release of sequestered calcium ion into cytosol by sarcoplasmic reticulum

Definition

Target type: biologicalprocess

The process in which the release of sequestered calcium ion by sarcoplasmic reticulum into cytosol occurs via calcium release channels. [GOC:mtg_muscle]

The release of sequestered calcium ions into the cytosol by the sarcoplasmic reticulum (SR) is a fundamental process in muscle contraction. Here's a detailed description:

1. **Action Potential Arrival:** The process begins with an action potential arriving at the neuromuscular junction, triggering the release of acetylcholine.

2. **Depolarization of T-Tubules:** Acetylcholine binds to receptors on the muscle fiber membrane, causing depolarization. This depolarization spreads down the transverse tubules (T-tubules), which are invaginations of the plasma membrane.

3. **Activation of Dihydropyridine Receptors:** Depolarization of the T-tubules activates dihydropyridine receptors (DHPRs) located within the T-tubules. These receptors are physically linked to ryanodine receptors (RyRs) on the SR membrane.

4. **Conformational Change in RyRs:** The activation of DHPRs causes a conformational change in RyRs. This change opens the RyR channels, allowing calcium ions to flow out of the SR and into the cytosol.

5. **Calcium Ion Release:** The SR is a specialized intracellular organelle that stores large amounts of calcium ions. When RyRs open, these ions rapidly diffuse out of the SR, driven by a concentration gradient.

6. **Calcium Ion Binding to Troponin:** The released calcium ions bind to troponin, a protein complex associated with thin filaments (actin) in the muscle fiber. This binding triggers a conformational change in troponin, moving tropomyosin away from the actin binding sites.

7. **Muscle Contraction:** The removal of tropomyosin from the actin binding sites allows myosin heads to bind to actin, initiating the cross-bridge cycle and muscle contraction.

8. **Calcium Ion Reuptake:** After contraction, calcium ions are actively pumped back into the SR by the SR Ca2+ ATPase (SERCA) pump. This reuptake lowers the cytosolic calcium concentration, allowing the muscle to relax.

9. **Regulation of Calcium Release:** The release of calcium from the SR is tightly regulated. The DHPR-RyR coupling ensures that calcium release occurs only when the muscle fiber is depolarized. Additionally, other regulatory proteins, such as calmodulin and phospholamban, can influence the activity of RyRs and SERCA pumps.

In summary, the release of sequestered calcium ions from the SR is a crucial step in muscle contraction, triggering the interaction between actin and myosin, leading to muscle fiber shortening. This process is highly regulated, ensuring efficient and coordinated muscle function.'
"

Proteins (3)

Compounds (27)