Also known as the ‘all or none law’, the Level 2 and Level 3 syllabuses, for some reason, always want you to know this rather arcane piece of anatomical knowledge regarding how neurons work.
First of all, you need to know that a motor unit refers to a motor neuron and all the muscle fibres it connects to. You need to know this because the all or none principle refers to something that happens within a motor unit.
Put simply, the all or none principle states that when a neuron activates a muscle, all the muscle fibres it attaches to will contract or none of them will.
Hopefully this picture will help to explain:
Therefore, the axon terminals that connect to the muscle fibre will either make them all contract together, or none of them will contract. There’s no situation where a few of them contract and the others don’t.
(And if you think this seems like an oddly specific and excessively scientific thing for a personal trainer to know, you’re almost certainly right. But, as we said, it almost always crops up in the exams!)
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Functional Fitness PTS 