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Before we discuss what the ‘all or none law’ is you must first understand what a motor unit is. A motor unit is made up of a single motor neuron and all the muscle fibres it innervates.
Motor units also vary in size, small motor units stimulate just a handful of fibres to do things that require low force production and fine movement control such as writing. Large motor units stimulate thousands of muscle fibres to do things that require a large force production such as deadlifting a heavy barbell. This is referred to as the innervation ratio i.e high force production may have an innervation ratio of 1:5000 (1 neuron, 5000 muscle fibres), low force production may have an innervation ratio of 1:50 (1 neuron, 50 muscle fibres. The important thing to remember is that is although the number if fibres vary there is always just one single neuron in a motor unit.
When an impulse is sent down a neuron, all the muscle fibres within a motor unit are activated (innervated). In other words the motor unit activates all of it’s muscle fibres or none at all.