Cardiac muscle
Identifiers
MeSH	D009206
TA98	A12.1.06.001
TA2	3950
FMA	9462
Anatomical terminology [edit on Wikidata]

Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusively 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 coming from the central nervous system.

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 inherent contractile activity is heavily regulated by the autonomic nervous system. If synchronization of cardiac muscle contraction is disrupted for some reason (for example, in a heart attack), uncoordinated contraction known as fibrillation can result.

This transmission of impulses makes cardiac muscle tissue similar to nerve tissue, although cardiac muscle cells are notably connected to each other by intercalated discs. Intercalated discs conduct electrochemical potentials directly between the cytoplasms of adjacent cells via gap junctions, in contrast to the chemical synapses used by neurons.

Specialized pacemaker cells in the sinoatrial node normally determine the overall rate of contractions, with an average resting pulse of 72 beats per minute.

The central nervous system does not directly create the impulses to contact the heart, but only sends signals to speed up or slow down the heart rate through the autonomic nervous system using two modes:

• (1) sympathetic nervous system (stress)
• (2) parasympathetic nervous system (restful) modulation

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.

In contrast to skeletal muscle, cardiac muscle cannot contract in the absence of extracellular calcium ions as well as extracellular potassium ions.

Cardiac muscle exhibits cross striations formed by alternation segments of thick and thin protein filaments which are anchored by segments called Z-lines. The primary structural proteins are actin and myosin.

The actin filaments are thin causing the lighter band appearance in muscle, while myosin is thicker and darker lending a darker appearance to the alternating bands in cardiac muscle as observed by a light enhanced microscope.

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, and cardiac muscle cells are mononucleated in humans.

In some animal species, 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.

Another histological differences between cardiac muscle and skeletal muscle is that 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.

• Indiana State University, Muscle action

• Overview at mcg.edu

• Heart
• Circulatory system
• Cardiac action potential