MICROANATOMY

Detailed explanation-1: -Depolarization of the SR causes the SR to release calcium into the muscle cytoplasm. The sudden presence of calcium in the cytoplasm causes the tropoin-tropomyosin complex to move away from blocking the myosin binding sites on the actin thin filament, allowing muscle contraction to occur.

Detailed explanation-2: -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.

Detailed explanation-3: -It is well accepted that a rise in intracellular Ca2+ concentration is the key requirement for vascular smooth muscle contraction. This rise results from Ca2+ influx from the extracellular space, Ca2+ release from intracellular stores, or both.

Detailed explanation-4: -Stimulation of the muscle fibre, causes a wave of depolarisation to pass down the t-tubule, and the SR to release calcium ions into the sarcoplasm. Calcium is pumped back up into the SR to lower calcium ion concentration in the sarcoplasm, to relax the muscle (turn off contraction).