Complete information on the mechanism of muscle contraction



Skeletal muscle is a voluntary muscle. It has a striated appearance when examined under light microscope. It is made up of multinucleated cells called muscle fibre. The cells are bounded by an electrically excitable plasmamembrane called sarcolemma. The muscle cell’ this cytosol contains many parallel myofibrils. The longitudinal section of myfibril in an electro micrograph displays a detailed structure. In a myofibril, a dark A-band and light I-band alternate regularly.

The central lighest area of the A-band is called H-zone. The I-band is bisected by a proteinous line called Z-line or krauses membrane. The part of the myofibril between Z-line to Z-line is called sarcomere. It is the functional unit of the myfibril. It repats every 2.3 µm long the fibril axis. The sacromere is made up of two kinds of protein filaments- the thick filament and the thin filament. The thick filament is made up of protein myosin where absent the thin filament contains acting, tropomtyosin and troponin complex.

In a cross section of myofibril it shows that each thin filament has three neighbouring thick filaments and each thick filament is surrounded by six thin filaments. Each myosin has two parts. The anterior globular part is called head and a long thread called tail.t head is otherwise called heavy meromyosin or HMM and tail called light meromyosin or LMM. The head with a short arm project outward at regular intervals. The globular head is an active ATPase enzyme, has binding site for ATP, and has active site for actin.

Mechanism of Muscle Contraction:

H.E. Huxley and A.F.Huxley (1950) and their colleagues proposed the slide filament theory of muscle contraction. The Mechanism of muscle contraction is best explained by this theory, which states that contraction of muscle fibre takes place by the sliding of thin filaments over the thick filaments.

When muscle contracts, there is no change in length of the thick and the thin filament but the Z-zone and the I-band shorten. Muscle contraction is initiated by central nervous system via motor nerves. When a nerve impulse or signal reaches neuromuscular junction, the nerve cell releases acetycholine to this junction. The acetyl choline generates on action potential in the sarcolemma of the muscle cell. The action potential spreads over the muscle fibre and stimulates sarcoplasmic reticulum to release calcium ions to the sarcoplasm (cytoplasm). The Ca++ ions bind to a subunit of troponin or calcium binding troponin or TPc on the actin filament. Now the actin is ready to bind with myosin because the active site in actin, for myosin binding, is uncovered. The myosin head hydrolyses ATP to ADP and Pi. These products remain bound to form myosin- ADP complex. This complex has energized and now it is called high-energy conformation. The Ca++ ions, troponin, tropomyosin and For-actin bind to the head of myosin and form the actin-myosin-ADP complex.

The Pi is released from the complex. This actin-myosin binding is called cross bridge. The complex initites power stroke. This is followed by release of ATP and is accompanied by conformation change in the head of myosin. Now the myosin head pulls the actin about 10 nm towards the center of the sacromere. This is called power storke.Ano ATP molecules bind to the head forming acting-myosin-ATP complex.

When ATP binds to the head of the myosin, the affininity for actin decreased and thus actin is realesed and cross bridge is broken. This step is the key component of the relaxation. ATP is again hydrolised by myosin head (myosin-ATP) and the cycle of cross bridge formation and breaking is repeated causing further sliding of the myosin over actin. During relaxation, the ca++ ion is pumped back to the lumen of the endoplasmic reticulum resulting covering (masking) of the active site or myosin-binding site of the actin filament.

The amount of ATP is myscle is very small.tu the muscle cells also needs high energy from phosphocreatine in vertebrates and phosphor-arginine in intervertebrates.

Phosphocreatine + ADP → ATP + creatinine

Phospho-arginine + ADP → ATP + Arginine

The compounds that carry high energy phosphate are known absent phosphagens.