Motor units in skeletal muscleSpec B3.3.4
In short
A motor unit is a single motor neuron together with all the skeletal muscle fibres it stimulates, connected by neuromuscular junctions. When the motor neuron fires, acetylcholine released at the junctions stimulates every fibre in the unit to contract together. Recruiting more motor units increases the force of a muscle contraction.
A motor unit consists of:
- One motor neuron, whose cell body is in the spinal cord or brainstem and whose axon branches at its end.
- The muscle fibres it controls: long, multinucleate cells packed with myofibrils.
- Neuromuscular junctions: the synapses where each axon branch meets a muscle fibre.
- An action potential travels along the motor neuron to its axon terminals.
- At each neuromuscular junction the neurotransmitter acetylcholine is released and diffuses across the gap.
- It binds to receptors on the muscle fibre membrane, which depolarises and passes an impulse along the fibre.
- Calcium ions are released inside the fibre and the sarcomeres contract.
All the fibres in one motor unit contract at the same time. Muscles that need fine control, such as those moving the eye, have small motor units with few fibres each. Large leg muscles have large motor units. The nervous system controls the strength of contraction by changing how many motor units are active.
(opens full size in a new tab)Written and checked against the IB Biology HL specification · Updated October 2026