Our heart is composed mostly of muscle cells that are somewhat similar to skeletal muscle cells yet retain certain fundamental differences. Although most of the events involved in contraction of heart (cardiac) muscle are the same as skeletal muscle, the heart is not attached to bone. Furthermore, our heart does not require the brain to tell it when to contract (beat). However, the brain certainly can play both a direct and indirect role in regulating the beating of our heart. The stimulus that invokes excitability in the heart comes from a specialized pacemaker region within our heart, called the sinoatrial node (SA node). The human heart may beat in excess of two billion times throughout a person’s life.
Unlike skeletal muscle, which pulls on bone when it contracts, the heart constricts in a wringing fashion when it contracts. As the heart contracts, the pressure of the blood inside the heart (ventricles) increases. This serves to propel blood out of the heart into the arteries. This increase in pressure also provides the driving force that forces blood to surge through our blood vessels. The dynamics of blood flow will be discussed in more detail in the final chapter.
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