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.
Three types of membrane junctions exist within an intercalated disc: fascia adherens, macula adherens, and gap junctions. Fascia adherens are anchoring sites for actin, and connects to the closest sarcomere. Macula adherens stop separation during contraction by binding intermediate filaments joining the cells together, also called a desmosome. Gap junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. When observing cardiac tissue through a microscope, intercalated discs are an identifying feature of cardiac muscle
The central nervous system does not directly create the impulses to contract the heart, but only sends signals to speed up or slow down the heart rate through the autonomic nervous system using two opposing kinds of modulation:
Since cardiac muscle is myogenic, the pacemaker serves only to modulate and coordinate contractions. The cardiac muscle cells would still fire in the absence of a functioning SA node pacemaker, albeit in a much less efficient manner. When another part of the conducting system takes over when there is damage to the SA node, it is then referred to as an ectopic pacemaker. Note that the heart can still beat properly even if its connections to the central nervous system are completely severed.
The reason for the calcium dependence is due to the mechanism of calcium-induced calcium release (CICR) from the SR that must occur under normal excitation-contraction (EC) coupling to cause contraction.
Like skeletal muscle, the primary structural proteins of cardiac muscle are actin and myosin. The actin filaments are thin causing the lighter appearance of the I bands in muscle, while myosin is thicker lending a darker appearance to the alternating A bands as observed by a light enhanced microscope.