GLOSSARY

How a muscle cell contracts is a chain of neural, electrical, chemical, and mechanical events that begins with a command from the brain and ends with sarcomere shortening and force production.

Steps in Order

• Neural command: an action potential in the motor neuron reaches the axon terminal.
• Acetylcholine release at the neuromuscular junction.
• Sarcolemmal depolarisation: acetylcholine binds receptors, sodium enters, and a muscle action potential is generated.
• Spread into the T-tubules and activation of voltage sensors.
• Calcium release from the sarcoplasmic reticulum.
• Tropomyosin moves aside: calcium binds troponin and exposes actin’s binding sites.
• Cross-bridge cycle: the myosin head attaches, performs the power stroke, detaches with a new ATP, and re-cocks.
• Filament sliding: actin is pulled towards the centre and the sarcomere shortens.
• Relaxation: the nerve signal ends, acetylcholine is broken down, calcium returns to the SR, and binding sites are covered again.

Key Teaching Point

A simple analogy: the nerve is the switch, calcium unlocks the lock, ATP is the fuel, and myosin heads are rowers pulling the actin rope. ATP is needed for the power stroke, for myosin detachment, and for the calcium pump.