The Cross Bridge Cycle: What Starts It?

What is the Cross Bridge Cycle?

The Cross Bridge Cycle, also known as the Sliding Filament Theory, is a fundamental concept in the field of human physiology. It is a process that is essential to muscle contraction. The cycle consists of four parts: binding of the myosin head to the actin filament, a power stroke, ATP hydrolysis, and a release of the myosin head.

How Does the Cycle Begin?

The Cross Bridge Cycle begins when a motor neuron releases the neurotransmitter, acetylcholine, at the neuromuscular junction. This neurotransmitter binds to receptors on the sarcolemma, the cell membrane of the muscle fiber, which causes an action potential to be conducted down the T-tubule of the muscle fiber. This action potential is then transferred to the sarcoplasmic reticulum, where it triggers the release of calcium ions.

What Happens Next?

The calcium ions then bind to troponin, which is a protein located on the actin filament. This binding causes a change in the structure of the tropomyosin, which exposes the myosin binding sites on the actin filament. The myosin heads can then form cross bridges with the actin filament.

The Power Stroke

The myosin head then undergoes a power stroke, where it bends and pulls the actin filament towards the center of the sarcomere. This causes a contraction of the muscle fiber. As the myosin head is pulling the actin filament, it is also utilizing ATP molecules to provide the energy it needs to complete the power stroke.

Hydrolysis of ATP and Release of Myosin Head

Once the power stroke is complete, the ATP molecule is hydrolyzed, and the myosin head is released from the actin filament. This completes one cycle of the Cross Bridge Cycle. The cycle then begins again when the motor neuron releases another action potential and the process starts over.