Sliding Filament Theory

According to the sliding filament theory, during a contraction, myosin filaments grab on to actin filaments by forming chemical bonds called crossbridges. By the use of these crossbridges, the thick filaments pull in the actins toward the center. Because the actins are attached to the Z-line, this sliding movement shortens the length of the entire sarcomere. Notice that during a contraction, the filaments maintain their original length and only the I band actually gets compressed. In a muscle fiber, the signal for contraction is synchronized over the entire fiber so that all of the myofibrils that make up the sarcomere shorten simultaneously.

At the molecular level, actin filaments (or thin filaments) are shaped like two strands of pearls twisted around each other. Imagine that each pearl is an actin molecule. Both strands of actins are also wrapped around with a strand of tropomyosin and interspersed toropnin. There are two structures in the grooves of each thin filament that enable them to slide along the thick ones: a long rod-like protein called tropomyosin and a shorter bead-like protein complex called troponin. Troponin and tropomyosin are the molecular switches that control the interaction of actin and myosin during contraction. The thick filaments are shafts of myosin molecules arranged in a cylinder. Myosin molecules are golf-club shaped. Their broader endings are called heads.

Running vertically down the Z-line is a small tube called the transverse or T-tubule, which is actually part of the cell membrane that extends deep inside the fiber. Stretching along the fiber's long axis, between T-tubules (Transverse tubule), is a membrane system called the sarcoplasmic reticulum. Calcium ions are stored in the sarcoplasmic reticulum. Energy (ATP) is required to maintain the calcium stored in the sarcoplasmic reticulum.

In the normal relaxed muscle fiber the intracellular calcium level of the sarcoplasma is maintained at a low concentration. When the calcium concentration is low, the regulatory proteins, troponin and tropomyosin, block the myosin binding sites on the actin proteins. When a muscle fiber is stimulated, calcium ions are released from storage (sarcoplasmic reticulum). The binding of calcium ions to the troponin complexes causes the regulatory proteins to shift over, opening up the binding sites along the actin protein. The myosin heads now move in contact with the actin binding sites and form chemical bonds called crossbridges. Initially, the crossbridges are extended with adenosine diphosphate (ADP) and inorganic phosphate (Pi) attached to the myosin. As soon as the crossbridge is formed, the myosin heads bend, thereby creating force and sliding the actin filament past the myosin. This process is called the power stroke. During the power stroke (which causes muscle shortening of only 1%), myosins release the ADP and Pi. Once ADP and Pi are released, a molecule of adenosine triphosphate (ATP) binds to the myosin. When the ATP binds, the myosin molecule releases the actin. When the actin is released, the ATP molecule gets split into ADP and Pi by the myosin. The energy from the ATP resets the myosin head to its original position. The process is repeated (up to when the muscle is shortened 35-50% of its relaxed length). The actions of the myosin molecules are not synchronized - at any given moment, some myosins are attaching to the actin filament, others are creating force and others are releasing the actin filament.