15.1.10—Muscle contraction stimulation

At the neuromuscular junction, a motor-neurone action potential causes Ca²⁺ entry at the presynaptic membrane. Acetylcholine (ACh) is released and binds receptors on the muscle-fibre sarcolemma.

1. ACh opens ion channels in the sarcolemma; Na⁺ entry depolarises the muscle membrane.
2. The action potential travels along the sarcolemma and down the transverse (T-) tubules.
3. This opens Ca²⁺ channels in the sarcoplasmic-reticulum membrane, so Ca²⁺ enters the sarcoplasm around the myofibrils.
4. Ca²⁺ binds to troponin, shifting troponin and tropomyosin and exposing myosin-binding sites on actin.
5. The sliding-filament contraction process can then begin.

The T-tubule and sarcoplasmic-reticulum arrangement carries the surface electrical event into the muscle fibre and converts it into a Ca²⁺ signal. This card ends when actin sites are exposed; sarcomere structure and the full cross-bridge/sliding-filament cycle are covered separately.

The neuromuscular junction is between a neurone and a muscle fibre, not between two neurones. ACh starts the muscle electrical response, while Ca²⁺ and troponin/tropomyosin provide the link to contractile-filament interaction.