What does contract mean in muscle?
In muscle physiology, contraction refers to the process by which muscles generate force and shorten or tense up. When a muscle contracts, it pulls on its attachment points, which can cause movement at a joint.
The basic mechanism of muscle contraction involves the sliding filament theory. Here's an overview of the process:
Actin and Myosin Filaments: Muscles are made up of long, parallel bundles of protein filaments called actin and myosin. Actin filaments are thin and contain a protein called actin, while myosin filaments are thicker and contain a protein called myosin.
Motor Units: Muscles are divided into numerous motor units, each consisting of a motor neuron and the muscle fibers it innervates. When a motor neuron receives a signal from the nervous system, it releases neurotransmitters that bind to receptors on the muscle fibers.
Calcium Release: The binding of neurotransmitters triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum, which is the calcium storage organelle within muscle cells.
Calcium Binding to Troponin: Calcium ions bind to a protein called troponin, which is located on the actin filaments. This binding induces a conformational change in troponin, which removes a blocking molecule called tropomyosin from the binding sites on the actin filaments.
Myosin Head Binding: The exposed binding sites on the actin filaments can now interact with the heads of myosin molecules. Each myosin head has a globular structure that contains an adenosine triphosphate (ATP) molecule, which serves as an energy source.
ATP Hydrolysis: The binding of myosin heads to actin filaments triggers the hydrolysis of ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi). This hydrolysis event provides the energy for muscle contraction.
Power Stroke: After ATP hydrolysis, a conformational change occurs in the myosin head, causing it to swivel and pull the actin filament toward the center of the sarcomere (the repeating unit of a muscle fiber). This pulling action, known as the power stroke, results in the shortening of the muscle fiber and the generation of force.
Relaxation: Once the action potential has ended, calcium ions are actively pumped back into the sarcoplasmic reticulum and detached from troponin. This leads to the rebinding of tropomyosin to the actin filaments, blocking the myosin binding sites and causing the muscle to relax.
By continuously repeating this cycle of contraction and relaxation, muscles can generate movement, maintain posture, and perform various bodily functions that require force production. The coordination of multiple motor units within a muscle and the input from the nervous system allow for precise control and regulation of muscle contractions.