Cardiac muscle tissue is only found in the heart.

There are seven steps involved in the signaling cascade from the initiation of contraction to the subsquent relaxation of muscle fibers.

1. Motor neurons release acetylcholine (ACh), which binds to receptors on the muscle fiber's cell membrane.
2. ACh receptor binding and activation creates an action potential that propagates along the muscle fiber membrane and down T tubules.
3. The action potential triggers the release of Ca2+ ions from the sarcoplasmic reticulum.
4. The released Ca2+ ions bind to troponin, which causes the displacement of tropomyosin, revealing the myosin-binding sites on the actin filament.
5. Myosin cross-bridges attach to actin at exposed binding sites and enter a cycle of shifts to crawl along the actin fiber. This causes the sarcomere to shorten. ATP is required for this reaction.
6. Cytosolic Ca2+ ions are removed and brought back into the sarcoplasmic reticulum by active transport (by a Ca2+ ATPase pump).
7. Tropomyosin blockage of myosin-binding sites is restored by Ca2+ ion removal). Contraction ends and the muscle fiber relaxes.

Muscle is a major type of tissue in the human body. They contract and enable movement. The three major types of muscle tissues include skeletal, cardiac, and smooth tissues.

During muscle contraction thick filaments (myosin) remain stationary, while thin filaments (actin) move towards one another. In the process, the H zone and I band shorten, as the Z lines get closer together. The A band, however, never changes in length.

The three types of muscle in the body are smooth, skeletal, and cardiac muscle. The type of muscle lining artery walls is smooth muscle, which is also present in many organs such as the intestines and the urinary bladder. Unlike skeletal muscle, smooth muscle control is involuntary and, unlike both skeletal and cardiac muscle, it is non-striated. Smooth muscle around the arteries and arterioles is important for regulating blood pressure and directing blood flow in the body.

Human muscle contraction relies on the action of the motor proteins actin and myosin. Muscles contract through the sliding of myosin and actin filaments along one another.

There are two compounds that are absolutely necessary for muscle contraction: ATP and calcium ions. Calcium ions bind to troponin, which removes tropomyosin from the binding site on actin. Only after this change occurs can myosin bind to actin and cause a contraction. ATP is hydrolyzed to alter myosin into a high energy state. This energy is released during the muscle contraction when myosin binds to actin. Even if ATP is present, the contraction cannot occur without calcium, and even if calcium is present it cannot occur without ATP.

During the contraction cycle, ATP binds to myosin in its low-energy state. ATP is converted to ADP, and the resulting energy is stored in the myosin head. The myosin then binds to actin, still carrying the ADP, and uses the energy to transition back to the low-energy state by pulling actin and shortening the sarcomere. This causes the ADP to be released from myosin. The binding of a new ATP to the myosin allows it to release actin, and the cycle begins again.

In smooth muscle tissue, contraction is involuntary and is caused by the sliding of actin and myosin filaments over one another. The function and mechanics of these contractions are generally the same in every organ; however, the physiological impact of each contraction may differ.

Muscle cells are called myocytes. "Myo" is a prefix for anything related to muscles. Terms such as myoblasts and myogenesis are related to muscle cells. "Cyte" means cell, and therefore "myo" and "cyte" means "muscle cell."

Melanocytes are found in the skin, and are responsible for producing the pigment melanin. Astrocytes are glial cells in the nervous system that create the blood-brain barrier. Acanthocytes are abnormal red blood cells that have become damaged, resulting in a change in their appearance.

Sarcomeres are not cells; they are a structure found in myocytes. Sarcomeres are composed of actin and myosin filaments, and are the fundamental contractile unit of the muscle fiber.

Cardiac muscle tissue is located only in the heart and contractions in this type of tissue are critical in propelling blood through the circulatory system. Contraction is involuntary, rhythmic, and continues through the entire life of a human.