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The human body is an intricate machine, with muscles that work both voluntarily and involuntarily to keep us alive. One of the most important involuntary muscles is the cardiac muscle, also known as the heart muscle or myocardium. This muscle is responsible for pumping blood throughout our bodies and is controlled unconsciously by regions of the brain stem and hypothalamus. The cardiac muscle is unique in its auto-rhythmicity, with its own pacemaker cells that set the rhythm of the heart's contractions. This fascinating muscle tissue forms the bulk of the heart and is responsible for keeping us alive from the moment our hearts first form in the womb.
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What You'll Learn
Cardiac muscle cells are located in the walls of the heart
The heart is an involuntary muscle, beating on its own without any conscious input. Cardiac muscle, also known as myocardium or heart muscle, is one of the three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscle cells are located in the walls of the heart and are responsible for pumping blood through the cardiovascular system.
Cardiac muscle forms the thick middle layer of the three-layered heart wall, between the inner endocardium and the outer epicardium or visceral pericardium. The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels connected to the heart. The outer epicardium forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
Cardiac muscle cells are unique to the heart and are specialised to pump blood powerfully and efficiently throughout a person's lifetime. They are striated, branched, and single-nucleated, with each cell containing a single nucleus. The presence of myofibrils and many mitochondria in cardiac muscle cells provides them with great strength and endurance for pumping blood.
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 made up of long proteins that organise into thick and thin filaments called myofilaments. The sliding of actin and myosin past each other during contraction produces the formation of "cross-bridges", which cause the contraction of the heart and the generation of force.
The concentration of calcium in the myocyte determines the force generated with each contraction. Calcium is released from the sarcoplasmic reticulum, the cell's internal calcium store, during a process called excitation-contraction coupling, which is triggered by electrical stimulation in the form of a cardiac action potential.
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Cardiac muscle is striated
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 striated due to the arrangement of actin and myosin into regular myofibrils. The presence of myofibrils and many mitochondria in cardiac muscle cells provides them with great strength and endurance to pump blood throughout a lifetime. 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 myofilaments slide past each other as the muscle contracts and relaxes. This process is activated by the release of calcium from the sarcoplasmic reticulum (SR) when delivering an action potential to the muscle, in a process called excitation-contraction coupling. The sliding of actin and myosin past each other produces the formation of “cross-bridges,” which causes contraction of the heart and generation of force.
The concentration of calcium in the myocyte is the critical factor that determines how much force is generated with each contraction. Cardiac muscle cells can increase contractility through beta-1 adrenergic receptors on the surface with a Gs G-protein. When stimulated by either the sympathetic nervous system or beta-1 agonist drugs, the Gs activate the enzyme adenylyl cyclase, which converts ATP to cAMP. Intracellular cAMP increases the activity of protein kinase A (PKA), which then phosphorylates calcium channels, permitting more calcium to enter the cell, leading to increased contraction.
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The heart is an involuntary muscle
Cardiac muscle cells are located in the walls of the heart and are under involuntary control. They are the only muscle cells found in the heart and are specialized to pump blood powerfully and efficiently throughout our entire lifetime. Cardiac muscle cells are intrinsically controlled, striated, branched, and single-nucleated. They are considered to be an involuntary tissue because they are controlled unconsciously by regions of the brain stem and hypothalamus.
The normal cardiac rhythm is set by specialized pacemaker cardiac muscle cells in the heart itself. These pacemaker cells are only weakly contractile without sarcomeres and are connected to neighboring contractile cells via gap junctions. They are located in the sinoatrial node (the primary pacemaker) on the wall of the right atrium. Other pacemaker cells are found in the atrioventricular node (secondary pacemaker). Pacemaker cells carry the impulses that are responsible for the beating of the heart.
Cardiac muscle contracts in a similar manner to skeletal muscle, although with some important differences. 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 concentration of calcium in the myocyte is the critical factor that determines how much force is generated with each contraction.
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Cardiac muscle is controlled unconsciously by the brain stem and hypothalamus
Cardiac muscle, or myocardium, is one of three types of muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the main tissue of the heart wall. The cardiac muscle is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs. These cells are responsible for the beating of the heart.
The cardiac muscle is under involuntary control, meaning that its movement is not consciously controlled by the brain. Instead, it is regulated by the autonomic nervous system (ANS), a component of the peripheral nervous system. The ANS functions continuously without conscious effort and is controlled by centres located in the spinal cord, brain stem, and hypothalamus. The ANS has two interacting systems: the sympathetic and parasympathetic systems. The sympathetic system prepares the body for energy expenditure, emergency, or stressful situations, i.e. the "fight or flight" response.
The brain stem, specifically the medulla oblongata, is responsible for regulating the body's blood pressure, pulse, and cardiac contractions based on the body's needs. It is also involved in managing other automatic processes such as coughing, sneezing, swallowing, vomiting, and maintaining balance. The hypothalamus, on the other hand, is involved in the production and release of vasopressin, or antidiuretic hormone, which helps regulate water retention and vasoconstriction. This hormone is released in response to changes in plasma volume, osmolarity, or an increase in cholecystokinin.
In summary, the cardiac muscle is controlled unconsciously by the brain stem and hypothalamus through the autonomic nervous system, which regulates cardiac muscle contraction, heart rate, and blood pressure, among other functions. The brain stem and hypothalamus work together to ensure the proper functioning of the cardiac muscle and maintain homeostasis in the body.
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Cardiac muscle cells are shorter than skeletal muscle cells
Cardiac muscle, also known as myocardium or heart muscle, is one of three types of muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. 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, also called cardiomyocytes, are shorter than skeletal muscle cells. They are the contractile myocytes of the cardiac muscle and are surrounded by an extracellular matrix produced by supporting fibroblast cells. The presence of myofibrils and many mitochondria in cardiac muscle cells provides them with great strength and endurance to pump blood throughout a lifetime.
Cardiac muscle cells are located only in the heart and are specialized to pump blood powerfully and efficiently. They receive a constant flow of blood to provide oxygen and nutrients, as contracting heart muscle uses a lot of energy. The cells are joined by intercalated discs, which lock them together tightly and allow the quick passage of electrochemical signals between cells.
Cardiac muscle contracts in a similar manner to skeletal muscle, but with some important differences. 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, which results in the contraction of the heart and the generation of force.
Cardiac muscle fibres have their own auto-rhythmicity and contain their own pacemaker cells, such as the sinoatrial (SA) node, which sets the rhythm of the heart contractions. These pacemaker cells are distributed throughout the heart and are responsible for several functions, including carrying the impulses that cause the heart to beat.
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Frequently asked questions
Yes, cardiac muscle is involuntary. It is controlled unconsciously by regions of the brain stem and hypothalamus.
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.
Cardiac muscle cells are found only in the heart. They are located in the walls of the heart and form a thick middle layer.
Cardiac muscle cells are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. Electrical stimulation triggers the release of calcium from the cell's internal store, which causes the cell's myofilaments to slide past each other and contract the muscle.
