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Cardiac muscle is one of three types of muscles in the human body and is found in the walls of the heart. It is made up of specialised cells called cardiomyocytes, which are long and cylindrical and contract at regular intervals to keep blood flowing through the heart. Cardiac muscle cells are connected by intercalated discs, which allow the muscle cells to contract together in a fluid, synchronised fashion to enable the heart to work as a pump. The contraction of cardiac muscle cells is controlled by specialised cardiac muscle cells called pacemaker cells, which are connected to other cardiac muscle cells and respond to signals from the autonomic nervous system.
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What You'll Learn
Cardiac muscle cells
The human body contains three types of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle tissue, or myocardium, is unique to the heart. It contains cardiac muscle cells, which work together to perform highly coordinated actions that keep the heart pumping and blood circulating throughout the body.
Cardiac muscle tissue gets its strength and flexibility from its interconnected cardiac muscle cells, or fibres. Most cardiac muscle cells contain one nucleus, but some have two. The nucleus houses all of the cell's genetic material.
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Cardiomyocytes
Cardiac muscle cells, also called cardiomyocytes, are the contractile cells of the cardiac muscle. They are the primary cell type in the myocardium, which is the cardiac muscle itself. Cardiomyocytes are striated, branched, and contain many mitochondria, which provide the energy needed for the cell in the form of adenosine triphosphate (ATP). This makes them highly resistant to fatigue.
Each cardiomyocyte needs to contract in coordination with its neighbouring cells to work effectively as a pump. This coordination is achieved through intercalated discs, which are complex cell junctions unique to cardiac cells. These discs enable the rapid transmission of electrical impulses through the network of cardiomyocytes, allowing them to act in a coordinated contraction of the myocardium.
The cardiac muscle must contract with enough force to pump blood into circulation and supply the metabolic demands of the entire body. This is termed cardiac output and is defined as heart rate multiplied by stroke volume. Cardiomyocytes are responsible for generating this contractile force in the intact heart. They contract autonomously and rhythmically, without instructions from the nervous system.
Specialised cardiomyocytes, known as pacemaker cells, set the rhythm of the heart contractions. These cells are only weakly contractile and are connected to neighbouring contractile cells via gap junctions. They are distributed throughout the heart and are responsible for generating and sending out electrical impulses that cause the heart to beat.
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Cardiac muscle fibres
Cardiomyocytes are surrounded by a membrane called the extracellular matrix, which is produced by supporting fibroblast cells. Fibroblasts are smaller but more numerous than cardiomyocytes, and they play a crucial role in responding to injury. They are also responsible for creating and maintaining the extracellular matrix, which is composed of proteins such as collagen and elastin, as well as polysaccharides known as glycosaminoglycans.
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Cardiac muscle vs skeletal muscle
The human body is made up of more than 600 muscles that help us move, breathe, swallow, and stay alive. There are three types of muscle tissue in the body: skeletal, smooth, and cardiac.
Cardiac muscle, also called myocardium, is a special type of muscle tissue found only in the heart. It is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. This is termed cardiac output and is defined as heart rate multiplied by stroke volume. Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. They are under involuntary control. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess. Intercalated discs connect cardiac muscle cells. Gap junctions inside these discs relay electrical impulses from one cardiac muscle cell to another.
Skeletal muscle, on the other hand, is present in the arms and legs and is attached to the bones of the body. Unlike cardiac muscle, skeletal muscle is under voluntary control. Skeletal muscle fibres are also striated in appearance.
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Diseases of the cardiac muscle
Cardiac muscle is one of three types of muscles in the human body and is found in the walls of the heart. It is made up of specialised cells called cardiomyocytes, which are long and cylindrical and contract at regular intervals to keep blood flowing through the heart.
Cardiac muscle diseases, also known as cardiomyopathies, are a group of disorders that affect the heart's ability to function properly. These diseases can have a significant impact on health worldwide, with ischemic heart disease being the leading cause of morbidity and mortality globally. Any insult or injury to cardiac muscle can be severe due to the muscle cells' minimal regeneration capabilities.
Cardiomyopathies cause the heart to lose its ability to pump blood effectively, and in some cases, the heart rhythm also becomes disturbed, leading to arrhythmias or irregular heartbeats. There are many causes of cardiomyopathy, including viral infections, complex congenital heart disease, nutritional deficiencies, uncontrollable fast heart rhythms, certain types of chemotherapy, and genetic defects. Hypertrophic cardiomyopathy, for example, occurs when the muscle of the left ventricle thickens, blocking blood flow to the rest of the body. This can also affect the heart's mitral valve, causing blood to leak backward through the valve.
Cardiomyopathic diseases can be fatal, but some conditions can be treated if detected early. Ongoing research is being conducted to stimulate in situ stem cell growth or to grow stem cells ex vivo and inject them into damaged hearts. These trials show promise but are still in the early stages of development.
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Frequently asked questions
Cardiac muscles are one of three types of muscles in the human body and are found in the walls of the heart. They are made up of specialised cells called cardiomyocytes.
Cardiomyocytes are rectangular, branching cells that typically contain only one centrally-located nucleus. They are surrounded by a membrane that contains salt ions, which allows them to generate an electrical signal called action potentials in order to contract.
Cardiac muscles contract and relax to pump blood through the heart and into the rest of the body. They are controlled by specialised cardiac muscle cells called pacemaker cells that directly control heart rate.
