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Cardiac muscle is a type of striated muscle found within the heart. Its function is to "pump" blood through the circulatory system by contracting.
Unlike skeletal muscle, which contracts in response to nerve stimulation, and like smooth muscle, cardiac muscle is myogenic, meaning that it stimulates its own contraction without a requisite electrical impulse. A single cardiac muscle cell, if left without input, will contract rhythmically at a steady rate; if two cardiac muscle cells are in contact, whichever one contracts first will stimulate the other to contract, and so on. This transmission of impulses makes cardiac muscle tissue similar to nerve tissue, although the cells are connected by intercalated discs, which conduct electrical potentials directly, rather than the chemical synapses used by neurons.
Specialized pacemaker cells normally determine the overall rate of contractions. The nervous system does contact the heart, but only sends signals to speed up or slow down the heart rate, rather than controlling each beat. Since cardiac muscle is myogenic, the pacemaker serves only to modulate the cells; the cardiac muscles would still fire in the absence of a pacemaker, albeit randomly, and the heart would go into fibrillation.
Cardiac muscle exhibits cross striations (these are the result of so called z-lines, which occur at regular intervals along each myofibril - usually about 2.5 micro meters apart) like those seen in skeletal muscle. A unique aspect of cardiac muscle is the number of nuclei found inside the cell. Skeletal muscle cells are multinucleated from the fusion of muscle cells and smooth muscle cells are strictly mononucleated, while cardiac muscle cells are mononucleated, binucleated and multinucleated. In the fetus and post parturition infant most cardiac muscle cells are mononucleated. Shortly after birth (within a few months) most cardiac muscles undergo a change of nucleation from mononucleated to primarily binucleated, and some go on to become multinucleated. Generally among species the cardiac muscle is 90% binucleated cells and 5% both mono and multinucleated cells, but exact numbers depend upon the species in question.
In addition, it may also be useful to appreciate some key histological differences between cardiac muscle and skeletal muscle. The T-tubules in cardiac muscle are shorter, broader and run along the Z-Discs and the sarcoplasmic reticulum of cardiac muscle lacks terminal cisternae.
See also
"Cardiac muscle"