Answer:
sarcoplasmic reticulum; sarcoplasm
Explanation:
The sarcoplasmic reticulum (RS). This is a membrane complex similar to the endoplasmic reticulum in other cells, but in the skeletal muscle it forms a tubular network around each myofibril. On each side of the T-tubule the sarcoplasmic reticulum widens and forms a chamber called a terminal cistern, which joins the T-tubule through a structure known as 'foot'. The combination of a pair of terminal cisterns and the transverse tubule is called a 'triad' and although their membranes are joined, the liquid contents are separated and different.
The intracellular concentration of calcium ions (Ca2 +) remains low due to 'pumps', which take them out when their concentration increases. Although skeletal muscle fibers 'pump' Ca2 + out of the cell, they also remove calcium from the sarcoplasm by transporting it to the terminal cistern of the sarcoplasmic reticulum. The sarcoplasm of a resting muscle fiber contains concentrations of Ca2 + around 10-7 molar / liter and its concentration within the terminal cistern can be up to 1000 times higher; In addition, the cistern contains a protein called calsecuestrin, which binds calcium ions reversibly. Including both free calcium and calcium bound to other molecules, the total concentration of Ca2 + inside the cistern can be up to 40,000 times that inside the surrounding sarcoplasm.
For the interaction between actin and myosin to produce the contraction to occur, there must be calcium, which after the contraction must be removed and the delivery and elimination of this ion is carried out by the combined work of the tubular-T system and the RS. The RS surrounds the myofibrils as a system of networks, one of them around the A-band and the other in the I-band, and where the two networks meet, at the junction of the A-and -I bands, the RS It forms a cistern. The RS controls the level of intracellular Ca2 + in the skeletal muscle, storing and releasing it. Initially the sarcoplasmic reticulum develops as an ER, but as the muscle differs it is enriched with specific proteins. Three proteins initially purified from the sarcoplasmic reticulum are, calcium ATPase (SERCA), calsecuestrin (CLQ) and ryanodine receptor (RyR). SERCA is responsible for pumping calcium into the RS light during relaxation, while CLQ is the most prominent of the calcium chelating intraluminal proteins and between them increases the capacity of the RS for calcium. The most abundant SR protein outside the tubule-T-RS junction is SERCA, which is normally distributed in tubular elements surrounding the Z-and -M lines, as well as in the elements aligned with the longitudinal axis of the myofibril. In the light of RS the most abundant protein is CLQ, an acidic protein that binds calcium with moderate affinity and high capacity.
