Cardiac muscle cell

Cardiac muscle cells or cardiomyocytes are the muscle cells that make up the cardiac muscle.

The regenerative capacity of heart muscle is limited with a turnover of not more than 1% per year.

Myocyte cells contain myofibrils, consisting of long chains of sarcomeres, the fundamental contractile units of muscle cells.

Myocytes show striations similar to those on skeletal muscle cells.

The majority of cardiomyocytes contain only one nucleus, although they may have as many as four.

Cardiomyocytes are about 100μm long and 10-25μm in diameter.

Cardiomyocytes have a high mitochondrial density.

Their high mitichondrial allows them to produce adenosine triphosphate (ATP) quickly, making myocytes highly resistant to fatigue.

Cardiomyocytes generate the majority of adenosine triphosphate via oxidative phosphorylation of adenosine diphosphate from reducing equivalents in the mitochondria.

Chemical energy in the form of ATP is converted into mechanical energy which allows myosin binds to actin and produce a power stroke resulting in sarcomere shortening/contraction.

Macrophages within the heart scavenge decaying and spent Mitochondria emitted from cardiomyocytes.
Insulin has a role in modulating the metabolism of cardiomyocytes.
Glucose is the preferred substrate for cardiomyocytes post prandially, but the heart prefers substrates for respiration in the fasted states are free fatty acids..
During protracted fasting, ketone bodies and amino acids contribute to the heart’s production of ATP.

There is flexibility in adaptive transition from one fuel substrate to another in myocellular homeostasis.

Two types of cells exist within the heart: cardiomyocytes and cardiac pacemaker cells.

Cardiomyocytes make up the atria and the ventricles.

Cardiomyocytes must be able to shorten and lengthen their fibers and the fibers must be flexible enough to stretch.

These characteristics are critical to a beating heart.

Cardiac pacemaker cells are distributed throughout the heart and carry impulses responsible for spontaneously generating electrical impulses for the beating heart.

Pacemaker cells are located in the sinoatrial node, which is the primary pacemaker, positioned on the wall of the right atrium, near the entrance of the superior vena cava.

Other pacemaker cells are found in the atrioventricular node, the secondary pacemaker.

Cardiac pacemaker cells also, must be able to receive and respond to electrical impulses from the brain, and must be able to transfer electrical impulses from cell to cell.

Pacemaker cells carry the impulses that are responsible for the beating of the heart. 

Pacemaker cells  are distributed throughout the heart and are responsible for  being able to spontaneously generate and send out electrical impulses. 

They also must be able to receive and respond to electrical impulses from the brain. 

They must be able to transfer electrical impulses from cell to cell.

Pacemaker cells in the sinoatrial and atrioventricular nodes are smaller and conduct at a relatively slow rate between the cells. 

Specialized conductive cells in the bundle of His, and the Purkinje fibers are larger in diameter and conduct signals at a fast rate.

The Purkinje fibers rapidly conduct electrical signals; coronary arteries to bring nutrients to the muscle cells, and veins and a capillary network to take away waste products.

Cardiomyocytes are connected by porous junctions-cellular bridges called intercalated discs.

Intercalated discs permit sodium, potassium, and calcium to diffuse from cell to cell allowing for easier depolarization and repolarization of the myocardium.

Cardiac myocytes contract through a cross-bridge cycle between the myofilaments, actin and myosin. 

These junctions and bridges allow the heart muscle to be able to act as a single coordinated unit.

Each cardiomyocyte needs to contract in coordination with its neighboring cells as a functional syncytium.

The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when looked at through a microscope, similar to skeletal muscle. 

These striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.

Cardiomyocytes contain T-tubules, pouches of cell membrane that run from the cell surface to the cell’s interior which improve the efficiency of contraction. 

The majority of these cells contain only one nucleus unlike skeletal muscle cells which contain many nuclei.

Cardiac muscle cells contain many mitochondria which provide the energy needed for the cell in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue.

Heart muscle cells increase in size as heart grows larger during childhood.

Cardiomyocytes are slowly turned over as we age.

Less than 50% of the cardiomyocytes are replaced during a normal life span.

Growth of individual cardiomyocytes occurs during normal heart development, and response to exercise, heart disease, or heart muscle injury such as after a myocardial infarction.

The adult cardiomyocyte has a cylindrical shape that is approximately 100μm long and 10-25μm in diameter.

Cardiomyocyte hypertrophy occurs by the creation of new sarcomere units in the cell.

With heart volume overload, cardiomyocytes grow through eccentric hypertrophy, extending lengthwise but have the same diameter, resulting in ventricular dilation.

With heart pressure overload, cardiomyocytes grow through concentric hypertrophy by growing larger in diameter but have the same length, resulting in heart wall thickening.

Myocardial cells possess the property of automaticity or spontaneous depolarization.

The very slow repolarization of the cardiac muscle cell membrane is responsible for the long refractory period.

With myocardial infarction, the loss of blood flow causes portions of cardiac tissue to die, with permanent damage.

The cardiomyopathies are characterized by disruptions to cardiac muscle cell growth and / or organization.

Cardiomyopathies can be caused by genetic, endocrine, environmental, or other factors.

Myocytolysis refers to cardiac muscle damage and is a type of cellular necrosis.

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