Sarcoplasmic Reticulum

The sarcoplasmic reticulum is a network of membrane enclosed tubules similar to smooth endoplasmic reticulum that extends throughout the sarcoplasm of muscle fibres.

Sarcoplasmic reticulum surrounds each myofibril and is physically separated from the sarcolemma.


Sarcoplasmic reticulum is wider at its two ends, which form terminal sacs, called cisternae. A functional unit is a triad, which consists of two cisternae of adjacent sarcoplasmic reticula and one transverse tubule in between. The distance between the terminal cisternae and the transervse tubule is 10-15 nm.


The functions of the sarcoplasmic reticulum within muscle tissue include:

  • Uptake of calcium from the sarcoplasm, and
  • Releasing calcium into the sarcoplasm to initiate muscle contraction, and the converse during relaxation.

See also more about the types of muscle contraction.

This section consists of short summaries about the structures that form the muscles of the body. This list is not exhaustive but is intended to be appropriate for students of A-Level Human Biology, ITEC courses in massage and related subjects, and other courses in health sciences. For more general information about muscles see the pages about:

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