How Cardiac Muscle Differs from Skeletal and Smooth Muscle
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.
About the Cardiac Muscle
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.
Connection and Organization
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.
Cardiac Muscle Structure and Cardiac Muscle Function
Let us look at the Cardiac Muscle Function and Cardiac Muscle Structure in detail, here.
Gross Anatomy
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
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
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.
Intercalated Discs
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.
FAQs on Cardiac Muscle: Essential Structure and Function
1. What is cardiac muscle?
Cardiac muscle, also known as myocardium, is one of the three types of muscle tissue found in the body. It is a highly specialised, striated muscle tissue that forms the walls of the heart. Its primary function is to contract and pump blood throughout the body. Unlike skeletal muscle, cardiac muscle is involuntary, meaning its contractions are not under conscious control.
2. What are the main characteristics of cardiac muscle tissue?
Cardiac muscle tissue has several distinct characteristics that are crucial for its function. According to the NCERT syllabus, these include:
Location: Found exclusively in the walls of the heart.
Structure: The cells (cardiomyocytes) are cylindrical, branched, and uninucleate (containing a single nucleus).
Appearance: They are striated, showing alternate light and dark bands, similar to skeletal muscles.
Control: Contractions are involuntary and rhythmic, controlled by the heart's own pacemaker cells.
Intercalated Discs: Cells are joined by specialised junctions called intercalated discs, which are unique to cardiac muscle.
3. How does cardiac muscle differ from skeletal and smooth muscles?
Cardiac muscle shares features with both skeletal and smooth muscles but is distinct. Here is a comparison based on key differences:
Control: Cardiac muscle is involuntary, like smooth muscle, whereas skeletal muscle is voluntary.
Structure: Cardiac muscle cells are branched and uninucleate. Skeletal muscle cells are unbranched and multinucleate, while smooth muscle cells are spindle-shaped and uninucleate.
Striations: Cardiac muscle is striated, like skeletal muscle. Smooth muscle is non-striated.
Junctions: Cardiac muscle has unique intercalated discs that connect cells, which are absent in the other two types. These discs allow for synchronized contraction.
4. Why is cardiac muscle considered involuntary?
Cardiac muscle is involuntary because its contraction is not controlled by conscious thought. Instead, it is regulated by the heart's own internal electrical conduction system. This system is led by specialised pacemaker cells in the Sinoatrial (SA) node, which generate rhythmic electrical impulses automatically. These impulses cause the heart to contract and pump blood. While the autonomic nervous system can modify the heart rate (speed it up or slow it down), it does not initiate the heartbeat itself.
5. How is the structure of cardiac muscle perfectly adapted for its function of continuous pumping?
The structure of cardiac muscle is intricately designed for its relentless, lifelong function. Firstly, its branched fibres form a complex network, allowing it to withstand pressure from multiple directions. Secondly, the intercalated discs contain gap junctions that allow electrical signals to pass rapidly from cell to cell, ensuring the entire heart wall contracts as a single, coordinated unit (a functional syncytium). Lastly, cardiac muscle cells are packed with a very high number of mitochondria to generate the constant supply of ATP needed for continuous, fatigue-resistant contractions.
6. What is the specific importance of intercalated discs in cardiac muscle function?
Intercalated discs are the most important structural feature unique to cardiac muscle. They serve two critical functions:
Strong Adhesion: They contain anchoring junctions (like desmosomes) that firmly bind the adjacent muscle cells together. This prevents the cells from separating during the forceful contractions of the heartbeat.
Rapid Communication: They contain gap junctions, which are tiny channels that allow ions and electrical impulses to flow directly and quickly between cells. This ensures that all cells in the atria or ventricles contract almost simultaneously, producing an efficient, coordinated pump.
7. What is the clinical significance related to cardiac muscle?
The health of cardiac muscle is vital for life, and its dysfunction is a leading cause of mortality. A primary example of its clinical significance is in Ischaemic Heart Disease. In this condition, blood flow to the cardiac muscle is reduced, typically due to narrowed coronary arteries. If the muscle is deprived of oxygen for too long, it leads to a myocardial infarction (heart attack), where a portion of the cardiac muscle tissue dies. Since cardiac muscle has very limited regenerative capacity, this damage is often permanent and can severely weaken the heart's pumping ability.
8. How do cardiac muscle cells get the continuous energy they need to function?
Cardiac muscle requires a constant and massive supply of energy in the form of ATP (adenosine triphosphate) to sustain its continuous contractions. It achieves this through several adaptations. The cells contain an extremely high density of mitochondria, the powerhouses of the cell, which carry out aerobic respiration. This process uses oxygen and nutrients delivered by a rich network of coronary arteries to efficiently produce ATP. This reliance on aerobic respiration is why a blockage in a coronary artery is so dangerous, as it cuts off the oxygen supply needed for energy production.
