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Cardiac muscle, also known as myocardium, is one of three major categories of muscles in the human body. It is composed of cardiac muscle cells or cardiomyocytes, which are electrically excitable, meaning they can initiate and conduct electrical impulses. The excitation of cardiac muscle cells results in the contraction of the heart, which is essential for pumping blood. This process, known as excitation-contraction coupling, involves the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum, triggering the cell's myofilaments to slide past each other. The coordination of these contractions is facilitated by gap junctions between adjacent cardiomyocytes, allowing for synchronized heartbeats. The SA node, containing the most excitable cells, sets the pace of the heart, with its cells spontaneously depolarizing due to a pacemaker potential. Thus, the cardiac muscle's ability to self-excite is fundamental to the heart's functioning, ensuring efficient blood pumping with each heartbeat.
| Characteristics | Values |
|---|---|
| Type of muscle | Cardiac muscle, also called the myocardium, is one of three major categories of muscles found within the human body, along with smooth muscle and skeletal muscle. |
| Involuntary control | Unlike skeletal muscle, cardiac muscle is under involuntary control. |
| Contraction | The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. |
| Contraction mechanism | The cardiac muscle contracts in a similar manner to skeletal muscle, but with some differences. |
| Electrical stimulation | Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. |
| Calcium release | The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. |
| Electrical impulses | T-tubules in cardiac muscle are bigger and wider than those in skeletal muscle, but fewer in number. They lie close to the cell's internal calcium store, the sarcoplasmic reticulum, and help regulate the concentration of calcium within the cell. |
| Electrical signals | Purkinje fibers rapidly conduct electrical signals through the heart. |
| Contraction coordination | Each cardiomyocyte needs to contract in coordination with its neighboring cells, working together to efficiently pump blood from the heart. |
| Shape | Viewed through a microscope, cardiac muscle cells are roughly rectangular. |
| Intercalated discs | Individual cardiac muscle cells are joined at their ends by intercalated discs to form long fibers. |
| Myofibrils | Each cell contains myofibrils, specialized protein contractile fibers of actin and myosin that slide past each other. |
| Excitation | Excitation of the cardiac muscle occurs through a small voltage change (about 0.1 V) called the cardiac action potential (AP). |
| Excitation-contraction coupling | The generation of a cardiac action potential is involuntary and proceeds via a process known as excitation-contraction coupling (ECC). |
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What You'll Learn
Cardiac muscle cells are electrically excitable
Cardiac muscle cells, also called cardiomyocytes, are electrically excitable. They can initiate and conduct electrical impulses, which is essential for their contractile function and, by extension, the pumping action of the heart.
Cardiac muscle cells are connected to each other via intercalated discs, forming long fibres. These discs contain gap junctions that allow the passage of ions between cells, facilitating synchronized contraction. The cardiac action potential, or the small voltage change (approximately 0.1 V) that occurs during excitation, triggers the release of calcium from the sarcoplasmic reticulum, the cell's internal calcium store. This release of calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling, resulting in contraction.
The Purkinje fibres, a component of the specialized conduction system of the heart, rapidly conduct electrical signals to the myocardial contractile cells in the ventricles. The electrical stimulus begins at the apex, and the contraction follows suit, travelling towards the base of the heart. This ensures that blood is pumped out of the ventricles and into the aorta and pulmonary trunk.
The sinoatrial (SA) node, positioned on the wall of the right atrium, contains the most excitable cells in the heart and sets the heart rate. The SA node initiates the action potential, which then spreads across the atria and reaches the atrioventricular (AV) node. After a brief delay, the impulse travels through the AV bundle and bundle branches to the Purkinje fibres, and the right papillary muscle via the moderator band. This sequence of events ensures the coordinated contraction of the heart, allowing it to function as a pump.
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Electrical coupling allows for coordinated action potentials
The cardiac muscle, or myocardium, is one of three major categories of muscles in the human body. It is composed of cardiac muscle cells, or cardiomyocytes, which are the contractile cells of the myocardium. These cardiomyocytes are responsible for the contractility of the heart, and therefore, its pumping action.
For the heart to pump blood efficiently, each cardiomyocyte must contract in coordination with its neighbouring cells. This coordination is achieved through electrical coupling, which allows for the propagation of coordinated action potentials from one cell to the next.
Action potentials are rapid, long-range electrical signals that can travel at a speed of up to one-third the speed of sound. In the context of the heart, these action potentials are generated through a process known as excitation-contraction coupling (ECC). ECC involves the transmission of electrical impulses, or action potentials, from the cell surface to the cell's core through structures called T-tubules. These T-tubules are highly branched invaginations of the cardiomyocyte membrane, or sarcolemma, and play a crucial role in ECC, action potential initiation and regulation, and maintaining the resting membrane potential.
Gap junctions, which are found at the intercalated discs connecting adjacent cardiomyocytes, facilitate electrical coupling between these cells. These gap junctions allow for the passage of ions between the cells, enabling synchronized contraction. Additionally, cardiac desmosomes, which are also found at the intercalated discs, anchor the cardiac muscle fibres together, contributing to the structural integrity of the heart.
The coordinated action potentials enabled by electrical coupling ensure that the sheets of cardiac muscle contract in a synchronized manner. This synchronized contraction allows the ventricles to squeeze in multiple directions simultaneously, maximising the amount of blood pumped out of the heart with each heartbeat.
