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Encyclopedia > Electrical conduction system of the heart

The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to (and stimulate) the myocardium (Cardiac muscle). After myocardium is stimulated, it contracts. It is the ordered stimulation of the myocardium that allows efficient contraction of the heart, thereby allowing blood to be pumped throughout the body. Image File history File links Gray501. ... Image File history File links Gray501. ... Image File history File links Download high-resolution version (700x800, 666 KB) Prinzip der EKG-Darstellung, langsam Summary en: Principle of ECG formation, schnell de: Prinzip der EKG-Darstellung, schnell Autor: Kalumet, selbst erstellt, 28. ... Image File history File links Download high-resolution version (700x800, 666 KB) Prinzip der EKG-Darstellung, langsam Summary en: Principle of ECG formation, schnell de: Prinzip der EKG-Darstellung, schnell Autor: Kalumet, selbst erstellt, 28. ... The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ... The heart and lungs, from an older edition of Grays Anatomy. ... Myocardium is the muscular tissue of the heart. ... Cardiac muscle is a type of involuntary striated muscle found within the heart. ... Human blood smear: a - erythrocytes; b - neutrophil; c - eosinophil; d - lymphocyte. ...

Contents

Requirements for effective pumping

In order to maximize efficiency of contraction and cardiac output, the conduction system of the heart has: Cardiac output is the volume of blood being pumped by the heart, in particular a ventricle in a minute. ...

  • Substantial atrial to ventricular delay. This allows the atria to completely empty their contents into the ventricles; simultaneous contraction would cause inefficient filling and backflow. The atria are electrically isolated from the ventricles, connected only via the AV node which briefly delays the signal.
  • Coordinated contraction of ventricular cells. The ventricles must maximize systolic pressure to force blood through the circulation, so all the ventricular cells must work together.
    • Ventricular contraction begins at the apex of the heart, progressing upwards to eject blood into the great arteries. Contraction that squeezes blood towards the exit is more efficient than a simple squeeze from all directions. Although the ventricular stimulus originates from the AV node in the wall separating the atria and ventricles, the Bundle of His conducts the signal to the apex.
    • Depolarization propagates through cardiac muscle very rapidly. Cells of the ventricles contract nearly simultaneously.
    • The action potentials of cardiac muscle are unusually sustained. This prevents premature relaxation, maintaining initial contraction until the entire myocardium has had time to depolarize and contract.
  • Absence of tetany. After contracting, the heart must relax to fill up again. Sustained contraction of the heart without relaxation would be fatal, and this is prevented by a temporary inactivation of certain ion channels.

In anatomy, the atrium (plural: atria) refers to a chamber or space. ... In the heart, a ventricle is a heart chamber which collects blood from an atrium (another heart chamber that is smaller than a ventricle) and pumps it out of the heart. ... The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the heart, which conducts the normal electrical impulse from the atria to the ventricles. ... Systolic is the adjective form of systole, typically referring to the contraction activity of the heart. ... The bundle of His is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the point of the apex of the fascicular branches. ... Tetany is the point at which signals from nerves (action potentials) are arriving to skeletal muscle rapidly enough in succession to cause a steady contraction, and not just a series of individual twitches. ...

Electrochemical Mechanism

See main article: Cardiac action potential

Cardiac muscle has some similarities to neurons and skeletal muscle, as well as important unique properties. Like a neuron, a given myocardial cell has a negative membrane potential when at rest. Stimulation above a threshold value induces the opening of voltage-gated ion channels and a flood of cations into the cell. The positively charged ions entering the cell cause the depolarization characteristic of an action potential. Like skeletal muscle, depolarization causes the opening of voltage-gated calcium channels and release of Ca2+ from the t-tubules. This influx of calcium causes calcium-induced calcium release from the sarcoplasmic reticulum, and free Ca2+ causes muscle contraction. After a delay (the absolute refractory period), Potassium channels reopen and the resulting flow of K+ out of the cell causes repolarization to the resting state. The cardiac action potential is a specialized action potential in the heart, with unique properties necessary for function of the electrical conduction system of the heart. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ... Look up Threshold in Wiktionary, the free dictionary. ... Voltage-gated ion channel is a ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse). ... An ion is an atom or group of atoms with a net electric charge. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Also called transverse tubules, t tubules are deep invaginations of the plasma membrane found in skeletal and cardiac muscle cells. ... Ryanodine receptors (RyRs) form a class of calcium channels in various forms of muscle and other excitable animal tissue. ... The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e. ... A top-down view of skeletal muscle A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) lengthens or shortens. ... It has been suggested that this article or section be merged into Refractory period. ... In cell biology, potassium channels are the most common type of ion channel. ... In neuroscience, repolarization refers to the change in membrane potential that returns the membrane potential to a negative value after the depolarization phase of an action potential has just previously changed the membrane potential to a positive value. ...



Note that there are important physiological differences between nodal cells and ventricular cells; the specific differences in ion channels and mechanisms of polarization give rise to unique properties of SA node cells, most importantly the spontaneous depolarizations necessary for the SA node's pacemaker activity.


Conduction pathway

Signals arising in the SA node stimulate the atria to contract and travel to the AV node. After a delay, the stimulus is conducted through the bundle of His to the Purkinje fibers and the endocardium at the apex of the heart, then finally to the ventricular epicardium.


Microscopically, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disk. The heart is a syncytium: electrical impulses propagate freely between cells in every direction, so that the myocardiam functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While normally advantageous, this property can be detrimental as it potentially allows the propagation of incorrect electrical signals. These gap junctions can close to isolate damaged or dying tissue, as in a myocardial infarction. gap junction A gap junction is a junction between certain animal cell-types that allows different molecules and ions to pass freely between cells. ... An intercalated disc is an undulating double membrane separating adjacent cells in cardiac muscle fibers. ... Acute myocardial infarction (AMI or MI), more commonly known as a heart attack, is a disease state that occurs when the blood supply to a part of the heart is interrupted. ...


