Muscles are the tissues in animals that convert chemical energy into mechanical energy and allow motion. Muscles are composed of cells called myocytes that have long, tubular shapes. Myocytes are arranged in bundles, and each bundle is wrapped in a connective tissue sheath. The myocytes within a muscle are connected to each other by structures called gap junctions. Gap junctions allow the passage of small molecules and electrical impulses between adjacent myocytes, which ensures coordinated contraction of the muscle.
The cell membrane of a myocyte is called the sarcolemma, and it is covered with tiny projections called sarcoplasmic reticulum (SR). The SR contains calcium ions (Ca2+), which play an important role in muscle contraction. When a muscle is stimulated to contract, Ca2+ is released from the SR into the sarcoplasm (the cytoplasm of a myocyte). This increase in Ca2+ concentration causes the actin filaments within the sarcoplasm to bind with myosin filaments, resulting in cross-bridge formation and muscle contraction.
Muscles can be classified based on their structure, function, or both. Three main types of muscles include skeletal muscles, cardiac muscles, and smooth muscles. Skeletal muscles attach to bones and are responsible for movement at joints. Cardiac muscle makes up the walls of the heart and pumps blood throughout the body. Smooth muscle lines internal organs such as blood vessels and digestive tract