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Structure of Cardiac Muscles

Structure of Cardiac Muscles

Cardiac muscle, or myocardium, is a specialized type of muscle found exclusively in the heart. Its unique structural and functional characteristics enable the heart to perform its vital role of pumping blood throughout the body continuously and rhythmically. The cardiac muscle cells, or cardiomyocytes, possess an endomysium and perimysium but do not have an epimysium.
Compared to skeletal muscles, cardiac muscle cells are small and mostly have a single nucleus. Additionally, they are usually...

Specialized Characteristics of Cardiac Muscles

Specialized Characteristics of Cardiac Muscles

The primary role of cardiac muscles is to propel blood throughout the cardiovascular system. The cardiac muscle cells, or cardiomyocytes, exhibit specialized characteristics that allow them to perform this function.
Cardiac muscle cells are smaller than skeletal muscles, averaging 10–20 mm in diameter and 50–100 mm in length. However, they have large energy demands for continuous contraction and relaxation. This energy is almost exclusively derived from aerobic metabolism of energy...

Electrophysiology of Normal Cardiac Rhythm

Electrophysiology of Normal Cardiac Rhythm

The normal cardiac rhythm is a synchronized electrical activity that facilitates the regular and coordinated contraction of the heart muscle. This process is essential for efficient blood circulation throughout the body. The fundamental elements involved in establishing and maintaining this rhythm include the unique electrical properties of cardiac muscle cells, the sinoatrial (SA) node's pacemaker function, the specialized conducting system, and the ionic mechanisms underlying each phase...

Mechanism of Cardiac Arrhythmias

Mechanism of Cardiac Arrhythmias

Arrhythmias are irregular heart rhythms occurring when the heart's electrical impulses become abnormal. These disturbances can lead to various symptoms, depending on their severity and the underlying cause. Some common factors contributing to arrhythmias include hypoxia, ischemia, electrolyte imbalances, excessive catecholamine exposure, drug toxicity, and muscle overstretching. Arrhythmias can be classified into two main types based on the rate and site of origin of abnormal heart rhythms.

Pathophysiology of Cardiac Performance

Pathophysiology of Cardiac Performance

Typical heart performance is influenced by heart rate, rhythm, myocardial contraction, and metabolism or blood flow. The cardiac muscle exhibits distinct electrophysiological features, including pacemaker activity and calcium channel control, which play a vital role in the heart's response to various drugs. The autonomic nervous system, comprising the sympathetic and parasympathetic branches, regulates heart rate. Sympathetic activation increases heart rate, while parasympathetic activation...

Cardiac Action Potential

Cardiac Action Potential

Cardiac action potentials are essential for proper heart function, enabling the rhythmic contractions needed for adequate blood circulation. Nodal cells and Purkinje fibers, specialized for electrical conduction, generate these action potentials.
The cardiac action potential process involves a series of phases characterized by the movement of ions across the cardiac cell membranes, leading to the depolarization and repolarization of the cardiac myocytes.
Ionic Basis of Cardiac Action Potentials

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Updated: May 5, 2026

Contractility Measurements on Isolated Papillary Muscles for the Investigation of Cardiac Inotropy in Mice

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Ionic basis of myocardial contractility.

Annual Review of Medicine

|January 1, 1977

Summary

This summary is machine-generated.

Mammalian cardiac muscle contractility is regulated by intracellular calcium (Ca) levels, influenced by surface-bound Ca and transport mechanisms across the cell membrane. Digitalis glycosides enhance contractility by inhibiting the Na-K pump, increasing intracellular Na and subsequently Ca delivery to myofilaments.

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Measurement of Heart Contractility in Isolated Adult Human Primary Cardiomyocytes

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Published on: August 9, 2022

Area of Science:

Cardiology
Molecular Physiology
Biophysics

Background:

Mammalian cardiac muscle physiology is less understood than skeletal muscle due to experimental challenges.
Extrapolation from skeletal muscle to cardiac muscle has often been misleading, particularly regarding ionic control of contractility.
Cardiac muscle requires intrinsic mechanisms to modulate force development, unlike skeletal muscle's motor unit recruitment.

Purpose of the Study:

To review the mechanisms regulating myocardial contractility, focusing on the role of calcium (Ca).
To elucidate the sources and transport pathways of Ca involved in cardiac muscle contraction.
To explain the mechanism of action of digitalis glycosides on cardiac contractility.

Main Methods:

Review of existing literature on cardiac muscle physiology and ionic control.
Discussion of experimental findings related to Ca sources and sarcolemmal transport.
Analysis of the effects of digitalis glycosides on intracellular ion concentrations and contractility.

Main Results:

Contractile-dependent Ca originates from the cell surface, with a significant portion bound to the external lamina complex, notably involving sialic acid.
Ca crosses the sarcolemmal membrane via an electrogenic 'pore' system and an electroneutral Na-coupled 'carrier' system.
Digitalis glycosides inhibit the Na-K pump, increasing intracellular Na, which enhances Na-Ca carrier activity and boosts myofilament Ca delivery, leading to a positive inotropic effect.

Conclusions:

Cardiac contractility is modulated by intracellular Ca, influenced by surface-bound Ca and specific transport systems.
The Na-Ca carrier system plays a crucial role in delivering Ca to myofilaments, particularly under the influence of digitalis glycosides.
Current understanding represents a working model, requiring further experimental validation with advanced tools.