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The heart is a hard-working muscle that beats thousands of times a day to keep us alive. The heart muscle is also known as the myocardium, and it forms the thickest layer of the heart wall. The heart wall is a three-layered structure with the myocardium sandwiched between the inner endocardium and the outer epicardium. The heart muscle is made up of cells called cardiomyocytes, which are striated, branched, and contain many mitochondria. These cardiomyocytes facilitate cell-to-cell communication through intercalated discs, allowing the heart to contract and pump blood. The heart is an involuntary muscle, meaning it beats on its own without our input, and is one of three major categories of muscles in the human body, alongside smooth and skeletal muscles.
| Characteristics | Values |
|---|---|
| Name of the heart muscle | Myocardium or cardiac muscle |
| Number of chambers in the heart | Four |
| Layers of the heart wall | Three |
| Inner layer of the heart wall | Endocardium |
| Middle layer of the heart wall | Myocardium |
| Outer layer of the heart wall | Epicardium or visceral pericardium |
| Muscle cells | Cardiomyocytes |
| Cardiomyocytes structure | Striated, branched, contain many mitochondria, and are under involuntary control |
| Cardiomyocytes nucleus | Uninucleated or rarely binucleated |
| Cardiomyocytes cell membrane | Sarcolemma |
| Sarcolemma composition | Voltage-gated calcium channels and other ion channels |
| Cardiomyocytes connection | Intercalated discs with gap junctions and desmosomes |
| Cardiomyocytes contraction | Synchronous |
| Pacemaker cells | Set the rhythm of heart contractions |
| Pacemaker cells location | Sinoatrial node on the wall of the right atrium |
| Heart rate determination | Depolarization of the sinoatrial node |
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What You'll Learn
The heart is a muscle
Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria. They are under involuntary control, meaning they contract without conscious input. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, which are specialised ion channels that skeletal muscles do not possess. Cardiomyocytes are connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously, enabling the heart to work as a pump.
The heart begins as a single chamber, but four separate chambers are created through the growth of various septa. The muscular ventricular septum originates from the bottom of the ventricle, with a membranous septum forming shortly after. The endocardial cushions separate the left and right atria and ventricles. Any structural changes or defects in these processes can lead to congenital heart disorders.
The primary function of the heart muscle is to pump blood into circulation by generating sufficient force. The contractile functions of the heart require adenosine triphosphate (ATP), which is obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Cardiomyocytes are the only heart muscle cells, and humans are born with a set number of them. These cells increase in size as the heart grows during childhood development and are slowly turned over during ageing.
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Cardiac muscle, or myocardium
Cardiac muscle is made up of sarcomeres that allow for contractility. It is composed of cells called cardiomyocytes, which are striated, branched, and contain many mitochondria. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma contains voltage-gated calcium channels, specialised ion channels that skeletal muscle does not possess. Cardiomyocytes are linked to the basement membrane by specialised glycoproteins called integrins.
The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base) and radially (becoming thicker from side to side).
Cardiac muscle is under involuntary control. It squeezes and relaxes to pump blood through the cardiovascular system. The heart is a hard-working muscle that beats thousands of times a day to keep us alive. Cardiomyocytes facilitate cell-to-cell communication through intercalated discs, which contain gap junctions and other cell adhesions. These discs reduce internal resistance, allowing action potentials to spread quickly throughout the heart muscle via electrically charged particles called ions. Thus, the heart muscle acts as a functional syncytium with rapid synchronized contractions responsible for pumping blood.
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The heart wall is a three-layered structure
The heart is a hard-working muscle that beats thousands of times a day to keep us alive. The heart wall is a three-layered structure, with a thick layer of myocardium (cardiac muscle) sandwiched between the inner endocardium and the outer epicardium (or visceral pericardium).
The myocardium is the middle and thickest layer of the heart wall. It is also the only place in the body where cardiac muscle is found. Cardiac muscle is made up of branched fibres of cardiomyocytes, which are connected via intercalated discs. These discs contain gap junctions and desmosomes, which allow the cardiomyocytes to contract together synchronously, enabling the heart to work as a pump. The myocardium is responsible for the contractile function of the heart, generating force to maintain adequate tissue and organ perfusion throughout the body.
The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels connected to the heart. The endocardium is the innermost layer of the heart wall.
The outer epicardium forms part of the pericardial sac, which surrounds, protects, and lubricates the heart. The heart's blood supply comes directly from the coronary arteries within the epicardial layer.
Within the myocardium, the sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base) and radially (becoming thicker from side to side).
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Cardiomyocytes
The heart is the first organ to develop in a foetus. Embryonic cardiomyocytes develop into a linear heart tube that subsequently undergoes rightward looping to initiate a primitive heart structure. The heart begins as a single chamber, but four separate chambers are created through the growth of various septa. The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium. The myocardium forms the bulk of the heart and is made up of cardiac muscle cells or cardiomyocytes.
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Heart muscle cells are involuntary
The heart is a muscle, and like all muscles, it is made up of muscle cells. The muscle cells of the heart are called cardiomyocytes, and they are involuntary. This means that they contract and relax without conscious input from the brain. This is also true of the smooth muscles that line the inside of some organs, including the female and male reproductive systems, urinary system, respiratory system, and digestive system.
Cardiac muscle cells are striated, branched, and contain many mitochondria. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscle does not possess. The calcium channels are necessary for muscle contraction. The cardiomyocytes are connected by intercalated discs, which contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously, enabling the heart to work as a pump.
The heart muscle, or myocardium, forms the thickest layer of the three-layered heart wall. The myocardium is sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. The outer epicardium forms part of the pericardial sac, which surrounds, protects, and lubricates the heart.
Within the myocardium, there are several sheets of cardiomyocytes. The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously. This coordinated contraction involves the cardiac muscle and electrical impulses and requires ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
Humans are born with a set number of heart muscle cells, which increase in size as the heart grows during childhood development. Evidence suggests that cardiomyocytes are slowly turned over during aging, but less than 50% of the cardiomyocytes present at birth are replaced during a normal lifespan.
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Frequently asked questions
The heart muscle is called the myocardium.
The myocardium is the middle and thickest layer of the heart wall. It is made up of cardiac muscle cells or cardiomyocytes.
Cardiomyocytes are striated, uninucleated muscle cells found exclusively in the heart. They are responsible for the contractile function of the heart.
The myocardium pumps blood into circulation by generating sufficient force.
The heart is surrounded by the musculoskeletal system, which includes the chest muscles, rib cage, and connective tissues.
