Dominic Stone
The human body contains three types of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle, also called myocardium or heart muscle, is a specialised tissue that exists only in the heart. It is responsible for keeping the heart pumping and blood circulating around the body. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium). The coronary arteries supply blood to the cardiac muscle, and cardiac veins drain this blood. The cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria. They are under involuntary control and contract in response to electrical impulses from the nervous system.
| Characteristics | Values |
|---|---|
| Name | Cardiac Muscle |
| Other Names | Heart Muscle, Myocardium |
| Type of Muscle Tissue | Involuntary, Striated |
| Composition | Individual cardiac muscle cells or cardiomyocytes joined by intercalated discs |
| Location | Thick middle layer of the heart wall between the outer layer (pericardium/epicardium) and the inner layer (endocardium) |
| Blood Supply | Coronary circulation, specifically coronary arteries |
| Blood Drainage | Coronary veins |
| Function | To pump blood into circulation by generating sufficient force through rhythmic, wave-like contractions |
| Contraction Mechanism | Electrical impulses trigger the release of calcium from the sarcoplasmic reticulum, causing myofilaments to slide past each other in a process called excitation-contraction coupling |
| Pacemaker Cells | Sinoatrial node (primary), Atrioventricular node (secondary) |
| Diseases | Cardiomyopathies (e.g., Ischemic conditions like angina and myocardial infarction), Hypertrophic Cardiomyopathy (HCM), Restrictive Cardiomyopathy (RCM) |
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What You'll Learn
- Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria
- The heart is made up of three layers—pericardium, myocardium, and endocardium
- The primary function of cardiomyocytes is to contract, generating the pressure needed to pump blood through the circulatory system
- The myocardial action potential occurs in five phases, beginning with rapid depolarization and ending with stabilization at the resting potential
- The cardiac muscle is supplied with blood by the coronary arteries
Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria
Cardiac muscle cells, also called cardiomyocytes, are striated, branched, and contain many mitochondria. They are the individual cells that make up the cardiac muscle, which is one of three types of muscle in the body, the others being skeletal and smooth muscle. The primary function of cardiomyocytes is to contract, generating the pressure needed to pump blood through the circulatory system.
Cardiac muscle cells are striated, meaning they have a striped appearance when viewed under a microscope. This is due to the regular organisation of myofibrils, which are composed of thick and thin filaments of the proteins actin and myosin. These filaments slide past each other during muscle contraction and relaxation, forming cross-bridges that cause the heart to contract and generate force. The cardiac muscle must contract with enough force to pump blood and supply the metabolic demands of the entire body. This is known as cardiac output and is determined by the contractile forces of the cardiac muscle and the frequency of their activation.
Cardiomyocytes are also branched, unlike skeletal muscle cells, which are linear and longitudinal. This branching structure allows cardiomyocytes to be connected in a network that functions as a syncytium, enabling the coordinated and efficient functioning of the myocardium during each heartbeat. The points of apposition between cardiac muscle cells contain specialised regions called intercalated discs, which contain gap junctions that facilitate the transmission of electrical signals and the regulated transport of molecules from myocyte to myocyte. This electrical coupling allows for the propagation of coordinated action potentials from one cell to the next, resulting in the synchronous contraction of cardiomyocytes and enabling the heart to work as a pump.
Cardiac muscle cells contain many mitochondria, which are essential for providing the energy required for muscle contraction. The high energy demand of contracting heart muscle requires a constant flow of blood to deliver oxygen and nutrients. Additionally, calcium plays a critical role in determining the force generated by each contraction. Voltage-gated calcium channels in the cell membrane, known as the sarcolemma, regulate calcium concentration within the cell through a process called excitation-contraction coupling.
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The heart is made up of three layers—pericardium, myocardium, and endocardium
The heart is a muscular organ that pumps blood throughout the body. It is housed in a pericardial sac, which is a protective layer that also assists with the mechanics of the heart. The heart itself is made up of three layers of tissue: the pericardium, myocardium, and endocardium.
The pericardium is the outermost layer, a fibrous sac that surrounds the heart. It consists of the epicardium, the pericardial space, the parietal pericardium, and the fibrous pericardium. The epicardium is the visceral layer of the serous pericardium, which adheres to the myocardium of the heart. The epicardium forms part of the pericardial sac and helps to protect the heart and reduce friction. It is composed primarily of loose connective tissue, including elastic fibres and adipose tissue. The coronary blood vessels are also found in this layer, supplying the heart wall with blood.
The myocardium is the thickest layer of the heart, made up of cardiac muscle cells or cardiomyocytes. These cells are responsible for the contractile functions of the heart, which require ATP. The myocardium is encased by collagen fibres and other substances that form the extracellular matrix. The cardiac muscle contracts to pump blood, generating the pressure needed to push blood through the circulatory system.
The endocardium is the innermost layer of the heart. It is not cardiac muscle but is made up of simple squamous epithelial cells. The endocardium lines the inner chambers and covers the valves of the heart, joining with the endothelium that lines the blood vessels connected to the heart. It has two layers: the inner layer of endothelial cells, and a subendocardial connective tissue layer that is continuous with the connective tissue of the myocardium.
