Cardiac Muscles

What are Cardiac Muscles?

Cardiac muscles are striated muscle fibres existing in the heart. They are special because they are rhythmically and involuntarily contracting in nature. This is because the primary function is pumping blood throughout the body. Cardiac muscle cells, also known as cardiomyocytes, interdigitate through special structures called intercalated discs, which enable synchronised contractions of the cardiomyocyte to maintain a brisk heartbeat.

Understanding cardiac muscles is important to understand how the heart operates and therefore supports and maintains circulation in the body. Research about the physiology of cardiac muscles enables the diagnosis and treatment of many conditions relating to the heart, including arrhythmias, heart attacks, and heart failures. Information regarding cardiac muscle functioning helps in the development of treatments and interventions geared at enhancing heart health and the general functionality of the cardiovascular.

General Characteristics of Cardiac Muscles

Cardiac muscles, also the myocardium, are muscle tissues that are unique to the heart. They are characterised by being striated and not under control, with the feature of rhythmical contraction. The cardiac muscles are active without stopping and control by the mind because the force of contraction is provided by intrinsic electrical excitation. They demonstrate intercalated discs with facilities to generate synchronised contractions.

Histology of Cardiac Muscles

The histology of Cardiac muscles are explained below-

Cardiomyocytes

The cardiomyocytes are the basic unit of functional cardiac muscle. They are cylindrical, striated muscle cells, generally with one nucleus. They are connected by intercalated discs which allow transmission of electrical signals so that the heart muscle can contract in unison.

Striations and Intercalated Discs

The striations in cardiac muscle are due to the arrangement of filaments made of actin and myosin. Intercalated discs are specialised attachments between the ends of cardiomyocytes. They contain gap junctions, through which the action potential conduction becomes very fast and desmosomes impart mechanical strength.

Types of Cells (Pacemaker vs Contractile)

Cardiac tissue consists of pacemaker cells and contractile cells. Pacemaker cells occur mainly in the sinoatrial node, the points that generate electric impulses for the sustenance of a heartbeat. While the contractile cells are responsible for the mechanical contraction of the heart.

Staining Techniques

Specific ways of staining are needed for histological studies of cardiac muscle to bring out the different components. Stains like hematoxylin and eosin can identify the striations, nuclei, and intercalated discs. Special stains and electron microscopy will give information about cellular structures and their junctions in detail.

Structure of Cardiac Muscles

The structure of Cardiac muscles is explained below-

Sarcolemma

The sarcolemma is the membrane of the cell coat, and the sarcoplasm is the cytoplasm, including myofibrils responsible for the cell's contraction. The sarcolemma corresponds to the plasma membrane of cardiomyocytes. It encloses the cell, thus providing the cell with its structure. More importantly, it is responsible for passing action potentials and maintaining the necessary ionic balance for muscle contraction.

Sarcoplasm

Sarcoplasm is the cytoplasm of the cardiomyocyte containing a large number of myofibrils, mitochondria, and other organelles. The nucleus contains the constituents necessary for the processes of muscle contraction and the production of energy, including enzymes and stored glycogen.

Nucleus

Each cardiomyocyte contains one nucleus, which is centrally located. The nucleus controls cellular activities and gene expression necessary for the maintenance of cardiac muscle health and function.

Mitochondria

The number of mitochondria in cardiomyocytes is more, indicative of the high energy requirements of these cells. Mitochondria are the site of ATP production via aerobic respiration and thus are crucial for maintaining cardiac muscle contraction.

Special Features of Cardiac Muscle Cells

Some of the features are unique to cardiac muscle cells.

Intercalated discs

These are specialised junctions between the cardiomyocytes that facilitate the mechanical and electrical coupling of adjacent cells. This ensures contractions are synchronised and hence efficient.

Gap junctions

Gap junctions in the intercalated discs create a pathway for the rapid movement of electrical impulses from one cell to another which makes it possible to co-ordinate contraction that enables the heart to be an efficient pump.

Desmosomes

Desmosomes are adhesive structures of the cardiomyocytes. They aid in holding tissues together to withstand the forces during contraction.

Myofibrils and Sarcomeres

The myofibrils within the cardiomyocyte are arranged into a sarcomere, and this arrangement is essentially the generic term for the basic contractile unit. This overlapping arrangement between the thick and thin filaments is responsible for the striated appearance and is the very essence of muscle contractility.

Function of Cardiac Muscles

The function of cardiac muscles are explained below:

Role in Circulatory System

The cardiac muscle contracts and propels blood from the heart's chambers into the arteries. In effect, the process is very crucial in the maintenance of the pressure of blood, hence facilitating circulation effectively.

Maintaining Blood Pressure

The pressure generated by the force of the cardiac muscle with its multiple contractions maintains the blood pressure within the arterial system, hence enabling the distribution of it to all tissue parts.

The cardiac cells contract starting from impulse conduction by pacemaker cells. Finally, the interaction of actin and myosin filaments by sliding in the sarcomeres.

Mechanism of Contraction

The mechanism of contraction includes the following steps:

Excitation-Contraction Coupling

Excitation-contraction coupling is the process by which the action potential, or electrical excitation, of the heart muscle, leads to its mechanical contraction.

Role of Calcium Ions

Calcium ions are important for cardiac muscle contraction. They combine with troponin molecules, activators that interact with the actin and myosin filaments. Adding to this effect is the sliding over each other of the filaments, hence muscle contraction.

