Cardiologists work with one of the most amazing organs in the entire human body: the human heart. One of the features that makes the heart so special is the cellular structure of its tissue. The cardiac muscle cells are what allow blood to flow through the heart and therefore feed the rest of the body’s tissues with oxygen and other nutrients. The pumping action of the heart is made possible by the unique structure of the cells; their repeated tension and relaxation is what lets the pumping action occur.

Electrical activity is what lies at the basis of cardiac muscle cells’ functioning. In the type of cells known as striated muscle cells, the contraction is triggered when a change in electrical voltage occurs on the surface of the cell membrane. Unlike other cells in the body, cardiac cells are able to generate rhythmic contractions.

All cells in the human body have an electrical potential (or voltage) across their membranes. These membrane potentials are caused by the separation of electrical charges across the membrane. Contrary to the way electricity travels through a wire, a current does not travel through the cells but instead is carried through them via ions in the cell membrane (ions are molecules that have either gained or lost an electron). Sodium, potassium, and calcium are the three ions that help relay electrical signals in the heart. Sodium and calcium are most concentrated in the fluid between cells, whereas potassium is more concentrated in the fluid within the cell itself. The cardiac cells are connected to each other by intercalated disks; an electrical current is transmitted throughout the heart at varying speeds, with each area of the heart having a different conduction velocity. An electrocardiogram (ECG) measures this voltage from different areas of the heart, with each area having a separate line on the ECG readout.

Surprising to most patients may be that the electrocardiogram is an exam that measures how the voltage between two points on the patient’s body changes over time as a result of the electrical events of the cardiac cycle. When the patient’s heart is beating normally, it is because the cells in one area of the heart are spontaneously acting as pacemakers of sorts – their contractions are rhythmic and consistent. The cells of the heart fire spontaneously at about 100 beats per minute; heart beat rate can be higher or lower than this, but only due to outside influences. The EKG exam that is so familiar to many patients is thus a way to track and measure the electrical signals that the heart sends. If there is a problem with a particular area of the heart or with its cellular function, the EKG will help locate the problem.

One final part of the heart’s functioning that may be unknown to patients is that the autonomic nervous system helps regulate the action of cardiac cells in the heart. The nervous system helps to control the depolarization (which alters the electrical charge) of cells in one area of the heart called the sinoatrial node. The sinoatrial node (known more simply as the SA node) is a small mass of specialized cardiac muscle fibers located on the front wall of the right atrium of the heart. The SA node acts as a pacemaker for the rest of the heart, generating the electric impulses of the heartbeat that travel through the rest of the heart.

All patients, and especially those with cardiac disease, should feel an immense sense of awe at the complexity and functioning of the human heart. The unique chemical, electrical, and cellular traits of the heart and its cells allow a healthy individual to adapt heart rate as needed, whether this is physical exercise, rest, or an intermediate activity. Cardiologists specialize in treating the heart when it becomes diseased, and are dedicated to detecting abnormalities as soon as these appear.

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