin heavy chains do slow oxidative and Fast fibres have?

Answer 33

slow: Type 1 fast: Type 2

Question 34

How is the ATP of slow oxidative formed?

Answer 34

oxidative phosphorylation

Question 35

How quickly is the conytraction and relaxation of Type 1 fibres?

Answer 35

SLOW

Question 36

What skeletal muscle fibres are fatigue resistant?

Answer 36

``` Slow oxidative (Type 1) Fast (Type iia) ```

Question 37

How is ATP of fast type muscle fibres produced?

Answer 37

oxidative phosphorylation.

Question 38

How quickly is the contraction and relaxation of Type iia fibres?

Answer 38

FAST

Question 39

What are the 2 types of fast type skeletal muscle fibres?

Answer 39

type iia | type iix

Question 40

Why is slow oxidative fibres dark in colour?

Answer 40

high myoglobin content

Question 41

How well vascularised are Type iia skeletal muscle fibres?

Answer 41

reasonably well vascularised

Question 42

Where is the ATP from type iix skeletal muscle fibre derived from?

Answer 42

glycolysis

Question 43

How well vascularised is Type IIx?

Answer 43

poorly vascularised

Question 44

How fast is type iia & iix contraction?

Answer 44

FAST

Question 45

Is Type iix fatigue resistant?

Answer 45

NO

Question 46

What fast skeletal muscle type is not present in mammals?

Answer 46

Type iib

Question 47

What motor units are most easily activated & trained by any muscle that activates the muscle?

Answer 47

slow motor units

Question 48

What order are alpha-MNs recruited?

Answer 48

by size allows dine and graded development of muscle | wider range of tensions developed

Question 49

What type of skeletal muscle fibre is recruited. first?

Answer 49

type 1 the iia then iix