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Calcium channels and calcium-induced calcium release
Calcium ions are the major signalling ions in cells, regulating muscle contraction, neurotransmitter secretion, cell growth, and migration. Calcium channels play a crucial role in the cardiac conduction system, and their mutation or dysfunction can lead to various diseases.
Cardiac muscle cells, or cardiomyocytes, have sarcolemma membranes that contain voltage-gated calcium channels. These channels are vital for the basic cellular electrophysiological properties and control of cardiac contractility. The T-tubules, which are part of the sarcolemma, play a role in excitation-contraction coupling (ECC). During the plateau phase (phase 2) of the action potential, voltage-sensitive dihydropyridine (DHP) receptors on the T-tubules allow the influx of calcium ions into the cell through L-type (long-lasting) calcium channels. This increase in intracellular calcium concentration triggers the sarcoplasmic reticulum to release more calcium through ryanodine receptors, known as calcium-induced calcium release.
The sarcoplasmic reticulum is the cell's internal calcium store, and it plays a crucial role in regulating the concentration of calcium within the cell. The calcium released from the sarcoplasmic reticulum causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling, leading to contraction. This process is similar to that of skeletal muscle, although there are some important differences.
The Purkinje fibres are additional myocardial conductive fibres that rapidly conduct electrical signals and spread the impulse to the myocardial contractile cells in the ventricles. They have a fast inherent conduction rate, allowing the electrical impulse to reach all ventricular muscle cells in a short time. The electrical stimulus begins at the apex, and the contraction follows suit, travelling towards the base of the heart. This allows for the efficient pumping of blood out of the ventricles and into the aorta and pulmonary trunk.
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Pacemaker cells set the rhythm of the heart
The heart is made up of cardiac muscle, also called the myocardium. This muscle is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force to supply blood to meet the metabolic demands of the entire body.
Cardiac muscle cells, or cardiomyocytes, are the contractile cells of the cardiac muscle. These cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Specialised modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. The pacemaker cells are only weakly contractile without sarcomeres, and are connected to neighbouring contractile cells via gap junctions.
Pacemaker cells are located in the sinoatrial node (SA node), positioned on the wall of the right atrium, near the entrance of the superior vena cava. The SA node is the primary pacemaker and regulates the heart's sinus rhythm. The SA node is a cluster of myocytes with pacemaker activity and is considered the heart's natural pacemaker. The SA node generates electrical impulses that set the rhythm and rate of the heart.
The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarises the other potential pacemaker cells (AV node) to initiate action potentials. This is the normal conduction of electrical activity in the heart. The key to the rhythmic firing of pacemaker cells is that, unlike neurons, these cardiomyocytes will slowly depolarise by themselves and do not need any outside innervation from the autonomic nervous system to fire action potentials.
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The SA node contains the most excitable cells in the heart
The SA node, or the sinus node, is a cluster of myocytes located in the upper part of the heart's right atrium. It is composed of specialized cardiac muscle cells that act as the heart's natural pacemaker. The SA node contains the most excitable cells in the heart, which are capable of spontaneously generating and conducting electrical impulses.
The SA node is responsible for initiating the electrical excitation signal that triggers each heartbeat. This signal travels through the heart's electrical conduction system, causing the myocardial contraction necessary for pumping blood. The SA node's pacemaker cells have a unique property called pacemaker potential, which allows them to automatically depolarize and generate electrical impulses without a resting phase.
The SA node's ability to generate electrical impulses is crucial for maintaining the heart's normal rhythm and rate. It ensures that the heart contracts in a coordinated manner, allowing it to efficiently pump blood throughout the body. The SA node's position in the upper part of the right atrium, near the superior vena cava, enables it to initiate the excitation signal and regulate the heart's electrical conduction system.
The SA node's excitable cells are connected to neighbouring contractile cells via gap junctions. These junctions facilitate the transmission of electrical impulses and ensure synchronized contractions. The SA node's unique characteristics, including its ability to spontaneously generate impulses and its continuous membrane potential changes, make it a critical component of the heart's electrical conduction system.
In summary, the SA node contains the most excitable cells in the heart due to their ability to spontaneously generate electrical impulses, maintain continuous membrane potential changes, and initiate the excitation signal for each heartbeat. These excitable cells play a vital role in the heart's electrical conduction system, ensuring the heart's normal rhythm and efficient blood pumping function.
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Frequently asked questions
Cardiac muscles are unique tissues located only in the heart. They are one of the three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle.
Cardiac muscles contract in a similar manner to skeletal muscles but with some key differences. Electrical stimulation triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Intercalated discs are critical structures that help support the synchronized contraction of the cardiac muscle. They consist of desmosomes, linking proteoglycans, tight junctions, and gap junctions that allow the passage of ions between the cells.
Purkinje fibers are additional myocardial conductive fibers that rapidly conduct electrical signals to the myocardial contractile cells in the ventricles. They help spread the impulse to the ventricular muscle cells, initiating contraction.
Yes, cardiac muscles are electrically excitable, meaning they can initiate and conduct electrical impulses. The electrical excitation of the cell membrane triggers the release of calcium, leading to muscle contraction.
