Learning Objective

By the end of this module, learners will be able to describe the ultrastructure of the sarcoplasmic reticulum (SR), identify its specialized regions, and explain its role in calcium storage and release during skeletal muscle contraction.

Overview

The sarcoplasmic reticulum (SR) is a specialized form of smooth endoplasmic reticulum found in muscle cells. It plays a critical role in regulating intracellular calcium (Ca²⁺), which is essential for excitation–contraction coupling.

Structurally, the SR forms a network that surrounds each myofibril and is divided into distinct regions:

Activity

Longitudinal SR

• Runs parallel to the myofibrils and contains calcium pumps (SERCA) that actively transport Ca²⁺ back into the SR during muscle relaxation.

Terminal Cisternae

• Enlarged sac-like regions are located adjacent to the T-tubules. These serve as major calcium storage sites and contain high concentrations of Ca²⁺.

Triad

• A structural complex formed by one T-tubule flanked by two terminal cisternae. In skeletal muscle, triads are located at the A–I junction and are essential for the rapid transmission of action potentials to trigger calcium release.

Activity

Key Functional Concepts

• The SR stores high concentrations of Ca²⁺ bound to proteins such as calsequestrin.
• Depolarization of the T-tubule triggers Ca²⁺ release via ryanodine receptors.
• Calcium reuptake into the SR via SERCA pumps allows muscle relaxation.

Activity