Sarcoplasmic Reticulum Definition

Sarcoplasmic Reticulum is a smooth endoplasmic reticulum type found in striated and smooth muscles that are reserves of calcium ions.

What is Sarcoplasmic Reticulum?

The endoplasmic reticulum (ER) occurs as an interlinked network of flattened sacs in the cytoplasm. The membrane extensions of the organelle are connected to the outer nuclear envelope and also extend into the plasma membrane.

The ER is of two types based on their appearance: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). RER has ribosomes on their surface while SER lacks them. Sarcoplasmic reticulum refers to specialized SER found in muscle cells.

Sarcoplasmic Reticulum Characteristics

The sarcoplasmic reticulum is present in myocytes as a membrane-bound network of tubules surrounding the myofibrils abounds in the myocyte. Sarcoplasmic Reticulum is located proximal to the transverse tubules in cardiac and skeletal muscle cells. A triad is formed by the union of the sarcolemma that is an extension of the cell membrane and components of Sarcoplasmic Reticulum.

Terminal cisternae are formed by the close association of parts of Sarcoplasmic Reticulum with transverse tubules. It is the primary site responsible for calcium release in the muscle cell.

Sarcoplasmic Reticulum has ion channel pumps that help in an influx of calcium ions present on the membrane. These pumps are termed SERCA and consist of 13 subunits. For Sarcoplasmic Reticulum to carry out the role of absorbing calcium ions, it has ion channel pumps in its membrane.

These calcium pumps in the Sarcoplasmic Reticulum are called SERCA and consist of 13 sub-units (M1-M10, N, P, and A). Some subunits are located outside (Eg: N, P, and A) while the rest is present inside the membrane.

ATP binds to the extracellular subunit while Calcium ions bind to the intracellular subunits. SERCA2a is generally found in muscle cells.

Calsequestrin binds to calcium and helps in its storage. This protein is located in terminal cisternae and can bind to 50 Calcium ions at a time. Ryanodine receptors (RyRs) help in the release of calcium ions via Sarcoplasmic Reticulum. The most common receptor is RyR3 present in the brain, while RyR2 is located in cardiac muscles. RyR1 is seen in skeletal muscles.

Sarcoplasmic Reticulum Functions

The Sarcoplasmic Reticulum is crucial for the contraction of myofibrils and functions in the absorption, storage, and release of calcium ions. These ions are released when muscles contract while in the relaxation phase they are absorbed.

Calcium levels need to be regulated to maintain homeostasis. A maintained increase in calcium levels can cause hardening, calcification, and even cell death. The calcium levels are kept constant through Sarcoplasmic Reticulum. As it concentrates the calcium ions inside, it decreases its intracellular levels.

Importance of Sarcoplasmic Reticulum

Calcium ion channel pumps ions into Sarcoplasmic Reticulum utilizing active transport as this is against the concentration gradient. Calcium is taken in when ATP along with 2 calcium ions binds to the cytosolic side of the pump.

The hydrolysis of ATP causes structural alterations of the pump. This leads to the opening of the channel through which the calcium ions pass through into the Sarcoplasmic Reticulum.

The calcium ions are released through the activation of RyR receptors at the terminal cisternae. This causes a surge in ion concentration causes calcium spark that can be spontaneous or evoked.

Calcium-induced calcium release is a mechanism for triggering calcium release. An action potential alters the shape of dihydropyridine receptors resulting in the opening of a calcium ion channel.

The resulting influx leads to the binding of calcium ions to RyR. This receptor is activated on binding with 4 calcium ions and this leads to calcium release. This results in the evoked calcium spark and is observed in the case of smooth and cardiac muscles.

In the case of skeletal muscle, RyR receptors open directly when dihydropyridine receptors change and open, and thus, this occurs without prior flooding of calcium ions. Both kinds of receptors are in proximity to each other, such that any change in dihydropyridine receptor shape triggers RyRs.

In cardiac and smooth muscles, the dihydropyridine receptors are not directly in contact with RyR, and hence calcium ion flooding and binding occurs is prerequisite for the release of calcium.

If no sort of action potentials are required for calcium release, then it is termed a spontaneous calcium spark. This mechanism is dependent on the high calcium ion concentration inside the SR.

The RyR opens by either direct binding of calcium ions or the detachment of calsequestrin from the receptor. If the concentration of calcium ions is low, then calsequestrin inhibits the opening of RyR by binding to it.