The heart consists of a collection of individual cells that work together to continuously pump blood through the circulatory system. Some heart cells form connective tissues, others grow into valves and the remaining, known as myocytes, give the heart the ability to pump. An average adult heart is the size of a fist and pumps approximately 100,000 times a day, and over 2000 gallons of blood, according to the American Heart Association. In order to perform this continuous pumping action, the heart tissue, also called cardiac tissue, exhibits several unique properties.
Contracts Without Neural Stimulation
Unlike most muscle cells of the body, cardiac muscle cells do not require neural stimulation to contract. This means that the heart is not controlled by the brain sending signals through nerves to the heart muscle. This property is called automaticity, according to "The Fundamentals of Anatomy and Physiology." Specialized cells in the sinus node, the internal pacemaker of the heart, send electrical impulses to initiate the contractions of the heart.
Contains Intercalated Discs
Myocytes, or heart muscle cells, provide the heart the ability to pump. Individual myocytes can pump, as described by Cells Alive, but for a heart to work effectively, all the muscle cells must beat in unison. In order to do this, the cells form an interconnected membrane between them, resulting in a sheet of cells known as intercalated discs, which beat in unison.
Within these intercalated discs, there are gap junctions which allow for ions and small molecules to move between cells. The presence of the gap junction ensures that the action potential, the energy of the pumping action, travels across the intercalated disc moving quickly from cell to cell as described in the "The Fundamentals of Anatomy and Physiology" by Dr. Frederic Martini.
Requires Extracellular Calcium
Cells, in general, contain valves in the cellular membrane which allow ions, such as sodium and potassium, to pass into and out of a cell. This exchange of ions works to contract the muscle. In heart cells, the rhythm of the cells depends upon this complex series of valves opening and closing, according to Cells Alive. Cardiac cells also require the presence of extracellular, meaning outside the cell, calcium. Calcium flowing into the cells allows for a sustained contraction, which is why cardiac cells contractions last approximately 10 times longer than other muscle cells, as reported by the Judith Brown School for Continuing Education.
Regeneration
Due to damage caused by heart attacks and other heart diseases still being present years after the onset of the event, scientists believed that once damage occurs it remains permanent. This led to the conclusion that cardiac tissue cannot regenerate, or repair, itself. New information, however, provides insight that the cardiac tissue can regenerate, but that the rate of regeneration is very slow. Information published by the National Institutes of Health and the National Institute on Aging suggests that heart muscle cells replicate in two regions of the heart.
National Geographic also provides information on studies conducted on those exposed to radiation from the 1950's nuclear bombs. This study found that heart muscle cells can renew themselves at a rate of approximately 1 percent per year at age 25. As a person ages, the rate drops to approximately 0.45 percent by age 75. Although slow, the fact that heart tissue regenerates provides promise for new treatments for heart damage.
References
- The American Heart Association: How the Heart Works
- Cells Alive: Myocyte -- The Beating of a Single Cell
- "The Fundamentals of Anatomy and Physiology" by Dr. Frederic Martini: Chapter 10 Cardiac Muscle Tissue
- Judith Brown School for Continuing Education: Cardiac Muscle
- National Geographic: Heart Cells Can Regerate -- Nuclear Bomb Evidence Shows
