Learning Objective

By the end of this module, learners will be able to describe the steps of cross-bridge cycling and explain the roles of calcium and ATP in initiating contraction and facilitating muscle relaxation in skeletal muscle.

Overview

Muscle contraction occurs through a process known as cross-bridge cycling, which converts chemical energy into mechanical force. This process begins when intracellular Ca²⁺ levels rise, and calcium binds to troponin C on the thin filament.

Calcium binding produces a conformational change in the troponin–tropomyosin complex, causing tropomyosin to shift away from the myosin-binding sites on actin. This exposes the active sites, allowing energized myosin heads to attach to actin and form cross-bridges.

Key Concepts in Cross-Bridge Cycling

1. Formation of Cross-Bridge:
   1. ATP is not required for myosin to bind to actin.
2. Power Stroke:
   1. The myosin head pivots, pulling the actin filament toward the center of the sarcomere, generating tension.
3. Detachment of Cross-Bridge:
   1. ATP binding to myosin is required to break the actin–myosin bond.
4. Reactivation of Myosin Head:
   1. ATP hydrolysis re-energizes the myosin head, allowing the cycle to repeat as long as calcium and ATP are available.

Termination of Cross-Bridge Cycling

Cross-bridge cycling continues until one of the following occurs:

• Withdrawal of Ca²⁺:
  Calcium is actively pumped back into the sarcoplasmic reticulum, preventing further actin–myosin interaction (normal resting state).
• ATP Depletion:
  Without ATP, myosin cannot detach from actin, resulting in sustained contraction (rigor mortis), a non-physiologic state.

Activity