sarcoplasmic reticulum (SR)

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Two adjoining myofibrils of skeletal muscle, with sarcoplasmic reticulum (SR) and t-tubules (T).
Figure 1: The autonomic nervous system. Actions on secretory functions (heart diagrams indicate release into the circulation). Sympathetic actions on the left and parasympathetic actions on the right. Solid arrows: stimulation (contraction or secretion); broken arrows: inhibition.

sarcoplasmic reticulum (SR)

membrane-bounded system within the cytoplasm of all muscle cells (particularly prominent in large, skeletal fibres), which on excitation releases calcium ions (thereby activating force-generation). In skeletal muscle this occurs when an action potential (AP) invades abutting t-tubes. If the AP, and thus the release process, is not repeated within a few tens of msec, the active reabsorption of Ca2+ into the SR by the calcium (Ca) pump leads to relaxation. See also muscle fibre; Figure 1.