Final answer:
The contraction cycle of skeletal muscle consists of four steps which begin with the release of ACh by a motor neuron, followed by a release of calcium from the sarcoplasmic reticulum leading to crossbridge formation between actin and myosin filaments, and ending with a power stroke that causes the muscle to contract.
Step-by-step explanation:
The sequence of events in the contraction cycle of skeletal muscle involves a meticulously choreographed interaction between various muscle components. This excitation-contraction coupling follows four primary steps:
Excitation: A motor neuron releases the neurotransmitter acetylcholine (ACh) which binds to receptors on the muscle fiber membrane (sarcolemma), leading to depolarization and an action potential.
Calcium Release: The action potential travels down the T-tubules and stimulates the sarcoplasmic reticulum to release calcium ions (Ca²⁺).
Formation of Crossbridges: Calcium binds to troponin on the thin actin filaments, causing a conformational change that exposes the myosin-binding sites, allowing myosin heads to attach and form crossbridges.
Power Stroke: Utilizing energy from ATP, the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere. This sliding of the filaments causes the muscle to contract.
The cycle repeats as long as calcium and ATP are present, enabling the muscle to sustain contraction.