Explain Cardiac Muscle or heart muscle?

Cardiac muscle, or heart muscle, is a striated muscle that occurs only in the vertebrate heart. The heartbeat is controlled by noncontractile cells in the heart called Purkinje fibers. These cells have no contractile proteins and are specialized for electrical conduction. The heartbeat is initiated by electrical activity of the pacemaker or sinoatrial node (S. A.) in the wall of the right atrium. How heart cells keep up their rhythmic contraction is not known, but heart cells in culture continue to beat, and even single heart cells beat rhythmically, independently of other muscle cells or external innervation.

An electrical impulse spreads from the sinoatrial node to all parts of the atrium and then to the atrioventricular node, which is located adjacent to the partition between the two ventricles. From here, the impulse spreads through the ventricles and triggers simultaneous contraction of the ventricles.

As in smooth muscle, cardiac muscle cells are joined by gap junctions, structures that contain tiny pores through which electrical currents and calcium can pass from one cell to another. Although the heart beats regularly without nervous intervention, there is a main nerve to the heart, the tenth cranial nerve, or vagus. The vagus nerve contains branches of both the sympathetic and autonomic nervous systems. The sympathetic activity speeds the heart rate, and the autonomic slows it. Cardiac muscle has many mitochondria, as would be expected from its constant activity.