What Are Cardiomyocytes and How Do They Work?

The heart beats approximately 100,000 times each day, an endurance made possible by highly specialized cells called cardiomyocytes. These are the muscle cells that form the walls of the heart, known as the myocardium. Their primary function is to contract and relax in a coordinated rhythm, generating the force required to pump blood and sustain every other cell in the body.

The Specialized Architecture of Heart Muscle Cells

Cardiomyocytes possess a unique, branched structure that allows them to connect with several neighboring cells, forming a complex, interwoven network. This physical arrangement enables the heart to work as a unified organ. A cardiomyocyte contains just one centrally located nucleus and is packed with mitochondria, which supply the constant stream of adenosine triphosphate (ATP) needed to fuel continuous cardiac contraction.

The contractile force of cardiomyocytes originates from structures called sarcomeres, which are repeating units of protein filaments, mainly actin and myosin. The arrangement of these filaments gives the cell a striped, or striated, appearance. The sliding action of these filaments past one another causes the muscle cell to shorten and generate force, driving the powerful contractions of the heart.

To ensure the network of cells contracts in a synchronized fashion, cardiomyocytes are linked by specialized junctions called intercalated discs. These structures contain desmosomes, which act like strong rivets holding the cells tightly together during forceful contractions. They also feature gap junctions, which are channels that allow electrical signals to pass rapidly from one cell to the next, ensuring the wave of contraction spreads evenly.

How Cardiomyocytes Keep the Heart Beating

The heart’s rhythmic beat results from a process known as excitation-contraction coupling, which converts an electrical stimulus into a mechanical force. The cycle begins when an electrical impulse, or action potential, is generated by pacemaker cells and travels across the cardiomyocyte’s outer membrane, the sarcolemma. This signal dives deep into the cell’s interior through a network of transverse tubules, or T-tubules.

This electrical signal triggers the release of calcium ions from an internal storage site known as the sarcoplasmic reticulum. The increase in intracellular calcium is the direct trigger for muscle contraction. Calcium ions bind to regulatory proteins on the actin filaments, allowing the myosin filaments to pull on them and cause the sarcomere to shorten. This collective shortening results in the forceful contraction of the cardiomyocyte.

This process is coordinated across millions of cells because the electrical impulse spreads almost instantaneously from one cardiomyocyte to the next through the gap junctions. This creates a functional syncytium, a network of cells that behave as a single unit. Following contraction, calcium is pumped back into storage, allowing the cell to relax and prepare for the next beat.

The Life Cycle of a Cardiomyocyte

During embryonic development, cardiomyocytes arise and organize to form the heart’s basic structure. After birth, the heart continues to grow to meet the body’s demands. This growth occurs differently than in most other tissues; instead of cells dividing (hyperplasia), individual cardiomyocytes get larger, a process known as hypertrophy.

A person is born with most of the cardiomyocytes they will ever have. While there is evidence of very slow regeneration and replacement of these cells throughout life, the rate is extremely limited. It is estimated that more than half of the cardiomyocytes present at birth continue to function for an entire lifetime.

Because adult cardiomyocytes have a restricted ability to divide and replace themselves, any significant loss of these cells is permanent. When cardiomyocytes die, as they do during a heart attack, the body cannot effectively replace them with new, functional muscle cells. Instead, the damaged area is repaired with non-contractile scar tissue. This limitation is a central challenge in treating heart disease.

Cardiomyocytes and Heart Health

The health and function of cardiomyocytes are directly linked to the overall health of the heart. A myocardial infarction, or heart attack, occurs when a blockage in a coronary artery cuts off the oxygen supply to a region of the heart muscle. This causes the death of cardiomyocytes in that area, and the resulting scar tissue permanently weakens the heart’s pumping ability.

In cases of chronic stress, such as high blood pressure or valve disease, healthy cardiomyocytes can undergo maladaptive changes. They may enlarge pathologically to compensate for the increased workload, a condition that can contribute to heart failure. In this state, the heart muscle is no longer able to pump blood efficiently to meet the body’s needs.

Diseases that directly affect the heart muscle are known as cardiomyopathies. These conditions can be caused by genetic factors, infections, or other stressors and result in cardiomyocytes that are weakened, enlarged, or stiff. Problems with the electrical properties of cardiomyocytes or their coordination can lead to arrhythmias, or irregular heartbeats, where the heart beats too fast, too slow, or erratically.