Definition
- A motor unit is a single motor neuron, its axonal branches, and all the muscle fibers supplied by them; discharge of the neuron contracts every fiber in the unit.
- Motor unit size ranges from fewer than 10 fibers per neuron (hand muscles) to hundreds or about 1000 fibers per neuron (back muscles).
Size of a Motor Unit
Differences between small and large motor units
Feature
Fibers per motor neuron
- Small motor unit (e.g., intrinsic hand muscles)
- Fewer than 10
- Large motor unit (e.g., back muscles)
- Hundreds to about 1000
Tension per unit activated
- Small motor unit (e.g., intrinsic hand muscles)
- Small rise in tension
- Large motor unit (e.g., back muscles)
- Large rise in tension
Tension regulation
- Small motor unit (e.g., intrinsic hand muscles)
- Graded, precisely controllable
- Large motor unit (e.g., back muscles)
- Coarse, added in large steps
Functional role
- Small motor unit (e.g., intrinsic hand muscles)
- Skilled, finely regulated movements
- Large motor unit (e.g., back muscles)
- Maintenance of posture, resisting gravity