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The primary function of cardiomyocytes is to contract, generating the pressure needed to pump blood through the circulatory system
The contractile functions of the heart require adenine triphosphate (ATP), which is generated by the mitochondria within the cardiomyocytes. The densely packed mitochondrial network in cardiomyocytes allows them to produce ATP quickly, making them highly resistant to fatigue. The concentration of calcium in the cardiomyocytes is critical in determining how much force is generated with each contraction. Calcium is released from the sarcoplasmic reticulum, causing the myofilaments to slide past each other in a process called excitation-contraction coupling, which results in the contraction of the heart and the generation of force.
Cardiomyocytes are under involuntary control, meaning they contract and relax independently of conscious input. This is in contrast to skeletal muscle, which requires neural input for contraction. Cardiac muscle has its own pacemaker cells, such as those in the sinoatrial node, that spontaneously depolarize to initiate contractions. These pacemaker cells can also receive input from the autonomic nervous system to increase or decrease the heart rate as needed.
The coordination of contractions among cardiomyocytes is essential for effective pumping. Cardiomyocytes are connected by intercalated discs that enable the rapid transmission of electrical impulses through the network, allowing for synchronized contractions of the myocardium. This coordination ensures that the ventricles can squeeze in multiple directions simultaneously to maximize the amount of blood pumped out of the heart with each heartbeat.
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The myocardial action potential occurs in five phases, beginning with rapid depolarization and ending with stabilization at the resting potential
The myocardial action potential is distinct from that of pacemaker cells and consists of five phases. It is important to note that cardiac muscle, or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle. Now, let's delve into the five phases of the myocardial action potential:
Phase 0: Rapid Depolarization
This phase involves a rapid, positive change in voltage across the cell membrane, known as depolarization. It is triggered by the opening of voltage-gated sodium (Na+) channels, resulting in a rapid influx of Na+ ions. This leads to a steep rise in the action potential waveform. The membrane potential typically changes from -70mV to +50mV during this phase.
Phase 1: Initial Partial Repolarization
In this phase, there is an inactivation of the previously opened voltage-gated Na+ channels, along with the activation of the transient outward potassium current (Ito). This results in a slight drop in the membrane electrochemical potential, initiating phase 2.
Phase 2: Plateau Phase
The plateau phase is characterized by the influx of Ca2+ ions through the opening of voltage-gated L-type Ca2+ channels. This calcium influx balances the K+ efflux, creating a plateau at around an electrochemical potential of +50mV. This phase is unique to the myocardial action potential and is not present in skeletal muscle.
Phase 3: Rapid Repolarization
During this phase, rapid repolarization occurs. The L-type Ca2+ channels close, while the slow delayed rectifier (IKs) K+ channels remain open, allowing for a net outward positive current. This corresponds to a negative change in membrane potential, enabling more types of K+ channels to open, including rapid delayed rectifier K+ channels (IKr) and inwardly rectifying K+ current (IK1).
Phase 4: Stabilization at Resting Potential
Also known as the resting phase, this phase is characterized by the stabilization of the membrane potential at the resting voltage. There is no spontaneous depolarization during this phase. The potassium channels, such as Kir, aid in maintaining the resting membrane potential.
These five phases of the myocardial action potential are crucial in coordinating the contraction and relaxation of the cardiac muscle, ensuring the efficient pumping of blood throughout the body.
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The cardiac muscle is supplied with blood by the coronary arteries
The cardiac muscle, 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 body, the other two being skeletal and smooth muscle. The myocardium forms a thick middle layer of the heart, sandwiched between the inner endocardium and the outer epicardium or visceral pericardium. The endocardium is composed of simple squamous epithelial cells and forms the inner lining of the heart chambers and valves. The epicardium, on the other hand, forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
The cardiac muscle is responsible for the heart's pumping action, generating the pressure required to pump blood through the circulatory system. This vital function necessitates a constant supply of oxygen and nutrients, which is provided by the coronary circulation. Specifically, the coronary arteries supply oxygenated blood to the myocardium, ensuring its metabolic demands are met.
The coronary arteries originate from the aortic root and lie on the outer or epicardial surface of the heart. They deliver oxygen-rich blood to the myocardium, ensuring the cardiac muscle cells, or cardiomyocytes, receive the necessary nutrients and oxygen for their energy-intensive contractions. These contractions occur involuntarily and are responsible for the heart's pumping action, propelling blood into circulation.
The cardiomyocytes are tubular structures composed of chains of myofibrils, which consist of repeating sections of sarcomeres. The sarcomeres are the fundamental contractile units of the muscle cells, composed of long proteins that form thick and thin filaments called myofilaments. The thick myofilaments contain the protein myosin, while the thin myofilaments contain actin. During muscle contraction, these myofilaments slide past each other, producing the formation of "cross-bridges" and resulting in the generation of force.
The cardiac muscle's contractility can be increased through beta-1 adrenergic receptors on the cell surface. Stimulation by the sympathetic nervous system or beta-1 agonist drugs activates adenylyl cyclase, increasing intracellular cAMP levels. This, in turn, enhances the activity of protein kinase A (PKA), which phosphorylates calcium channels, allowing more calcium to enter the cell and leading to increased contraction.
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Frequently asked questions
Cardiac muscle, also called heart muscle or myocardium, 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 primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
The coronary arteries supply blood to the cardiac muscle, and the cardiac veins drain this blood.
