31. When the action potential reaches the sarcoplasmic reticulum it triggers the release of Ca2 + from the Ca2 + channels. 32. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction. 33. Concentrations of intracellular calcium can be increased by increasing influx into the cell or stimulating release from the sarcoplasmic reticulum . 34. At high heart rates, phospholamban is phosphorylated and deactivated thus removing most from the cell back into the sarcoplasmic reticulum . 35. The sarcoplasmic reticulum , a specialized type of smooth endoplasmic reticulum, forms a network around each myofibril of the muscle fiber. 36. T tubules are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction. 37. This depolarisation triggers the entry of calcium from the sarcoplasmic reticulum to cause contraction ( tensing ) of the muscle. 38. It is found that there is an increase with calcium reuptake because of changes within sarcoplasmic reticulum morphology and structure. 39. The so-called delayed after-depolarizations are thought to be due to an increased Ca 2 + filling of the sarcoplasmic reticulum . 40. In March 1902, he provided the first accurate description of the reticular network ( sarcoplasmic reticulum ) in skeletal muscle fibers.