Motor Unit
Motor Unit: A motor unit includes a motor nerve (nerve cell axon) and all of the muscle cells (fibers) it innervates. More specifically, the motor unit consists of 1 anterior horn cell, its axon, and all the muscle fibers innervated by that motor neuron.
For more on muscle cells, check out these courses:
Motor Units and Force Production
All-or-none Principle: When a muscle cell is stimulated, it is completely stimulated, and contraction is carried out to the best of the cell's ability. Regardless of the strength of the impulse or the intensity of the external stimulus, an action potential is either initiated or not, and that action potential results in the contraction of the stimulated muscle cell or does not.
Motor Unit Recruitment: The all-or-none principle begs the question of how we control the force output of any muscle. How do we pick up a pencil and not impart the same force we would impart on a 30lb dumbbell? The answer is motor unit recruitment. Force is modified by adjusting the size, number, and type of motor units recruited.
- The Smallest Motor Units: The smallest motor units are in the muscles of the eyes. These include a single motor nerve and just 3-6 muscle cells (fibers). These smaller motor units are well-suited for the fine motor skill of tracking objects with the eyes.
- The Largest Motor Units: The largest motor units are found in the muscles of the legs and back and may include 1 motor nerve innervating 1000s of muscle fibers. These motor units are well suited for generating massive amounts of force that do not require the same fine motor control provided by smaller motor units.
- Fiber Type of Motor Unit: An interesting characteristic of motor units is that they will generally only include motor units of a single muscle fiber type (Type I, Type IIa, Type IIx, and transitional fibers). Different muscle fibers are uniquely adapted for endurance (type I), strength (type IIa), and power (type IIb). This topic is covered further in Muscle Fiber Types .