Cardiac Muscles

Cardiac muscles - or myocardium, lies between an outer layer of epicardium and an inner layer of endocardium - they contract rhythmically and involuntarily. The sinoartial node or pacemaker of the heart regulates their rhythmic contractions.

The cardiac output of the heart - amount of blood pumped by the heart each minute- and the frequency at which it occurs, is determined by the contractile force of the myocardium.

Cardiac muscles are composed of individual cardiomyocytes connected by intercalated discs and held together in bands of muscle by collagen fibers. These intercalated discs promote the rapid transmission of action potentials.

Action potentials (electrical signals) trigger the sarcoplasmic reticulum to release calcium ions. This causes the actin and myosin filaments that are arranged into contractile units to slide past each other. In each contractile unit, its sarcomere stretches from one Z-line to the next Z-line, creating a cross-striated pattern.

When a cardiac muscles contracts, the bands of muscle it is a part of contracts. This in turn causes the contraction of the heart chamber which then pumps blood to its intended destination.