Structure of the Contractile Apparatus
Muscle fibers consist primarily of myofibrils, which are divided into contractile units called sarcomeres. The boundaries of a sarcomere are the Z-discs, to which thin filaments of the protein actin anchor.
Regulatory proteins are distributed along the actin filament:
- Thin tropomyosin filaments.
- The protein troponin, which overlays the tropomyosin filaments and blocks myosin-binding sites at rest.
Thick myosin filaments occupy the center of the sarcomere. They feature specialized projections with heads that interact with troponin on the actin filament during contraction. An ATPase enzyme is also present on the myosin heads.
Role of Calcium and Initiation of Contraction
The signal for contraction is an action potential that spreads along the cell membrane and penetrates deep into the cell via the system of transverse T-tubules.
Membrane permeability changes in response to excitation, causing a massive influx of $Ca^{2+}$ ions into the sarcoplasm. In skeletal muscle, the sole source of calcium is the sarcoplasmic reticulum, whereas in the myocardium, ions come from both the reticulum and the T-tubules.
Calcium triggers the process by binding to troponin. This electrostatic interaction causes troponin to shift position, exposing the myosin-binding sites on actin.
Cross-Bridge Cycle (Power Stroke)
Once active sites are exposed, contraction proceeds through distinct phases:
- Cross-bridge formation: The myosin head, already bound to an ADP and inorganic phosphate (or carrying ATP), firmly attaches to actin. The ATPase enzyme transitions to an active state.
- ATP hydrolysis: ATPase hydrolyzes ATP into ADP and phosphate, releasing energy. ADP is released into the cytoplasm.
- Power stroke: The released energy alters the spatial (quaternary) structure of the myosin head. It pivots, pulling the actin filament toward the center of the sarcomere.
- Detachment: For myosin to detach from actin, a new ATP molecule must bind to the head (supplied by mitochondria via oxidative phosphorylation).
- Resetting: After the cross-bridge breaks, the myosin head is "cocked" back to its original position without consuming energy, ready for a new cycle.
Sliding Filament Theory
Through repeated cycles of cross-bridge formation, myosin filaments remain stationary while actin filaments slide past them, much like oars propelling a boat.
As a result, the Z-discs move closer together, and each sarcomere slightly shortens. The cumulative shortening of thousands of sarcomeres across all myofibrils results in the contraction of the entire muscle fiber and muscle as a whole. The force of this contraction depends on the number of cross-bridges formed and the frequency of the sliding cycles.