influx causes a small local increase in intracellular . The increase of intracellular is detected by RyR2 in the membrane of the sarcoplasmic reticulum, which releases in a positive feedback physiological response. This positive feedback is known as calcium-induced calcium release and gives rise to calcium sparks ( sparks). The spatial and temporal summation of ~30,000 sparks gives a cell-wide increase in cytoplasmic calcium concentration. The increase in cytosolic calcium following the flow of calcium through the cell membrane and sarcoplasmic reticulum is moderated by calcium buffers, which bind a large proportion of intracellular calcium. As a result, a large increase in total calcium leads to a relatively small rise in free . The cytoplasmic calcium binds to Troponin C, moving the tropomyosin complex off the actin binding site allowing the myosin head to bind to the actin filament. From this point on, the contractile mechanism is essentially the same as for skeletal muscle (above). Briefly, using ATP hydrolysis, the myosin head pulls the actin filament toward the centre of the sarcomere. Following systole, intracellular calcium is taken up by the sarco/endoplasmic reticulum ATPase (SERCA) pump back into the sarcoplasmic reticulum ready for the next cycle to begin. Calcium is also ejected from the cell mainly by the sodium-calcium exchanger (NCX) and, to a lesser extent, a plasma membrane calcium ATPase. Some calcium is also taken up by the mitochondria. An enzyme, phospholamban, serves as a brake for SERCA. At low heart rates, phospholamban is active and slows down the activity of the ATPase so that does not have to leave the cell entirely. At high heart rates, phospholamban is phosphorylated and deactivated thus taking most from the cytoplasm back into the sarcoplasmic reticulum. Once again, calcium buffers moderate this fall in concentration, permitting a relatively small decrease in free concentration in response to a large change in total calcium. The falling concentration allows the troponin complex to dissociate from the actin filament thereby ending contraction. The heart relaxes, allowing the ventricles to fill with blood and begin the cardiac cycle again. Invertebrate Circular and longitudinal muscles In annelids such as earthworms and leeches, circular and longitudinal muscles cells form the body wall of these animals and are responsible for their movement. In an earthworm that is moving through a soil, for example, contractions of circular and longitudinal muscles occur reciprocally while the coelomic fluid serves as a hydroskeleton by