Motor units and recruitment

Motor units, Henneman's size principle, recruitment and firing rate: how muscle grades its force.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Motor unit: what is it?
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One motor neuron and all the muscle fibres it supplies.
Its fibres are all of the same type.
To lift a glass then a pack of water, how does the muscle increase force?
- It recruits more motor units and raises their firing rate
- It changes fibre type
- It lengthens its sarcomeres
- It recruits its large units before the small ones
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It recruits more motor units and raises their firing rate
Recruitment and rate grade force.
Size principle: what is it?
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Small motor units are recruited before large ones as required force rises (Henneman).
Type I first, IIx last.
Why are IIx fibres little used in an easy walk?
- Their smaller motor neuron is less excitable
- Their large units are recruited only for high forces
- They only contract eccentrically
- They are too slow to keep up with the walking pace
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Their large units are recruited only for high forces
That is the size principle: the large motor neurons of IIx fibres have the highest activation threshold.
Spatial recruitment: what is it?
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Raising force by activating more motor units.
The first way to grade force.