Question

Describe the sequence of heart excitation.

Answer 1

Heart excitation: The conduction system within the heart is very important because it permits a rapid and organized depolarization of ventricular myocytes that is necessary for the efficient generation of pressure during systole.

• The cardiac muscles act like a physiological syncytium.
• The heart is formed by two syncytia, the atrial syncytium, and the ventricular syncytium.
• So, the action potential is transmitted from atria to ventricles only through the fibers of a specialized conductive system.

Normal activation sequence.

Normal activation sequence	Structure	conduction velocity(m/sec)	Pacemaker rate (beats/min)
1	SA node	<0.01	60-100
2	Atrial myocardium	1.0-1.2	None
3	AV node	0.02-0.05	40-55
4	Bundle of His	1.2-2.0	25-40
5	Bundle branches	2.0-4.0	25-40
6	Purkinje network	2.0-4.0	25-40
7	Ventricular myocardium	0.3-1.0	None

Steps of contraction:

SA node: SA node is a small strip of modified cardiac muscle situated in the superior part of the lateral wall of the right atrium just below the superior vena cava.

• the fibers of this node don't have contractile elements
• these fibers are continuous with the atrial muscles, so that impulse from SA node can spread rapidly through atria.
• Even though other parts of the heart like the AV node, atria, and ventricle can produce impulses, SA node is called the pacemaker because the rate of production of impulse is more here.
• SA node generates spontaneous action potential (autorythmic tissue)
• The action potential generated by SA spread throughout the atria primarily via cell to cell conduction at a velocity of around 0.5 m/sec.
• After initial sinoatrial nodal excitation, depolarization spreads throughout the atria.

Atrial Myocardium:

The impulse from the SA node to /av node is conducted by three bundles of Atrial fibers.

1. Anterior internodal fibers of Bachman,
2. Middle internodal fibers of Wenkebach,
3. Posterior internodal fibers of Thorel.

The spread throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system.

General excitation spreads from cell to cell throughout the entire atrial myocardium via the specialized connections between cells, the gap junctions, that typically exist between all myocardial cell types.

The conduction through atrial muscle cells is rapid enough that the two atria are depolarized and contract at essentially the same time.

All these fibers converge towards the AV node and interdigitate with the fibers of AV node.

AV node:

• The link between atrial depolarization and ventricular depolarization is a portion of the conducting system called the atrioventricular (AV) node, which is located at the base of the right atrium.
• The action potential spreading through the muscle cells of the right atrium causes depolarization of the AV node.
• This node has a particularly important characteristic: the propagation of action potentials through the AV node is relatively slow (requiring approximately 0.1 s).
• This results in a delay that allows atrial contraction to be completed before ventricular excitation occurs.

Bundle of HIS:

• After leaving the AV node, the impulse enters the wall—the interventricular septum—between the two ventricles.
• This pathway has conducting-system fibers termed the bundle of His (or atrioventricular bundle) after its discoverer (pronounced Hiss).
• AV node and the bundle of His constitute the only electrical link between the atria and the ventricles.
• Within the interventricular septum the bundle of His divides into right and left bundle branches, which eventually leave the septum to enter the walls of both ventricles.

Bundle branches:

• The left bundle branch splits into fascicles as it travels down the left side of the ventricular septum just below the endocardium.
• Its fascicles extend for a distance of 5 to 15 mm, fanning out over the left ventricle.
• the left bundle separates into two major divisions, the anterior and posterior branches.
• These divisions extend to the base of each papillary muscle as well as the adjacent myocardium.
• the right bundle branch continues inferiorly, as if it were a continuation of the bundle of His, traveling along the right side of the muscular interventricular septum and then dividing into fibers that spread throughout the right ventricle.

Purkinje fibers:

• The complex network of conducting fibers that extend from either the right or left bundle branches is composed of the rapid conduction cells known as Purkinje fibers.
• Purkinje fibers in both the right and left ventricles act as preferential conduction pathways to provide rapid activation.
• It coordinates the excitation pattern.
• These fibers travel within the trabeculations of the right and left ventricles, as well as within the myocardium itself.
• Finally, the Purkinje fibers make contact with ventricular myocardial cells, by which the impulse spreads through the rest of the ventricles.

Ventricular Myocardium:

• The rapid conduction along the Purkinje fibers and the diffuse distribution of these fibers cause depolarization of all right and left ventricular cells more or less simultaneously and ensure a single coordinated contraction.
• depolarization and contraction begin slightly earlier in the bottom (apex) of the ventricles and spread upward.