A motor unit consists of one somatic motor neuron and all skeletal muscle fibers innervated by that neuron's axon branches.

One muscle contains many motor units. When a motor neuron fires, its action potentials reach all fibers in its motor unit and can activate those fibers nearly together.

Motor-unit size trades precision for force

A motor neuron can innervate relatively few fibers or many fibers.

• Small motor units allow fine increments of force because activating one additional neuron adds only a small number of fibers.
• Large motor units add more force per recruited neuron and are useful where precise control is less important.

The number of muscle fibers in a motor unit is therefore a property of the neural organization of a muscle, not the number of fibers that happen to contract on one occasion.

Recruitment grades whole-muscle force

The nervous system can increase force by activating additional motor units. This is motor-unit recruitment.

few active motor units  → low whole-muscle force
more active motor units → greater whole-muscle force

In many movements, motor units are recruited in a broadly ordered pattern from lower-threshold, typically smaller units toward higher-threshold, typically larger units as force demand rises. This tendency is called the size principle.

Recruitment and firing rate are distinct control mechanisms

Whole-muscle force can be increased in two different ways:

1. recruitment: activate more motor units;
2. rate coding: increase action-potential frequency in already active motor neurons, increasing twitch summation in their fibers.

These mechanisms can operate together.

Example

Suppose a simplified muscle has four motor units capable of contributing approximately 5, 10, 20 and 40 N under a given activation condition. Recruiting only the first two could provide about

$$5+10=15\ \mathrm{N},$$

whereas recruiting all four could provide about

$$5+10+20+40=75\ \mathrm{N}.$$

Real motor-unit forces are not fixed constants: firing frequency, muscle length, contraction velocity and sustained prior activity can all alter the force observed.

Motor units therefore explain how an all-or-none action potential in each axon can participate in continuously graded force at the level of the whole muscle.