Excitation-contraction coupling in a muscle cell. In response to stimulation of the nerve, an action potential occurs in the muscle, which travels stimulates the release of calcium from the sarcoplasmic reticulum, enabling contraction of the muscle. The site where the T-tubule and calcium-filled sarcoplasmic reticulum interact is called the junctional membrane complex. Image by OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons