Muscle

Motor unit

• Functional unit of contraction.
• A single anterior horn alpha-motor neuron, its axon and all the muscle fibres it innervates.
• The smallest possible amount of muscle contraction in response to excitation of a single neuron is all the fibres supplied by that neuron.

Muscle fibre type

See Types of skeletal muscle

• Each spinal motor neuron innervates only one kind of muscle fibre, so that all of the muscle fibres in a motor unit are of the same time.
• Motor units with slow, fatigue-resistant muscle fibres are recruited first and most frequently used.
• Motor units with fast, fatigable fibres are only recruited in more forceful movements

Size principle

Muscle units with slow muscle fibres are innervated by small, slow motor neurons

Muscle units with fast muscle fibres are innervated by large, fast motor neurons