Sarcoplasmic reticulum: Structure, Types and functions

Muscle cell Endoplasmic Reticulum

Sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum (ER) of muscle cells of striated muscles. The Sarcoplasmic reticulum is located in the immediate vicinity of the myofibrils. The SR is not well developed in the cardiac muscles as compared to skeletal muscles. 

The sarcoplasmic reticulum (SR) constitutes the principal intracellular calcium kept in striated muscle and performs a crucial function within the law of excitation-contraction-coupling (ECC) and of intracellular calcium concentrations all through contraction and relaxation.

Structure of Sarcoplasmic reticulum (SR)

The structure resembles a smooth Endoplasmic Reticulum. SR lacks ribosomes unlike the endoplasmic reticulum which functions as the synthesis of protein for the cell. Its structures are subdivided into:

  • terminal cisterns, which enclose myofibrils in a semicircle ,and 
  • longitudinal ducts, which connect adjacent terminal tanks. 

The terminal cisterns of the SR are adjoined by T- tubules – deep invaginations of the outer membrane. The number of T- tubules roughly corresponds to the number of sarcomeres. A T- tubule is formed by the inward invaginations of the sarcolemma of muscle cells and it is attached to sarcoplasmic reticulum.

Types of Sarcoplasmic Reticulum (SR)

There are forms of sarcoplasmic reticulum (SR) in fish cardiomyocytes:

jSR paperwork peripheral couplings with both the sarcolemmal or t-tubular network. In those peripheral couplings in which the jSR profile aligns with the sarcolemma or t-tubule, rows of proteins recognised as ‘feet’ are seen among the 2 membranes. These proteins are the Ca2+-launch ryanodine receptor channels. fSR isn’t carefully related to every other membrane system, and may be located all through the cardiomyocyte.

Function of Sarcoplasmic Reticulum (SR)

Sarcoplasmic reticulum (SR) serves as a depot of calcium ions. The concentration of ions of calcium in the SR may reach 10 -3 mol, in the time both in the cytosol of the order of 10 -7 mol of ( a state of rest ).

When the neurotransmitter acetylcholine is exposed to the muscle cell, the outer membrane of the muscle cell and the membrane of the T- tubules are depolarized. Calcium channels (ryanodine receptors ) are located at the points of contact of the T- tubules with the SR. When T- tubes are depolarized, they open, and calcium ions leave the sarcoplasmic reticulum into the cytoplasm of the muscle fiber. 

Previously, it was believed that there is a protein domain between the calcium channels and the T- tube membrane, which provides mechanical coupling of depolarization with the opening of the channel. However, now the point of view prevails, according to which, during depolarization, calcium channels on the T- tubule, the first portions of calcium ions enter the cell from the outside, bind to the cytoplasmic domain of the CP calcium channels and cause them to open. Over time, the order of milliseconds, the concentration of calcium in the cytoplasm sharply increases, which causes the simultaneous reduction of sarcomeres ( By binding to troponin, ions of calcium causes a change conformation tropomyosin, which causes the formation of actomyosin complexes and reducing muscle ).

After repolarization of the fiber membrane, calcium ions are pumped out of the cytoplasm into the sarcoplasmic reticulum, and within a time of about 30 ms, their concentration returns to the initial one. This process is provided by a protein – calcium pump ( Ca- ATPase, Ca 2 + – ATPase), which is contained in large quantities in the CP membrane.

Ca – ATPase is activated when two calcium ions bind from the cytoplasmic side, so that its activation occurs when the concentration of calcium ions in the cytoplasm increases.

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