Skeletal muscles are used for voluntary movement of the body. They are striated and have multiple nuclei at the periphery of each cell.

Cardiac muscle is used for involuntary contractions of the heart, allowing for the movement of blood. Cardiac muscles are striated and have a single, central nucleus in each cell.

Sarcomeres represent the basic contractile unit of the muscle and can be used to show the process of contraction. Numerous sarcomeres make up myofibril, which combine to form a muscle fiber (myocyte), which combine to form a fasciculus, which combine to form muscle.

Skeletal muscle is attached to bones by tendons, allowing you to move around. These are the muscles that you can consciously control.

Smooth muscle and cardiac muscle are not controlled consciously. You cannot consciously control your heart beat or your blood vessels constriction.

Swallowing involves both skeletal muscle and smooth muscle. While skeletal muscle is voluntary (somatic), smooth muscle is involuntary (autonomic). Swallowing involves both of these systems. One can voluntarily induce swallowing, but will also swallow involuntarily, such as while sleeping.

Most of the muscles in the esophagus are smooth muscle; these will be the muscles largely responsible for counter-acting gravity in the example. The upper region of the esophagus (the pharynx) and the tongue are composed of skeletal muscle, allowing us to induce the swallowing motion voluntarily.

A muscle consists of millions of tiny subunits called sarcomeres. This is the functional unit of the muscle cell responsible for shortening and causing contractile force. Sarcomeres combine to form myofibrils, which form together to become a single muscle fiber, or muscle cell. Multiple muscle fibers form a muscle belly, the macrostructure of the muscle.

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.

Calcium ions facilitate the translocation of the troponin-tropomyosin complex, which inhibits cross-bridge formation between the myosin head (thick filament) and actin (thin filament). In order to inhibit muscle contraction, the troponin-tropomyosin must slide back into its original position, thereby inhibiting physical and chemical contact between the thick and thin filaments. In order to accomplish this, the calcium ions must be actively pumped out of the cytoplasm and into the sarcoplasmic reticulum (SR). It is important to remember that in a muscle fiber, the SR acts as a reservoir to store calcium ions. In order for them to be stored in a concentration greater than that in the cytoplasm, active transport must be involved to pump the ions against their gradient.

In order for muscle contraction to occur, myosin binding sites must be exposed on actin filaments. This is accomplished by releasing calcium ions from the sarcoplasmic reticulum. Calcium interacts with troponin, which in turn allows the myosin binding site to be exposed via interaction with tropomyosin. Troponin is not degraded during this process, only used to move tropomyosin out of the way. Myosin can then bind actin, hydrolyze ATP, and begin contraction.

GTP is not involved in the process and actin does not have an ATPase function.

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.