Cardiac Muscle Fiber
Cardiac muscle is only found in the heart. The muscle fibers (cells) in cardiac muscle have only a single nucleus located near the center of the cell (unlike skeletal muscle that can have several nucleus). Cardiac muscle fibers (called cardiomyocytes) also have many more mitochondria than skeletal muscles. This allows the heart muscle to operate continually without growing tired. The heart has the greatest endurance of any muscle in the body, thanks primarily to large amounts of oxygenated blood flow and a continual supply of ATP.
Cardiac muscle has striations like skeletal muscle, but it is unique in that it is not under voluntary control like skeletal muscle. Cardiac muscle is under the control of the autonomic nervous system and is therefore an involuntary muscle. But the heart is also self-regulating. Pacemaker cells in the heart provide the stimulus to start a contraction. The autonomous nervous system can request a different contraction rate from the pacemaker cells. Pacemaker cells will then automatically initiate contractions in the heart at the new rate.
Within the heart muscle the individual muscle fibers are much shorter than in most skeletal muscle. The fibers are also not a simple long cylindrical shape. Instead these cells have a branching organization so that the end of one cell may connect with the ends of one or more other cells. An Intercalated Disc occurs at the point where the ends of two cells interconnect. This disc provides two structures important to heart operation. The first is the Desmosomes which provide strong connections to hold the ends tightly together so they can withstand the pressures of a continual and rapidly beating heart. The second structure is a gap junction. This junction provides a way for contraction signals to move or propagate from one muscle fiber to another.
Cardiac Muscle Contractions
Once a pacemaker cell in the right atrium initiates a contraction the contraction propagates rapidly through other muscle fibers of the heart in a rhythmic pattern. This rhythmic pattern causes the upper portions of the heart muscle to contract first forcing blood out of the two atrium chambers downward into the ventricles. After a short delay the heart muscle surrounding the ventricles begins to contract. This process begins from the bottom of the ventricles and propagates upward. A helical method is also employed so contractions concurrently proceed from left to right as well. This pushes blood up and out of the heart via an efficient pumping action in a manner not dissimilar from wringing out a mop.
An important difference between cardiac and skeletal muscle is that cardiac muscles do not immediately release following a contraction. They sustain a contraction for some period of time so that blood does not flow back into the heart while other muscle fibers are still in the process of contracting.
Like skeletal muscles the cardiac muscle relies on sarcomere to perform contractions. The T-tubules in cardiac muscles are wider than those in skeletal muscle to more reliably communicate contraction signals. Unlike skeletal muscle, sarcomere are not stimulated to contract solely due to nerve action. Within the heart, sarcomere contractions begin when a cell receives calcium ions from an adjacent cell. This spurs a sudden release of calcium ions from the sarcoplasmic reticulum of the receiving cell. This process is commonly called the calcium induced calcium release and is the fundamental mechanism by which sarcomere contract in a controlled and rhythmic pattern in the heart.
Cardiac muscle cells depend largely on aerobic production of ATP. But in moments of spontaneous need the cells may rely on the anaerobic process to provide additional ATP.
This additional reliance on aerobic ATP production requires an increased number of mitochondria in heart muscle. Roughly 25% of muscle tissue in the heart is mitochondria, while only 2% of skeletal muscle tissue contains mitochondria.