Week 14: Motor Units and Muscle Fibre Flashcards
What is a motor unit?
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
What comprises the neural element of the motor system?
upper motor neurone (brain)
low motor neurone (soma within brain & ventral horn of brain)
How is LMNs activity monitored?
UMNs
propioreceptors & interneurones
What are propioreceptors?
provide info about the body in space
What are interneurons?
form part of a reflex arc
What is the role of LMNs?
command muscle contraction & form the final command pathway, no intervening axon
What are alpha-motor neurones?
innervate the bulk of fibres within a muscle that generate force.
What are gamma motor neurones?
innervates a sensory organ within the muscle known as a muscle spindle
pairs of muscle work together in the arm?
- Biceps brachii and brachialis work together as synergists.
- Triceps brachii and anconeus are synergistic muscles
- Oppose each other
What pair of muscles are flexors?
Biceps brachii and brachialis
What pair of muscles are extensors?
Triceps brachii and anconeus
How do LMNs exit the spinal cord?
ventral roots (or via cranial nerves)
What is the dorsal root axon?
axons of sensory nerve fibres, entering cord
What is the ventral root axon?
axons of motor neurones
What does the ventral root join with?
Dorsal-root ganglion
forming a mixed spinal nerve
What does the mixed spinal nerve?
30 pairs containing sensory and motor fibres
What is a motor neurone pool?
The collection of α-MNs that innervate a single muscle
How is the force of muscle contraction graded? What 2 mechanism?
- Frequency of action potential discharge of the α-MN
- The recruitment of additional motor units
Do motor units have to be coordinated?
YES
motor neurone pool for one bicep?
What distinct horn do the cell bodies LMNs have a distinct distribution of?
ventral (anterior) horn
What 3 sources of input regulate alpha-MN activity?
- Central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
- UMNs in the motor cortex and brain stem
- Spinal interneurones
What is the axial and medial muscles?
bicep= axial tricep= distal
What is relationship between LMNs innervating axial muscle and distal muscle?
LMNs innervating axial muscles medial to those innervating distal muscles
What are the 4 types of muscle fibre contraction from least forceful to most?
Twitch 5Hz
Wave summation 10Hz
Unfused tetanus 20 Hz
fused tetanus 40Hz
What 3 things does muscle strength activation depend on?
- The firing rates of LMNs involved
- The number of a LMNs that are simultaneously active
- The co-ordination of the movement
what 2 things does the force production by innervation muscle fibres depend on?
- Fibre size (hypertrophy)
- Fibre phenotype (“fast” or “slow” contracting muscle)
Can the number of fibres a motor unit stimulates be different eh between X and Y?
YES
e.g. X motor neurone effects 5 fibres (smaller tension)
Y motor neurone effects 9 fibres (tension greater)
What is the different effects of small and large muscle fibres?
- 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
What is the difference in size between small alpha-MNs?
Small motor units are innervated by small α-MNs (soma diameter) whereas the converse is true for large motor units
What is the features of a aplha-MNs innervating fast type muscle fibres?
tend to be larger and have faster conducting axons than those of slow units
Can a muscle type be both fast and slow type?
NO
How many motor axons innervate a single muscle fibre?
- Each muscle fibre is innervated by a single motor at the endplate
- Some extraocular muscle fibres have multiple endplates)
What myosin heavy chains do slow oxidative and Fast fibres have?
slow: Type 1
fast: Type 2
How is the ATP of slow oxidative formed?
oxidative phosphorylation
How quickly is the conytraction and relaxation of Type 1 fibres?
SLOW
What skeletal muscle fibres are fatigue resistant?
Slow oxidative (Type 1) Fast (Type iia)
How is ATP of fast type muscle fibres produced?
oxidative phosphorylation.
How quickly is the contraction and relaxation of Type iia fibres?
FAST
What are the 2 types of fast type skeletal muscle fibres?
type iia
type iix
Why is slow oxidative fibres dark in colour?
high myoglobin content
How well vascularised are Type iia skeletal muscle fibres?
reasonably well vascularised
Where is the ATP from type iix skeletal muscle fibre derived from?
glycolysis
How well vascularised is Type IIx?
poorly vascularised
How fast is type iia & iix contraction?
FAST
Is Type iix fatigue resistant?
NO
What fast skeletal muscle type is not present in mammals?
Type iib
What motor units are most easily activated & trained by any muscle that activates the muscle?
slow motor units
What order are alpha-MNs recruited?
by size allows dine and graded development of muscle
wider range of tensions developed
What type of skeletal muscle fibre is recruited. first?
type 1 the iia then iix
