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Cardiac muscle, also known as myocardium or heart muscle, is one of three types of muscle tissues found in vertebrates, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the bulk of the heart, keeping it pumping and relaxing normally. The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The individual cardiac muscle cells or cardiomyocytes are joined by intercalated discs to form long fibres. The rhythmic contractions of the cardiac muscle are regulated by the sinoatrial node of the heart and are thus not under voluntary control.
| Characteristics | Values |
|---|---|
| Muscle type | Cardiac muscle is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. |
| Location | Cardiac muscle is found only in the heart. |
| Function | Cardiac muscle keeps the heart pumping and relaxing normally. |
| Involuntary movement | Cardiac muscle contracts and releases involuntarily. |
| Contraction | The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of the blood into the pulmonary and systemic vessels. |
| Rhythm | Cardiac muscle exhibits rhythmic contractions and is not under voluntary control. |
| Structure | Cardiac muscle forms a highly branched cellular network in the heart. |
| Composition | Cardiac muscle is composed of individual cardiac muscle cells joined by intercalated discs, and encased by collagen fibers and other substances that form the extracellular matrix. |
| Shape | Cardiac muscle cells are roughly rectangular. |
| Size | Cardiac muscle cells measure 100–150μm by 30–40μm. |
| Nucleus | The majority of cardiac muscle cells contain only one nucleus. |
| Mitochondria | Cardiac muscle cells contain mitochondria, which provide the energy needed for the cell in the form of adenosine triphosphate (ATP). |
| T-tubules | T-tubules are microscopic tubes that run from the cell surface to deep within the cell. They are bigger and wider but fewer in number than those in skeletal muscle. |
| Calcium | Calcium is released from the cell's internal calcium store, the sarcoplasmic reticulum, triggering the contraction of the cardiac muscle. |
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What You'll Learn
Cardiac muscle forms the bulk of the heart
Cardiac muscle, also called myocardium or heart muscle, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The individual cardiac muscle cell (cardiomyocyte) is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes. This process activates from the release of calcium from the sarcoplasmic reticulum (SR) when delivering an action potential to the muscle, in a process called excitation-contraction coupling. The sliding of actin and myosin past each other produces the formation of “cross-bridges,” which causes contraction of the heart and generation of force.
Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end to end by intercalated discs and are organized into layers of myocardial tissue that are wrapped around the chambers of the heart. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of the blood into the pulmonary and systemic vessels.
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It is one of three types of vertebrate muscle tissues
Cardiac muscle, also called myocardium or heart muscle, is one of three types of vertebrate muscle tissues. The other two types are skeletal muscle and smooth muscle. Cardiac muscle is found only in the heart, constituting the main tissue of the heart wall.
The heart wall is a three-layered structure, with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The cardiac muscle forms both the atria and the ventricles of the heart. The myocardium found in the ventricles is thick to allow for forceful contractions, while the myocardium in the atria is much thinner. The individual cardiac muscle cells are joined by intercalated discs to form long fibres.
Cardiac muscle tissue works to keep the heart pumping through involuntary movements. This differentiates it from skeletal muscle tissue, which is under voluntary control. The rhythmic contractions of cardiac muscle are regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels.
Cardiac muscle cells contain many mitochondria, which provide the energy needed for the cell in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue.
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Cardiac muscle cells are connected by intercalated discs
Cardiac muscle tissue, or myocardium, is a type of muscle tissue that forms the heart. It is one of three types of muscle tissues in the human body, the others being skeletal muscle and smooth muscle. Cardiac muscle tissue is found only in the heart and is responsible for keeping the heart pumping and relaxing normally. It contracts and releases involuntarily, keeping the heart pumping blood around the body.
Cardiac muscle cells, or cardiomyocytes, are the individual cells that make up cardiac muscle. They are connected end to end by intercalated discs, forming a highly branched cellular network in the heart. Intercalated discs are small connections that join cardiac muscle cells to each other. They contain gap junctions and desmosomes. Gap junctions are responsible for transferring stimulation from one cardiac muscle cell to the next, allowing the cardiomyocytes to contract together synchronously. Desmosomes are intercellular structures that anchor cardiac muscle fibres together, maintaining the structural integrity of the heart.
Cardiac muscle cells are roughly rectangular in shape when viewed through a microscope. They contain mitochondria, which are organelles that convert oxygen and glucose into energy in the form of adenosine triphosphate (ATP). This energy is used to power the contraction of the cardiac muscle cells. Each cell also contains myofibrils, which are composed of thick and thin filaments of the proteins myosin and actin. These filaments slide past each other during contraction, causing the cell to shrink. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance, similar to skeletal muscle.
The outside of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma, which acts as a barrier between extracellular and intracellular contents. The sarcolemma contains voltage-gated calcium channels, which are involved in the release of calcium during an action potential. This calcium release triggers the contraction of the cardiac muscle cell.
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The rhythmic contractions are regulated by the sinoatrial node
The heart is a pump made of muscle tissue. Its rhythmic contractions are regulated by the sinoatrial node (SA node), a small mass of specialised tissue located in the upper right chamber (atria) of the heart. The SA node acts as the heart's pacemaker, generating electrical impulses that stimulate the heart to contract and pump blood.
The SA node is an oval-shaped region of cardiac muscle, approximately 15mm long, 3-4mm wide, and 1mm thick. It is located directly below and to the side of the superior vena cava, a large vein that brings oxygen-poor blood from the body to the heart. The SA node is composed of specialised cells known as pacemaker cells, which produce electrical impulses known as cardiac action potentials.
These electrical impulses travel through the heart's electrical conduction system, causing the heart to contract. The conduction system consists of nodes, cells, and signals that work together to control the heartbeat. The electrical impulse travels from the SA node to the atrioventricular node (AV node), where it is slowed down briefly before continuing through the conduction pathway to the ventricles.
The AV node is about 5mm long and 5mm wide and is located in the centre of the heart. From the AV node, the electrical impulse travels through the bundle of His, which divides into right and left bundle branches to stimulate the right and left ventricles. The ventricles then contract, pumping blood out to the body.
The rate at which the SA node produces electrical impulses determines the heart rate. At rest, a healthy heart rate is between 60 and 100 beats per minute. This is influenced by two sets of nerves: parasympathetic nerves slow down impulse production, while sympathetic nerves speed it up. The autonomic nervous system also controls the speed at which the SA node sends electrical signals.
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Cardiac muscle tissue is responsible for keeping the heart pumping
Cardiac muscle tissue, also known as myocardium, is a type of muscle tissue that forms the heart. It is one of three types of muscle tissues in the human body, the other two being skeletal and smooth muscle. The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (or visceral pericardium).
The individual cardiac muscle cells are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other during contraction and relaxation. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed under a microscope.
The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels. This process is vital for pumping blood throughout the cardiovascular system.
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Frequently asked questions
Cardiac muscle is a type of muscle tissue that forms the heart. It is one of three types of muscle tissues in the body, the others being skeletal and smooth muscle.
Cardiac muscle is only found in the heart. It is the main tissue of the heart wall and makes up the thick middle layer of the three-layered heart wall structure.
Cardiac muscle is responsible for the heart's pumping action. It contracts and relaxes involuntarily, keeping the heart pumping blood around the body.
Cardiac muscle cells are connected and work together to contract in a coordinated manner. This coordination is regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker.
Doing aerobic exercise, such as cardio workouts, can help keep cardiac muscle strong and healthy.
