A motor unit consists of one motor neuron and all the skeletal muscle fibres it innervates. The all-or-none principle means that once the neuron's stimulus reaches threshold, every muscle fibre in that motor unit is activated; if threshold is not reached, the motor unit does not respond.
How the Mechanism Works
- A nerve impulse, or action potential, travels along a motor neuron to the neuromuscular junction.
- The motor neuron releases the neurotransmitter acetylcholine.
- If depolarization of the muscle fibre reaches the threshold, an action potential spreads along the sarcolemma and into the transverse tubules.
- Calcium ions are released within the muscle fibre, allowing actin and myosin to form cross-bridges and produce contraction.
- Because one motor neuron branches to all fibres in its motor unit, those fibres are activated together.
The principle does not mean that an entire muscle can only be fully contracted or fully relaxed. This is a common misconception. A whole muscle contains many motor units, so its total force can be varied.
| Mechanism | Effect on muscular force |
|---|---|
| Motor unit recruitment | Activating more motor units increases the number of contracting muscle fibres and therefore increases force. |
| Rate coding | Increasing the frequency of motor neuron impulses produces temporal summation and greater tension. |
| Motor unit size | Small motor units allow precise control; large motor units produce greater force. |
For example, holding a pencil requires relatively few, small motor units. Lifting a heavy barbell requires the recruitment of many motor units, including larger units, together with a higher firing frequency.
Exam Technique
For IB SEHS subtopic B.1.3, define a motor unit precisely and distinguish the response of one motor unit from the graded response of the whole muscle. Examiners may expect the terms threshold, motor unit recruitment, and all-or-none, plus a clear correction of the misconception that all-or-none applies to the entire muscle.