motor unit

Imagine a single light switch wired to a particular set of bulbs in a hall. Flip that switch and exactly those bulbs come on, no others. A motor unit is the body's version: one nerve cell and all the muscle fibres it personally controls. When that nerve cell fires, every one of its fibres contracts together, all or nothing — that is the smallest amount of muscle you can deliberately turn on.

More precisely, a motor unit is one motor neuron (a nerve cell whose body sits in the spinal cord) plus the muscle fibres it reaches through its branching nerve fibre. The fibres of one unit may be scattered through a muscle, and one muscle contains many separate units. The body grades force in two ways: it switches on more units when more force is needed (recruitment), and it makes each active unit fire faster (rate coding). Small, precise muscles such as those that move the eye or fingers have units controlling only a handful of fibres each, giving fine control; big power muscles such as the calf have units controlling hundreds, trading precision for force.

The motor unit is the meeting point of nerve and muscle, which makes it central to rehabilitation. Strength is not only about muscle bulk — early gains from exercise often come from the nervous system learning to recruit more units and fire them better, before the muscle itself visibly grows. When the controlling nerve is injured (as in nerve damage or motor neuron disease), the muscle fibres lose their 'switch' and waste away even though the muscle tissue itself was healthy. This is also why the motor unit is the unit the electromyography needle is built to listen to.