There are many sources for information on the motor unit. In Enoka's textbook, the motor unit and its characteristics are described on pages 278-280. Enoka states "A motor unit is defined as the cell body and dendrites of a motor neuron, the multiple branches of its axon, and the muscle fibres that it innervates." In the simple drawing below, there are two motor units, a blue and a green. The blue motor unit innervates 3 muscle fibres whereas the green motor unit innervates 4 muscle fibres.

We know that nerves can transmit electrical signals according to the all-or-none principle. When we record the electrical activity associated with contracting muscle, this electrical signal in the nerve (A) arrives at the motor enplate (B) as is transfered to the sarclemma (C) and the subsequent release of calcium results in muscle contraction (D). This is shown schematically in the image below.