The Phases Of Crossbridge Cycling

It doesn't take much thought to flex your biceps or kick a soccer ball down a field, but at the biochemical level, the process of muscle contraction is actually quite complex. The central event in muscle contraction is a process that takes place inside the muscle fibers called crossbridge cycling. You can divide crossbridge cycling into four separate phases.

Low-Energy Configuration

Inside a muscle fiber, thin filaments made of a protein called actin lie above and below thick filaments made of a protein called myosin. The thick filament is like a staggered bundle of myosin molecules, each of them with a globular "head" region projecting out of the filament. In the first phase of crossbridge cycling, the head is in its low energy configuration and bound to a molecule of adenosine triphosphate, or ATP.

Crossbridge Formation

Myosin hydrolyzes or splits the ATP into ADP and a free phosphate group. The energy released by this reaction alters the shape of the myosin head and converts it to its high-energy configuration, aligning it with a binding site on the actin filament. In this new configuration, the myosin head is able to bind to the actin filament, forming a "cross-bridge" between the thick and thin filaments.

Power Stroke

Binding between the myosin head and the actin filament alters the myosin head again, causing it to release the inorganic phosphate produced by ATP hydrolysis. Next, the myosin head releases the other product of ATP hydrolysis, a molecule called adenosine diphosphate or ADP. These two steps return the myosin head back to its low-energy configuration, pulling the actin filament in a horizontal direction in the process. This power stroke works a little like yanking on a lever while it's attached to a rope; the actin filament ends up sliding while the thick filament does not move.

End of Cycle

The myosin head is now in its low energy configuration, but it's still attached to the actin filament. It frees itself by binding a molecule of ATP. Once the myosin head is bound to ATP, it separates from the actin filament and can hydrolyze the ATP, repeating the cycle all over again. This last phase of crossbridge cycling is actually linked to a phenomenon that often crops up in detective novels: rigor mortis, the stiffness of a recently-deceased corpse. After you die, your muscles cease to synthesize ATP. Without ATP, the myosin in your muscle fibers becomes unable to detach itself from the actin filaments, so your muscles remain rigid and stiff.