Action Potential Propagation

Action potentials developed in the sinoatrial node get propagated through the conduction system of the heart—the atrioventricular node, the bundle of His, and the Purkinje fibres. The spread is well-coordinated, and this brings forth a well-synchronized contraction of the heart chambers.

Differences in Contraction Compared to Skeletal and Smooth Muscles

Contractions in cardiac muscle are both involuntary and rhythmic, this is in contrast to the voluntary contractions of skeletal muscles. In response to a single action potential, cardiac muscle tissue remains contracted 10 to 15 times longer than skeletal muscle tissue. The long contraction is due to prolonged delivery of Ca2+ into the sarcoplasm.

Skeletal muscle tissue contracts only when stimulated by acetylcholine released by a nerve impulse in a motor neuron. In contrast, cardiac muscle tissue contracts when stimulated by its own autorhythmic muscle fibers. This continuous, rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.

Cardiac Muscles NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Histology of Cardiac Muscles

• Special Features

• Mechanism of Contraction

• Difference in Contraction

Practice Questions for NEET

Q1. Cardiac muscles are

1. Cylindrical, voluntary, uninucleate

2. Cylindrical, striated, involuntary

3. Involuntary, uninucleate, cylindrical

4. Involuntary, multinucleate, cylindrical

Correct answer: 3) Involuntary, uninucleate, cylindrical

Explanation:

Cardiac muscles, which are essential components of the heart, are characterized by their involuntary and striated nature. These muscles facilitate the systematic contraction and relaxation of the cardiac chambers, thereby enabling efficient blood circulation throughout the organism. The key attributes of these muscles are as follows:

1. Involuntary Action: Cardiac muscles operate independently of conscious control, contracting automatically to maintain the rhythmic beating of the heart.
2. Striated Appearance: The presence of actin and myosin filaments arranged in a regular pattern gives cardiac muscles a striped or striated look, akin to skeletal muscles.
3. Mononucleated Nature: Typically, each cardiac muscle cell possesses a single nucleus, though some may exhibit two or more due to the fusion of cells during development.
4. Branched Configuration: The cells of the cardiac muscle display a unique branched structure, which allows for interconnection among them at specific points known as intercalated discs.
5. Intercalated Discs: These are crucial for the rapid conduction of electrical impulses, which synchronizes the contraction of heart muscle cells and coordinates the heart's pumping action.
6. Exceptional Endurance: Cardiac muscle cells have a high fatigue resistance, enabling the heart to contract continuously without rest throughout an individual's lifespan.
7. Autogenic Excitability: The sinoatrial (SA) node, often referred to as the heart's natural pacemaker, is responsible for the generation of electrical impulses that initiate the contraction process in cardiac muscle cells.

Hence, the correct answer is option 3) Involuntary, uninucleate, cylindrical.

Q2. Which is the feature of Cardiac muscles only

1. Branched fibres

2. Multinucleated

3. A and I bands

4. Intercalated fibres

Correct answer: 4) Intercalated fibres

Explanation:

Cardiac muscles are responsible for the involuntary movements of the heart, enabling it to contract and pump blood throughout the body. These muscles are unique due to the presence of intercalated discs, specialized structures that connect individual cardiac muscle cells (cardiomyocytes). Intercalated discs contain gap junctions and desmosomes, which allow electrical signals to pass quickly from one cell to another, ensuring coordinated contraction of the heart. This feature is essential for the heart's rhythmic and synchronized pumping action. Unlike skeletal muscles, cardiac muscles work automatically without conscious control, making them essential for sustaining life.

Hence, the correct answer is option 4) Intercalated discs.

Q3. Cardiac muscles contract

1. Slowly and get fatigued

2. Quickly and do not get fatigued

3. Slowly and do not get fatigued

4. Quickly and get fatigued

Correct answer: 2) Quickly and do not get fatigued

Explanation:

Cardiac muscles, which are essential for the circulation of blood, operate independently of voluntary control, performing their pumping function with remarkable efficiency. The hallmark of these muscles is their rhythmic and automatic nature, facilitating the consistent beating of the heart without the need for conscious thought. This is made possible by the presence of specialized structures known as intercalated discs, which connect cardiac cells and enable swift conduction of electrical impulses. This coordinated effort results in synchronized contractions that propel blood through the circulatory system.

Intercalated discs are unique to cardiac muscle tissue and serve as critical junctions between individual muscle cells, or myocardial fibres. These discs are crucial for the rapid transmission of electrical signals, which initiate and coordinate the contraction process. The structural integrity and functional proficiency of these discs contribute significantly to the heart's ability to contract as a cohesive unit, ensuring the synchronized pumping action that is characteristic of this vital organ.

Another key feature of cardiac muscles is their exceptional endurance. Unlike skeletal muscles, which tire easily with use, cardiac muscles are continuously active from birth to death, beating approximately 100,000 times per day. This tireless performance is a direct result of their high fatigue resistance, a trait that is essential for maintaining the steady flow of blood and oxygen to the body's tissues. The inherent endurance of cardiac muscles allows them to withstand the demands placed upon them throughout an organism's lifespan, thereby supporting the continuous operation of the circulatory system and overall bodily functions.

Hence, the correct answer is option 2) Quickly and do not get fatigued.

Also Read:

• MCQs on Cardiac Muscles

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