Motor units and muscle spindles Flashcards
Where can upper motor neurones be found?
Brain
Where can lower motor neurones be found?
Soma of brain stem and ventral horn of spinal cord
What is the relationship between upper and lower motor neurones?
UMN supply input to LMN to modulate their activity
From which sources do LMNs receive input?
UMN
Proprioception
Interneurons
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
Where are the spinal cord enlargements?
Cervical (C3-T1)
Lumbar (L1-S3)
What is a motor unit?
An alpha motor unit and all of the skeletal muscle it innervates
What is a motor neurone pool?
The collection of alpha motor neurones that innervate a single named muscle
What determines the force of muscle contraction?
Frequency of action potential discharge of the alpha motor neurone
Recruitment of additional, synergistic motor units
Where can the cell bodies of LMNs be found?
Ventral horn
Where can LMNs that innervate axial muscles be found?
Medial to those innervating distal muscles
Where can MNs supplying flexors be found?
Dorsal to those supplying extensors
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
What does muscle strength depend on?
Activation of muscle fibres
Force production by innervated muscle fibres
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
What does the force production of innervated fibres depend on?
Fibre size - hypertrophy
Fibre phenotype - fast or slow contracting muscle
What does a single action potential in an alpha motor neurone result in?
Twitch in muscle fibre
What does a sustained twitching of muscles result in?
Tetany
Describe the variable sizes of motor units
Small - extraocular eye muscles
Large - postural antigravity muscles
Describe the relationship between the size of a cell body of an alpha motor neurone and excitability
Smaller cell bodies are more excitable
What is each muscle fibre innervated by?
Single motor axon at endplate (neuromuscular junction) which is usually at the centre of the fibre
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
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
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
Describe a fast type 2a muscle fibre
ATP derived from oxidative phosphorylation
Fast contraction and relaxation
Fatigue resistant
Red and well vascularised
Describe a fast type 2b muscle fibre
ATP derived from glycolysis
Fast contraction but not fatigue resistant
Pale in colour and poorly vascularised
White
What are the 3 types of motor units?
Fast fatiguing
Fatigue resistant
Slow
Describe a fast fatiguing motor unit
High tension Large alpha motor neurone High threshold Type 2b fibres Utilised in burst power
Describe a fatigue resistant motor unit
High tension Slow fatiguing Intermediate alpha motor neurone Intermediate threshold Type 2a fibres Utilised in sustained locomotion