Structure of the Heart Cell

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The wonder of anatomy known as the heart might be thought of as the one part of your body that absolutely cannot take a break. While your brain is the control center of the rest of you, its moment-to-moment functioning is exceptionally diverse and in some ways largely passive. In any event, "thinking," or interpreting and dispatching electrochemical signals is neither as obvious nor as dramatic as the beating of your heart, which an all likelihood you can feel by placing a hand over the left side of your chest at this moment.

As befits such an unusual and vital structure, the wiring and overall operating of the heart is unique within the human body. Like all organs and tissues, the heart is made up of tiny cells.

In the case of heart cells, called cardiomyocytes, the level of specialization of these cells and the tissues to which they contribute is as profound as it is exquisite.

Overview of the Cardiovascular System

If someone asked you, "What is purpose the heart?" you might instinctively respond, "To pump blood throughout the body." Technically, you'd be right. But why does the body need to be continually bathed in blood in the first place?

There are actually a number of reasons. The blood distributes oxygen and glucose to the body's tissues, but relatedly, and just as important, it picks up carbon dioxide and other metabolic waste products.

The heart's activity also gets hormones (natural chemical signalers) to their target tissues, and helps promote homeostasis, or a more-or-less-constant internal environment in terms of chemistry, fluid balance and temperature.

The heart has four chambers: two atria (singular: atrium) that receive blood from the veins and operate as primer pumps, and two ventricles, which are by far the stronger pumps and eject blood into the arteries. The right side of the heart gives and receives blood to and from the lungs only, while left side heart services the rest of the body.

Arteries are strong-walled vessels that get blood from the heart to capillaries, the tiny, thin-walled exchange points where materials can enter and leave the circulatory system. Veins are the collecting tubes, and these are what are "poked" when you are asked to give a blood sample because the blood pressure in these vessels is considerably lower than it is in the arteries.

Basic Heart Anatomy

The heart is not a uniform organ. It is known for being mainly muscle, but also contains other vital elements to protect it and make its job easier in various ways.

The heart has an outer layer called the pericardium (or epicardium), which itself includes an outer fibrous layer and an inner serous, or watery, layer. Underneath this protective and lubricating layer is the thick myocardium, discussed in detail shortly. Next is the endocardium, which contains adipose (fat), nerves, lymph and other diverse elements, and is continuous with the valves.

The heart includes four distinct valves, one each between the left and right atrium and ventricle, one between the right ventricle and the pulmonary arteries to the lungs, and one between the left ventricle and the large aorta, the artery that essentially serves the entire body at root level.

The fibrous skeleton runs throughout the various layers and tissues of the heart to give it solidity and anchor points for other tissues. Finally, the heart has a unique and complex conduction system that includes as its major features the sinoatrial (SA) node, the atrioventricular (AV) node and the Purkinje fibers running through the septum, or wall, between the atria and the ventricles.

Structure of the Cardiomyocyte

The primary cells of the heart are cardiac muscle cells, or cardiomyocytes. ("Myocyte" means "muscle cell.") The cardiac muscle cell organelles (membrane-bound components) are fundamentally the same as those found in other mammalian cells, but this is a lot like saying that a well-worn kid's bike on display at a yard sale has the same parts as a Tour de France racing bike.

Cardiac muscle cells are elongated and somewhat tubular, like muscles themselves. The basic unit of a cardiomyocyte is the sarcomere, which consists mostly of contractile proteins and mitochondria – tiny "power plants" that generate a fuel molecule called adenosine triphosphate (ATP) when oxygen is present. There is also a network of tubules called the sarcoplasmic reticulum, which is rich in calcium ions (Ca2+), these ions being indispensable for proper muscle contraction.

The proteins in the cardiomyocyte are arranged in parallel bundles and include both thick filaments and thin filaments, which overlap with each other to form the physical basic for an actual muscle contraction. This area of overlap is darker than the rest of the cell and is known as the A-band.

The very middle of a sarcomere contains only thick filaments because thin filaments do not extend completely inward from the two ends of the sarcomere, regions called Z-lines. Finally, the area extending in both directions from any Z-line, toward the centers of adjacent sarcomeres, is called the I-band.

The Myocardium

At a more gross (macro) level than the cardiomyocytes reveal, the myocardium itself, or the muscular substance of the heart, differs from skeletal muscle in four important ways:

  1. Cardiomyocytes often branch; regular myocytes form linear chains of cells and do not.
  2. The myocardium features prominent connective tissue in its substance, whereas regular muscle is anchored to bones, ligaments and tendons.
  3. The nuclei of cardiomyocytes are in the middle of the cell and have a perinuclear halo.
  4. Cardiomyocytes have intercalated discs running across them at branching points, and these structures allow for the coordinated contraction of various heart muscle fibers at once. 

Structures called T-tubules extend from the cell membrane to the interior of cardiomyocytes, which allows electrical impulses to reach the inside of the sarcomeres. The myocardium contains a high density of mitochondria, which is perhaps expected of a muscle that speeds up and slows down, but never stops working altogether.

Cardiac Physiology

A discussion of the heart's mechanical marvels could fill an entire chapter, but the basic things to know are that the factors determining how much blood the heart will pump include the heart rate, the preload (i.e., the amount of blood filling the heart from the lungs and body), the afterload (i.e., the pressure the heart is pumping against) and characteristics of the myocardium itself.

Excessive dilation of the heart's main pumping chamber, the left ventricle (and can you figure out why this one is the strongest and most important of the four cardiac chambers?), is often a sign of a "flabby" heart that does not pump a significant amount of the blood, filling it with each stroke, causing a back-up of fluid throughout the body, including the lungs and gravity-affected areas such as the ankles.

This condition a type of cardiomyopathy called congestive heart failure, or CHF, and it can usually be controlled with drugs and dietary modifications.

The Cardiac Action Potential

The heart beats as a result of electrical activity that is generated at the SA node and then propagated down to the AV node and through the Purkinje fibers in a highly coordinated manner even at very high heart rates (exceeding 200 per minute, or three per second).

The heart cell membrane has a resting electrical potential that is slightly more negative than the membrane potential of other body cells. When the membrane is sufficiently perturbed, various ion channels open, allowing the influx and outflow of potassium (K+) and sodium (Na+) ions in addition to calcium.

The sum of this electrochemical activity is responsible for the characteristic pattern of an electrocardiogram (EKG or ECG; EKG is based on the German version of the word), a vital tool in clinical medicine used to assess various disorders of the heart.

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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