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Cardiac muscles are found only in the heart and are characterised by their branched and cylindrical shape. They are also known as cardiomyocytes and are involuntary muscles. The cardiac muscles' cells are uninucleate, which is a characteristic shared with skeletal muscles.
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What You'll Learn
Cardiac muscle cells are also called cardiomyocytes
Cardiac muscle cells, also called cardiomyocytes, are a type of muscle tissue found only in the heart. They are involuntary muscles, meaning they are not under voluntary control. The heart consists mostly of cardiac muscle cells, also known as myocardium, which makes up the thick middle layer of the heart.
Cardiac muscle cells are tubular structures 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. The thick filaments contain the protein myosin, while the thin filaments contain actin. The sliding of these filaments past each other during muscle contraction and relaxation produces the formation of "cross-bridges," resulting in the contraction of the heart and the generation of force.
The contraction of individual cardiac muscle cells produces force and shortening in the muscle bands, leading to a decrease in heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels. These cells are connected end-to-end by intercalated discs, forming layers of myocardial tissue wrapped around the chambers of the heart. Gap junctions within the intercalated discs facilitate the transfer of stimulation from one cardiac cell to the next, enabling the muscle to contract in a coordinated manner.
Cardiac muscle cells have distinct features that allow them to contract in a synchronized and rhythmic manner. They possess their own pacemaker cells, such as the sinoatrial (SA) node, which spontaneously depolarizes to initiate contractions at a consistent pace. Additionally, these cells can receive input from the autonomic nervous system to adjust the heart rate according to the body's requirements.
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Cardiac muscles are branched and cylindrical in shape
Cardiac muscle cells are much smaller than skeletal muscle cells, with a diameter of 10-20 μm and a length of 50-100 μm. They are connected to each other via intercalated discs, which contain desmosomes and gap junctions. The intercalated discs facilitate a synchronized heartbeat by allowing the rapid propagation of action potentials across the entire myocardium.
The cylindrical shape of cardiac muscle cells is approximately 100 μm long and 10-25 μm in diameter. The growth of these cells occurs during normal heart development and in response to various stimuli such as extensive exercise, heart disease, or heart muscle injury. For example, during heart volume overload, cardiomyocytes grow longer while maintaining the same diameter, resulting in ventricular dilation.
The branching and cylindrical shape of cardiac muscles is unique to the heart and is an important structural feature that distinguishes them from skeletal muscles, which are long and multinucleated. The branched nature of cardiac muscles, along with the intercalated discs, enables the heart to contract and relax as a single unit, allowing for efficient pumping of blood.
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Cardiac muscles are involuntary
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 cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
Cardiac muscle cells are also called cardiomyocytes. They are the contractile myocytes of the cardiac muscle. The cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiac muscle cells appear striated or striped under a microscope. These stripes occur due to alternating filaments that comprise myosin and actin proteins. The dark stripes indicate thick filaments that comprise myosin proteins, while the thin, lighter filaments contain actin. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, which causes the cell to shrink.
The contraction and relaxation of cardiac muscle are similar to that of skeletal muscle but with some differences. Cardiac muscle cells are specialized excitable cells that can induce electrical impulses and give rise to the action potential. 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. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Cardiac muscle tissue produces involuntary movements, meaning they are automatic and a person cannot control them. The heart also contains specialized types of cardiac tissue containing "pacemaker" cells. These contract and expand in response to electrical impulses from the nervous system. Pacemaker cells generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax. The pacemaker cells control heart rate and determine how fast the heart pumps blood.
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Cardiac muscle contraction and relaxation is similar to skeletal muscles
Cardiac muscle contraction and relaxation are similar to skeletal muscles in many ways. Both cardiac and skeletal muscles are derived from the mesoderm, specifically the paraxial mesoderm in the case of skeletal muscle and the lateral splanchnic mesoderm for cardiac muscle. They are both striated muscles, meaning they contain actin and myosin filaments that give them a striped appearance and power their contraction. This contraction is controlled by the phosphorylation of troponin, which is also true of skeletal muscle.
The contractile apparatus of both muscle types consists of two main proteins: actin and myosin. The actin and myosin filaments are arranged into repeating arrays called sarcomeres, which are organised into myofibrils. These are found in both skeletal and cardiac muscle cells. The contraction of both muscle types is dependent on calcium, which enters the cell and binds to calmodulin, activating myosin light chain kinase (MLCK). MLCK phosphorylates myosin head light chains and increases myosin ATPase activity, leading to contraction.
However, there are also important differences between the two muscle types. For example, skeletal muscle is under voluntary control, whereas cardiac muscle is under involuntary control by the body's autonomic nervous system (ANS). Additionally, the influx of calcium that initiates contraction occurs through L-type calcium channels in skeletal muscle, whereas in cardiac muscle, the influx of calcium occurs through the plasma membrane and elicits further calcium release from the sarcoplasmic reticulum via ryanodine receptor (RyR) channels.
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Cardiac muscle cells are uninucleate and involuntary
Cardiac muscle cells, also called cardiomyocytes, are found only in the heart. They are cylindrical and branched in shape, with long cells that have pointed ends. Importantly, cardiac muscle cells are uninucleate, meaning they contain a single nucleus. This is in contrast to other types of muscle cells, such as skeletal muscle cells, which can be multinucleate, containing multiple nuclei.
The uninucleate characteristic of cardiac muscle cells is significant because it relates to their function and structure. Cardiac muscle cells are involuntary, meaning they contract and relax without conscious control. Specifically, they are specialized excitable cells that can induce electrical impulses and generate action potentials. This unique property of cardiac muscle cells allows them to play a crucial role in maintaining the rhythmic contractions of the heart.
The fact that cardiac muscle cells are uninucleate and involuntary has important implications for their function and behavior. The presence of a single nucleus in each cell ensures uniformity in the cell's structure and function, contributing to the synchronized contractions of the heart muscle. The involuntary nature of cardiac muscle cells allows for the automatic regulation of heart contractions, ensuring a consistent and steady supply of oxygenated blood to the body.
Additionally, the uninucleate structure of cardiac muscle cells may have evolutionary advantages. It is suggested that the presence of a single nucleus allows for more efficient and rapid communication within the cell, enabling faster contraction and relaxation of the heart muscle. This efficiency is crucial for the heart's ability to respond to changing demands and maintain homeostasis in the body.
In summary, cardiac muscle cells being uninucleate and involuntary is a fundamental characteristic that distinguishes them from other types of muscle cells. This unique combination of features is essential for the proper functioning of the heart and the maintenance of overall physiological balance in the body. The uninucleate structure contributes to the synchronized contractions, while the involuntary nature allows for automatic regulation, ensuring the continuous and vital work of the heart.
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Frequently asked questions
Yes, cardiac muscles are uninucleate.
Cardiac muscles are also involuntary and found only in the heart.
Cardiac muscle cells are also called cardiomyocytes.
Cardiac muscles are cylindrical and unbranched in shape.
The cardiac cells of the heart are specialized excitable cells that can induce electrical impulses and give rise to action potentials.
Cardiac muscle is a type of involuntary muscle.
