Cardiac muscle, or myocardium, forms the thick middle layer of the heart wall. It pumps blood through the circulatory system by sustained, rhythmic contractions without conscious effort. Unlike skeletal muscle, cardiac muscle is involuntary and innervated by the autonomic nervous system.
Cardiocytes are short, thick, branched cells packed with cellular machinery designed for continuous aerobic work. A large portion of the cell volume is taken up by mitochondria, reflecting the heart's constant need for ATP and high resistance to fatigue. The contractile proteins actin and myosin are arranged in repeating units called sarcomeres, giving the tissue a striated appearance under the microscope.
Sarcoplasmic reticulum is less developed in cardiocytes compared to skeletal muscle, meaning extracellular calcium ions play a major role in triggering contraction.
Cardiocytes are joined end-to-end by thick, dark staining regions called intercalated discs. These structures contain two vital mechanical and electrical elements:
Because of gap junctions, electrical stimulation applied to any single part of the tissue spreads rapidly to all neighboring cells. This allows the atria or ventricles to contract as a single coordinated unit, known as a functional syncytium.
The cardiac action potential is prolonged compared to nerve or skeletal muscle, lasting up to 300 milliseconds. This extended duration prevents tetany, ensuring the heart has time to relax and fill with blood between beats.
Excitation-contraction coupling is the process by which an electrical action potential triggers physical muscle shortening. When an action potential sweeps across the sarcolemma and down the T-tubules, it triggers voltage-gated calcium channels to open. Calcium enters the cell from the extracellular fluid and stimulates the release of even more calcium from the sarcoplasmic reticulum, a process called calcium-induced calcium release.
Free calcium binds to troponin, shifting tropomyosin away from binding sites on actin. Myosin heads then attach to actin, pulling the filaments inward to shorten the sarcomere.
The heart muscle has an absolute requirement for aerobic metabolism to sustain its continuous work cycle. Under normal resting conditions, the myocardium derives the majority of its energy from fatty acids, though it readily utilizes glucose, lactate, and ketone bodies depending on availability and metabolic state.
Because of its heavy reliance on oxygen, any interruption in coronary blood flow quickly leads to ischemia, ATP depletion, and cellular injury within minutes.
Contractility refers to the strength of cardiac muscle contraction at any given muscle fiber length. This property is heavily influenced by the autonomic nervous system and circulating hormones.