Mechanism of Muscle Contraction.
During muscle contraction, the laterally projecting heads (cross bridges) of the thick myosin myofilaments come in contact with the thin actin myofilaments and rotate on them. This pulls the thin myofilaments towards the middle of the sarcomere past the thick myofilaments. The Z lines come closer together and the sarcomere becomes shorter. Length of the A band remains constant. Myofilaments stay the same length. Free end of actin myofilaments moves closer to the center of the sarcomere, bringing Z lines closer together. I bands shorten and H zone narrows. A similar action in all the sarcomeres results in shortening of the entire myofibril, and thereby of the whole fibre and the whole muscle. A contracted muscle becomes shorter and thicker, and its volume remains the mile.
Molecular Basis of Muscle Contraction.
The sarcomere, is the functional unit of the myofibril. Each sarcomere contains thick filaments and thin filaments, which are anchored to the Z-disc. The thin filament is made up of actin. The thick filament is made up of the protein myosin. Myosin molecules consists of two globular heads with a long tail i.e. two heavy chains. Myosin heads also contain four smaller light chains. Thus, each myosin molecule consists of six polypeptide chains. Myosin molecules are arranged in the thick filament so that the tails point inward towards the center of the sarcomere, and the heads decorate the outer ends of each thick filament. The myosin heads are known as cross-bridges because they can bind to and move along actin in the thin filament.
It is this actin-myosin interaction that is the molecular basis for force generation and movement in muscle cells. When muscle cells contract, the thick and thin filaments do not change their size. Instead, the interaction between the myosin heads and actin pulls the thin filaments past the thick filaments. As stated above, cross-bridge cycling forms the basis for movement and force production in muscle cells. Each cycle of myosin binding to actin and movement of the thin filament involves the hydrolysis of one ATP molecule. ATP binding causes the dissociation of myosin from actin. ATP hydrolysis causes a shape change so that the myosin head is cocked.
The products of ATP hydrolysis (ADP and inorganic phosphate) remain bound. Cocking of the myosin head puts it in line with a new binding site on the actin filament. Myosin binds to actin and the power stroke occurs. Initial weak binding releases inorganic phosphate. Stronger binding triggers the power stroke and the release of ADP. The power stroke involves the return of the myosin head to its low-energy conformation. The power stroke generates force, pulling the thin filament toward the center of the sarcomere. Binding of another ATP molecule causes dissociation of myosin from actin and the cycle repeats
Control of Contraction by Calcium and Regulatory Proteins.
Calcium and ATP are cofactors (non-protein components of enzymes) required for the contraction of muscle cells. Calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the binding of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks the binding of myosin to actin. In the above analogy of pulling shelves, tropomyosin would get in the way of your hand as it tried to hold the actin rope. For myosin to bind actin, tropomyosin must rotate around the actin filaments to expose the myosin-binding sites.
Specifically, troponin (the smaller protein) shifts the position of tropomyosin and moves it away from the myosin-binding sites on actin, effectively unblocking the binding site. Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to begin cross-bridge cycling. Then the sarcomere shortens and the muscle contracts. In the absence of calcium, this binding does not occur, so the presence of free calcium is an important regulator of muscle contraction.
Initiation of Muscle Contraction.
The axons of the nerve cells of the spinal cord branch and attach to each muscle fibre forming a neuromuscular junction. An action potential travels along a motor nerve to its endings on muscle fibres. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine. The acetylcholine acts on a local area of the muscle fibre membrane to open multiple “acetylcholine-gated” cation channels through protein molecules floating in the membrane. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fibre membrane. This causes a local depolarization that initiates an action potential at the membrane and referred to as excitation of fibre.
The action potential travels along the muscle fibre membrane in the same way that action potentials travel along nerve fibre membranes. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fibre. Here, it causes the sarcoplasmic reticulum to release large quantities of Ca2+ that have been stored within this reticulum. The Ca2+ initiates attractive forces between the actin and myosin filaments, causing them to slide along side each other, which is the contractile process. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along. This removal of calcium ions from the myofibrils causes the muscle contraction to cease.
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