Which event initiates the formation of crossbridges between actin and myosin during muscle contraction?

• A. An electrical impulse directly shortening actin and myosin
• B. ATP hydrolysis in mitochondria
• C. Release of chemical mediators at the motor end plate
• D. Calcium influx in the sarcoplasmic reticulum

Answer: (C)

The signal that starts the chain leading to crossbridge formation begins with the release of chemical mediators at the motor end plate. When a nerve impulse reaches the neuromuscular junction, acetylcholine is released into the synaptic cleft. This chemical signal binds to receptors on the muscle fiber, triggering an electrical impulse that travels along the membrane and into the muscle fiber. That electrical activity then causes calcium to be released from the sarcoplasmic reticulum into the cytoplasm. The calcium binds to troponin, moving tropomyosin away from actin’s binding sites, so myosin heads can attach to actin and form crossbridges, enabling contraction.

So, the initial chemical signal at the motor end plate sets off the cascade that culminates in crossbridge formation. The other options miss this sequence: an electrical impulse alone doesn’t directly shorten the filaments; ATP hydrolysis supplies energy for the cycle rather than initiating it; calcium release is the pivotal step that enables crossbridges, not calcium influx into the sarcoplasmic reticulum.