Sequence of Excitation

To reiterate, the SA node is the normal pacemaker for the entire heart. Its depolarization normally generates the current that leads to depolarization of all other cardiac muscle cells, and so its discharge rate determines the heart rate, the number of times the heart contracts per minute.

The action potential initiated in the SA node spreads throughout the myocardium, passing from cell to cell by way of gap junctions. The spread

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Superior vena cava

Sinoatrial node

Atrioventricular node

Bundle of His

Superior vena cava

Sinoatrial node

Right bundle branch

Purkinje fibers

The Sequence Excitation The Heart

Left bundle branch

FIGURE 14-16

Conducting system of the heart.

Right bundle branch

Purkinje fibers

Left bundle branch

FIGURE 14-16

Conducting system of the heart.

throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system. The spread is rapid enough that the two atria are depolarized and contract at essentially the same time.

The spread of the action potential to the ventricles is more complicated and involves the rest of the conducting system (Figures 14-16 and 14-17). 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 right atrium causes depolarization of the AV node. This node manifests a particularly important characteristic: For several reasons related to the electrical properties of the AV-node cells, the propagation of action potentials through the AV node is relatively slow (requiring approximately 0.1 s). This delay allows atrial contraction to be completed before ventricular excitation occurs.

After leaving the AV node, the impulse enters the wall—the interventricular septum—between the two ventricles via the conducting-system fibers termed the bundle of His (or atrioventricular bundle) after its discoverer (pronounced Hiss). It should be emphasized that the AV node and the bundle of His constitute the only electrical link between the atria and the ventricles. There are no others because a layer of nonconducting connective tissue, pierced by the bundle of His, completely separates each atrium from its ventricle.

Within the interventricular septum the bundle of His divides into right and left bundle branches, which

Heart Sequence Excitation

FIGURE 14-17

Sequence of cardiac excitation. The blue color denotes areas that are depolarized. Impulse spread from right atrium to left atrium is via the atrial muscle cells where the atria contact each other in their shared wall.

Adapted from Rushmer.

FIGURE 14-17

Sequence of cardiac excitation. The blue color denotes areas that are depolarized. Impulse spread from right atrium to left atrium is via the atrial muscle cells where the atria contact each other in their shared wall.

Adapted from Rushmer.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions eventually leave the septum to enter the walls of both ventricles. These fibers in turn make contact with Purk-inje fibers, large conducting cells that rapidly distribute the impulse throughout much of the ventricles. Finally, the Purkinje fibers make contact with nonconducting-system ventricular myocardial cells, via which the impulse spreads through the rest of the ventricles.

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. Actually, depolarization and contraction begin slightly earlier in the bottom (apex) of the ventricles and spread upward. The result is a more efficient contraction, like squeezing a tube of toothpaste from the bottom up.

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Responses

  • Awate
    What is the purpose of the sequence of excitation in the heart?
    7 years ago
  • luam
    What are the sequences of excitation of the heart?
    7 years ago
  • hilda burrows
    What part of the conducting mechanism is located in the right atrium?
    6 years ago
  • Rudolph Goldworthy
    Which of these sequences is the normal sequence of the spread of cardiac excitation?
    1 year ago
  • Holfast
    What is the normal sequence of cardiac exvitation?
    4 months ago
  • Temshe
    What is the correct sequence of excitation that occurs within the heart?
    15 days ago

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