Cardiac muscle has some of the properties of both voluntary and involuntary muscle. It resembles the former in that it appears striped under the microscope, and the latter in that is capable of contracting without being stimulated by nerve impulses. The branching network of cardiac muscleenables waves of contraction to spread through the heart tissue. The rhythm of the heart is set by a specialized group of muscle in the upper part of the heart, called the sino-atrial node. This is the natural pacemaker of the heart. The cells contract spontaneously and rhythmically and the waves of muscle contraction spread throughout the heart. The influence of the parasympathetic nervous system on the heart rate is to decrease it. An increase in heart rate is caused by the sympathetic system and by the systemic release of adrenaline from the adrenal glands. Like involuntary muscle, cardiac muscle responds to stretching by contracting – the greater the stretch, the more forcefully the muscle contracts. As more enters the atria, and thereby stretches the cardiac muscle, the heart pumps the away more powerfully. Each contraction of a cardiac muscle cell is followed by a relatively long period when it cannot be stimulated. This is important to prevent the heart rate from racing.
The energy for cardiac contraction comes from stores of glycogen and fatty acids in the muscle, which are converted by the mitochondria inside the cells. The coronary arteries supply oxygen and nutrients to the heart. If the arteries become blocked or damaged, the cardiac muscle is starved of oxygen, stops beating, and the result is a heart attack.