Explain in detail how muscle tissue functions

Muscles are really bundles of different-sized fibers (thin containing
actin and thick containing myosin) that are called sarcomeres. These
sarcomeres expand and contract as a result of an action potential
traveling down a nerve.  Once the action potential stimulates the
sarcomere, a modified endoplasmic reticulum inside the muscle cell
releases calcium ions, which bind to the thin fibered actin. The actin
(with the calcium ions attached) is drawn in by the thick-fibered
myosin, causing a contraction.  ATP dissasociates (gives up one high
energy phosphate) to relax the sarcomere, thus allowing the process to
repeat itself.