Merge: date=March 2008

The cardiac muscle is a type of involuntary striated muscle found in the walls of the heart. As it contracts, it propels blood into the heart and through the blood vessels of the circulatory system.

Metabolism

Cardiac muscle is adapted to be highly resistant to fatigue: it has a large number of mitochondria, enabling continuous aerobic respiration via oxidative phosphorylation, numerous myoglobins (oxygen-storing pigment) and a good blood supply, which provides nutrients and oxygen. The heart is so tuned to aerobic metabolism that it is unable to pump sufficiently in ischaemic conditions. At basal metabolic rates, about 1% of energy is derived from anaerobic metabolism. This can increase to 10% under moderately hypoxic conditions, but, under more severe hypoxic conditions, not enough energy can be liberated by lactate production to sustain ventricular contractions.

Under basal aerobic conditions, 60% of energy comes from fat (free fatty acids and triacylglycerols/triglycerides), 35% from carbohydrates, and 5% from amino acids and ketone bodies. However, these proportions vary widely according to nutritional state. For example, during starvation, lactate can be recycled by the heart. This is very energy efficient, because one NAD⁺ is reduced to NADH and H⁺ (equal to 2.5 or 3 ATP) when lactate is oxidized to pyruvate, which can then be burned aerobically in the TCA cycle, liberating much more energy (ca 14 ATP per cycle).

In the condition of diabetes, more fat and less carbohydrate is used due to the reduced induction of GLUT4 glucose transporters to the cell surfaces. However, contraction itself plays a part in bringing GLUT4 transporters to the surface. This is true of skeletal muscle, but relevant in particular to cardiac muscle, since it is always contracting.

Unlike skeletal muscle, which contracts in response to nerve stimulation, specialized pacemaker cells at the entrance of the right atrium termed the sinoatrial node display the phenomenon of automaticity and are myogenic, meaning that they are self-excitable without a requisite electrical impulse coming from the central nervous system. However, the sinoatrial node is modulated by the autonomic nervous system. The rest of the myocardium conducts these action potentials by way of electrical synapses called gap junctions. It is because of this automaticity that an individual's heart does not stop when a neuromuscular blocker (such as succinylcholine or rocuronium) is administered, such as during general anesthesia.

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.