Cardiac muscle, which is also called myocardium, in the vertebrates, is one of three major muscle types that is only found in the heart. Cardiac muscle is the same as the skeletal muscle, the other major muscle type. In that, it possesses the contractile units called sarcomeres. However, these features of cardiac muscle also distinguish it from smooth muscle, which is the third muscle type. Cardiac muscle varies from the skeletal muscle in that it exhibits rhythmic contractions and not under voluntary control. The cardiac muscle's rhythmic contraction is regulated by the sinoatrial node of the heart that serves as the pacemaker of the heart.
Mostly, the heart consists of cardiac muscle cells (otherwise called myocardium). Contractility, which is the foundation for the contraction's rhythmicity, and pumping action are two of the heart's most notable characteristics. The amount of blood pumped by the heart per minute (which is the cardiac output) differs from meeting the metabolic needs of peripheral tissues, specifically the kidneys, skeletal muscles, skin, brain, heart, liver, and gastrointestinal tract.
The contractile force produced by cardiac muscle cells, as well as the frequency at which they are stimulated (rhythmicity), can be used to describe cardiac output. The force and frequency of heart muscle contractions are important factors in determining the normal heart's pumping efficiency and response to changes in demand.
In the heart, cardiac muscle cells form a highly branched cellular network. The intercalated discs bind them end to end and arrange them into myocardial tissue layers that wrap around the heart chambers. Individual cardiac muscle cell contractions trigger force and shortening of these muscle bands, resulting in a reduction in the heart's chamber size and blood ejection into the systemic and pulmonary vessels.
The plasma membrane and transverse tubules in the registration with Z lines, the longitudinal terminal cisternae and sarcoplasmic reticulum, and the mitochondria are all essential components of any cardiac muscle cell involved in the metabolic and excitation recovery processes. The thin (troponin, actin, and tropomyosin) and thick (myosin) protein filaments are arranged into the contractile units, with sarcomere, extending from Z line - Z line, that has a characteristic cross-striated pattern same as that, which is seen in skeletal muscle.
The conduction of electrical information from one area of the heart to another, as well as the electrical properties of the cardiac muscle cells, determine the rate at which the heart contracts and the coordination of ventricular and atrial contraction needed for efficient blood pumping. The action potential (or the activation of the muscle) is divided into five phases. Every phase of the action potential is caused by the time-dependent change in the plasma membrane's permeability to sodium ions (Na+), calcium ions (Ca2+), and potassium ions (K+).
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The above diagram shows the cross-section of the four-chambered mammalian heart.
Let us look at the Cardiac Muscle Function and Cardiac Muscle Structure in detail, here.
Cardiac muscle tissue is also called the myocardium, and forms the heart's bulk. A thick layer of myocardium is sandwiched between the outer epicardium (also known as visceral pericardium) and the inner endocardium, forming the heart wall. The inner endocardium lines the cardiac chambers, which cover the cardiac joins and valves, with the endothelium, which lines the blood vessels that connect to the heart. Whereas, on the outer aspect of the myocardium is the epicardium that forms part of the pericardium, which is the sack that protects, surrounds, and lubricates the heart.
Cardiac muscle cells or the cardiomyocytes are given as the contracting cells, which allow the heart to pump. Every cardiomyocyte needs to contract in coordination with its neighbouring cells - called a functional syncytium that is working to efficiently pump blood from the heart. If this coordination breaks down, then, despite the individual cells contracting, the heart may not pump at all, such as can take place during abnormal heart rhythms like ventricular fibrillation.
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T-tubules are the microscopic tubes, which run from the surface of the cell to deep within the cell. These are continuous with the cell membrane and are composed of a similar phospholipid bilayer. They are open at the cell's extracellular fluid surface that surrounds the cell. T-tubules present in the cardiac muscle are wider and bigger than the ones in skeletal muscle, but some in number. In the cell's centre, they join together by running into and along with the cell as a transverse-axial network. They lie close to the cell's internal calcium store inside the cell, the sarcoplasmic reticulum. A single tubule is paired with a terminal cisterna from the sarcoplasmic reticulum in a diad combination.
The cardiac syncytium is a network of cardiomyocytes linked by intercalated discs that allow for the rapid transmission of electrical impulses across a network by allowing the syncytium to participate in the synchronised contraction of the myocardium. There are a ventricular syncytium and an atrial syncytium, which are connected by cardiac connection fibres.
1. What are Fibroblasts?
Answer: Cardiac fibroblasts are explained as the vital supporting cells within cardiac muscle. These are unable to provide forceful contraction of the heart such as cardiomyocytes, but instead, they are largely responsible for both creating and maintaining the extracellular matrix that forms the mortar, where cardiomyocyte bricks are embedded. Fibroblasts play an essential role in responding to injury, like myocardial infarction.
2. What is the Extracellular Matrix?
Answer: Continuing the analogy of heart muscle is like a wall, and the extracellular matrix is the mortar, which surrounds the fibroblasts and cardiomyocyte bricks. The matrix is formed of proteins such as elastin and collagen along with polysaccharides (sugar chains) called glycosaminoglycans. These substances, together, give strength and support to the muscle cells, keep the muscle cells hydrated, and create elasticity in cardiac muscle by binding water molecules.
3. Give the Clinical Significance of the Cardiac Muscle?
Answer: Diseases that affect the cardiac muscle are of immense clinical significance, and they are the leading cause of death in many developed nations. Ischaemic heart disease is the most common condition affecting cardiac muscle, where the blood supply to the heart is reduced. The coronary arteries become narrowed by atherosclerosis in ischaemic heart disease. If these narrowings become severe enough gradually to partially restrict blood flow, the syndrome of angina pectoris may take place.
4. Differentiate between Atria and Ventricles?
Answer: Cardiac muscle forms atria and the heart ventricles. Although this muscle tissue is the same between the cardiac chambers, a few differences exist. The myocardium that is found in the ventricles is thick to allow forceful contraction of the heart, whereas the myocardium in the atria is thinner. The individual myocytes, which make up the myocardium, also vary between the cardiac chambers. Ventricular cardiomyocytes are wider and longer, having a denser T-tubule network.