Depolarization and the ECG

See also: Electrocardiogram
The EKG complex. P=P wave, PR=PR interval, QRS=QRS complex, QT=QT interval, ST=ST segment, T=T wave

“QRS” redirects here. ... Image File history File links Size of this preview: 525 × 600 pixel Image in higher resolution (700 × 800 pixel, file size: 664 KB, MIME type: image/gif) Prinzip der EKG-Darstellung, schnell en: Priniple of ECG formation de: Prinzip der EKG-Darstellung Autor: Kalumet, selbst erstellt, 28. ... Image File history File links Size of this preview: 525 × 600 pixel Image in higher resolution (700 × 800 pixel, file size: 664 KB, MIME type: image/gif) Prinzip der EKG-Darstellung, schnell en: Priniple of ECG formation de: Prinzip der EKG-Darstellung Autor: Kalumet, selbst erstellt, 28. ... A schematic of the EKG complex with labels. ...

SA node: P wave

Under normal conditions, electrical activity is spontaneously generated by the SA node, the physiological pacemaker. This electrical impulse is propagated throughout the right and left atria, stimulating the myocardium of the atria to contract. The conduction of the electrical impulse throughout the atria is seen on the ECG as the P wave. In anatomy, the atrium (plural: atria) is the blood collection chamber of a heart. ... Myocardium is the muscular tissue of the heart. ... ECG may also refer to the East Coast Greenway Lead II An Electrocardiogram (ECG or EKG, abbreviated from the German Elektrokardiogramm) is a graphic produced by an electrocardiograph, which records the electrical voltage in the heart in the form of a continuous strip graph. ...


As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the AV node. The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the heart, which conducts the normal electrical impulse from the atria to the ventricles. ...


AV node/Bundles: PR interval

The AV node functions as a critical delay in the conduction system. Without this delay, the atria and ventricles would contract at the same time, and blood wouldn't flow effectively from the atria to the ventricles. The delay in the AV node forms much of the PR interval on the ECG. And part of atrial repolarization can be represented by PR segment. In anatomy, the atrium (plural: atria) is the blood collection chamber of a heart. ... In the heart, a ventricle is a heart chamber which collects blood from an atrium (another heart chamber that is smaller than a ventricle) and pumps it out of the heart. ... ECG may also refer to the East Coast Greenway Lead II An Electrocardiogram (ECG or EKG, abbreviated from the German Elektrokardiogramm) is a graphic produced by an electrocardiograph, which records the electrical voltage in the heart in the form of a continuous strip graph. ...


The distal portion of the AV node is known as the Bundle of His. The Bundle of His splits into two branches in the interventricular septum, the left bundle branch and the right bundle branch. The left bundle branch activates the left ventricle, while the right bundle branch activates the right ventricle. The left bundle branch is short, splitting into the left anterior fascicle and the left posterior fascicle. The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant to ischemic damage. The bundle of His is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the point of the apex of the fascicular branches. ... In the heart, a ventricle is a chamber which collects blood from an atrium (another heart chamber) and pumps it out of the heart. ... The right ventricle is one of four chambers (two atria and two ventricles) in the human heart. ...


Purkinje fibers/ventricular myocardium: QRS complex

The two bundle branches taper out to produce numerous Purkinje fibers, which stimulate individual groups of myocardial cells to contract. Purkinje fibers (or Purkyne tissue) are located in the inner ventricular walls of the heart, just beneath the endocardium. ...


The spread of electrical activity through the ventricular myocardium produces the QRS complex on the ECG. The QRS complex is a record of the measurement of the movement of electrical impulses through the lower heart chambers (ventricles). ... ECG may also refer to the East Coast Greenway Lead II An Electrocardiogram (ECG or EKG, abbreviated from the German Elektrokardiogramm) is a graphic produced by an electrocardiograph, which records the electrical voltage in the heart in the form of a continuous strip graph. ...


Ventricular repolarization: T wave

The last event of the cycle is the repolarization of the ventricles. In the heart, a ventricle is a heart chamber which collects blood from an atrium (another heart chamber that is smaller than a ventricle) and pumps it out of the heart. ...


Pathology

An impulse (action potential) that originates from the SA node at a rate of 60 - 100 beats/minute (bpm) is known as normal sinus rhythm. If SA nodal impulses occur at a rate less than 60 bpm, the heart rhythm is known as sinus bradycardia. If SA nodal impulse occur at a rate exceeding 100 bpm, the consequent rapid heart rate is sinus tachycardia. These conditions are not necessarily bad symptoms, however. Trained athletes, for example, usually show heart rates slower than 60bpm when not exercising. If the SA node fails to initialize the AV Junction can take over as the main pacemaker of the heart. The AV Junction "surrounds" the AV node (the AV node is not able to initialize its own impulses) and has a regular rate of 40 to 60 bpm. These "Junctional" rhythms are characterized by a missing or inverted P-Wave. If both the SA node and the AV Junction fail to initialize the electrical impulse, the ventricles can fire the electrical impulses themselves at a rate of 20 to 40 bpm and will have a QRS complex of greater than .12ms. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ... Sinus bradycardia is a rhythm that originates from the sinus node and has a rate of under 60bpm. ... Sinus tachycardia is a rhythm with elevated rate of impulses originating from the SA node, defined as a rate greater than 100 beats/min in an average adult. ...

See also: Cardiac arrhythmia

Cardiac arrhythmia is any of a group of conditions in which the electrical activity of the heart is irregular or is faster or slower than normal. ...

External links

  • Conduction system of the heart - Merck Source

See also


 
 

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