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Electrical conduction system of the heart

Electrical conduction system of the heart

Overview
The normal intrinsic electrical conduction of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 allows electrical propagation to be transmitted from the Sinoatrial Node
Sinoatrial node
The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...

 through both atria and forward to the Atrioventricular Node
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...

. Normal/baseline physiology allows further propagation from the AV node to the ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

 or Purkinje Fibers
Purkinje fibers
For the nervous cells, see Purkinje cellPurkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium...

 and respective bundle branches and subdivisions/fascicles. Both the SA and AV nodes stimulate the Myocardium. Time ordered stimulation of the myocardium allows efficient contraction of all four chambers of the heart, thereby allowing selective blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 perfusion through both the lungs and systemic circulation.

Cardiac neurons innervating the myocardium bear limited similarities to those of skeletal muscle as well as other important differences.
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Encyclopedia
The normal intrinsic electrical conduction of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 allows electrical propagation to be transmitted from the Sinoatrial Node
Sinoatrial node
The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...

 through both atria and forward to the Atrioventricular Node
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...

. Normal/baseline physiology allows further propagation from the AV node to the ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

 or Purkinje Fibers
Purkinje fibers
For the nervous cells, see Purkinje cellPurkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium...

 and respective bundle branches and subdivisions/fascicles. Both the SA and AV nodes stimulate the Myocardium. Time ordered stimulation of the myocardium allows efficient contraction of all four chambers of the heart, thereby allowing selective blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 perfusion through both the lungs and systemic circulation.

Electrochemical mechanism


Cardiac neurons innervating the myocardium bear limited similarities to those of skeletal muscle as well as other important differences. Cardiac neurons are uniquely subject to influence by the sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 and parasympathetic influence of the autonomic nervous system
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

 unlike skeletal muscle.

Like a neuron, a given myocardial cell has a negative membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...

 when at rest. Stimulation above a threshold value induces the opening of voltage-gated ion channel
Voltage-gated ion channel
Voltage-gated ion channels are a class of transmembrane ion channels that are activated by changes in electrical potential difference near the channel; these types of ion channels are especially critical in neurons, but are common in many types of cells....

s with inducted flow of cations into the cell. The positively charged ions entering the cell cause the depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...

 characteristic of an action potential. After depolarization, there's a brief repolarization that takes place with the eflux of potassium through fast acting potassium channels. Like skeletal muscle, depolarization causes the opening of voltage-gated calcium channels - meanwhile potassium channels have closed - and are followed by a titrated release of Ca2+ from the t-tubules. This influx of calcium causes calcium-induced calcium release
Calcium-induced calcium release
Calcium-induced calcium release is a process whereby calcium can trigger release of further calcium from the muscle sarcoplasmic reticulum. Originally proposed for skeletal muscle in the 1970s, subsequent research has revealed that it is even more pronounced in the cardiac muscle...

 from the sarcoplasmic reticulum, and free Ca2+ causes muscle contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

. After a delay, slow acting Potassium channels reopen and the resulting flow of K+ out of the cell causes repolarization
Repolarization
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. Repolarization results from the movement...

 to the resting state.

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 pacemaker activity.

Conduction pathway


Action potentials arising in the SA node (and propagating to the left atrium via Bachmann's bundle
Bachmann's bundle
right|thumb|350px|Image showing Bachmann's bundleBachmann's bundle, also known as the anterior interatrial band, is a broad band of atrial muscle that runs just behind the ascending aorta and connects the top of the right atrium with the top of the left atrium. Bachmann's bundle is, during normal...

) cause the atria to contract. In parallel, action potentials travel to the AV node via internodal pathways. After a delay, the stimulus is conducted through the bundle of His
Bundle of His
The bundle of His, known as the AV bundle or atrioventricular bundle, is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node to the point of the apex of the fascicular branches...

 to the bundle branches and then to the purkinje fibers
Purkinje fibers
For the nervous cells, see Purkinje cellPurkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium...

 and the endocardium at the apex of the heart, then finally to the ventricular myocardium.
      • The pathway of the nerve impulse pass through the heart muscles is ;
  • SA node-> internodal pathway->transitional fibers->AV node->penetrating fibers->distal fibers->Bundle of his/AV bundle->right &left bundle branches->Purkinji fibers.

the total time taken by the nerve impulse pass from SA node to the ventricular myocardium is 0.19 seconds.
Microscopically, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disk. The heart is a functional syncytium (not to be confused with a true "syncytium" in which all the cells are fused together, sharing the same plasma membrane as in skeletal muscle). In a functional syncytium, electrical impulses propagate freely between communicating cells via gap junctions, so that the myocardium 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 (e.g., via an ectopic pacemaker
Ectopic pacemaker
An ectopic pacemaker or ectopic focus is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the human heart. Acute occurrence is usually non-life threatening, but chronic occurrence can progress into tachycardia, bradycardia or ventricular...

). Gap junctions can close, e.g., after a cardiac ischemic event such as myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...

, thus isolating damaged or dying tissue in the myocardium, which then no longer participate in synchronous myocardial contractility
Contractility
Myocardial contractility is the intrinsic ability of the heart to contract independent of preload and afterload. Changes in the ability to produce force during contraction result from different degrees of binding between myosin and actin filaments...

.

Depolarization and the ECG




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 atrium, and through Bachmann's bundle
Bachmann's bundle
right|thumb|350px|Image showing Bachmann's bundleBachmann's bundle, also known as the anterior interatrial band, is a broad band of atrial muscle that runs just behind the ascending aorta and connects the top of the right atrium with the top of the left atrium. Bachmann's bundle is, during normal...

 to the left atrium, stimulating the myocardium of both atria to contract. The conduction of the electrical impulse throughout the left and right atria is seen on the ECG as the P wave.

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.

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 segment on the ECG. Part of atrial repolarization can be represented by the PR segment.

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 left posterior fascicle transmits impulses to the papillary muscles, leading to mitral valve closure. As the left posterior fascicle is shorter and broader than the right, impulses reach the erection muscles just prior to depolarization, and therefore contraction, of the left ventricle myocardium. This allows pre-ejaculating of the chordae tendinae, increasing the resistance to flow through the mitral valve during left ventricular contraction.

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.

The spread of electrical activity (depolarization) through the ventricular myocardium produces the QRS complex
QRS complex
The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...

 on the ECG.

Ventricular repolarization: T wave


The last event of the cycle is the repolarization of the ventricles. The transthoracically measured PQRS portion of an electrocardiogram is chiefly influenced by the sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

. The T (and occasionally U) waves are chiefly influenced by the parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

 guided by integrated brainstem control from the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

 and the thoracic spinal accessory ganglia.

An impulse (action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

) that originates from the SA node at a relative rate of 60 - 100bpm is known as normal sinus rhythm. If SA nodal impulses occur at a rate less than 60bpm, the heart rhythm is known as bradycardiac sinus. If it occurs with a rate greater than 100bpm, it is called tachycardiac sinus.