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Difficulty: MediumCirculatory and Transport Systems in Animals

During a normal mammalian cardiac cycle, electrical excitation coordinates the synchronized contraction of the heart chambers. What is the correct sequence of electrical impulse propagation through the cardiac tissues starting from the initiation of the heartbeat?

  1. 1Spontaneous depolarization generated at the sinoatrial (SA) node
  2. 2Spread of electrical impulse across the atrial myocardium
  3. 3Brief conduction delay at the atrioventricular (AV) node
  4. 4Rapid propagation along the Bundle of His and Purkinje fibers
  5. 5Widespread contraction of the ventricular myocardium starting from the apex

Answer

The correct sequence of cardiac conduction begins with depolarization at the sinoatrial (SA) node, followed by the spread of excitation across the atrial myocardium, a brief delay at the atrioventricular (AV) node, rapid signal conduction through the Bundle of His and Purkinje fibers, and finally contraction of the ventricular myocardium.
In the mammalian heart, the electrical signal originates spontaneously at the sinoatrial (SA) node in the right atrium. The impulse spreads across the atrial myocardium, causing atrial systole. It then reaches the atrioventricular (AV) node, where conduction is delayed to allow complete ventricular filling. From the AV node, the impulse travels rapidly through the Bundle of His and Purkinje fibers, causing coordinated contraction of the ventricular myocardium from the apex upward.

Step-by-Step Solution

1
Identify the primary pacemaker site where the cardiac action potential originates.
The heartbeat starts at the sinoatrial (SA) node located in the right atrium wall.
The SA node possesses the intrinsic capability for spontaneous rhythm generation.
2
Trace the path of electrical excitation across the upper chambers.
The signal spreads through the muscle cells of both atria, causing atrial contraction.
Atrial muscle fibers are electrically connected via intercalated discs.
3
Determine the role and location of the secondary conducting node.
The signal reaches the atrioventricular (AV) node, where its transmission is momentarily delayed.
This delay ensures ventricular filling occurs prior to ventricular systole.
4
Follow the specialized rapid conducting pathway down the ventricles.
The signal travels down the Bundle of His and through Purkinje fibers in the ventricular walls.
Standard muscle conduction would be too slow to produce coordinated ventricular pumping.
5
Identify the final mechanical response of the lower chambers.
The ventricular myocardium depolarizes and contracts from the apex toward the base of the heart.
This upward squeezing motion efficiently ejects blood into the major arterial trunks.

Key Concept

Mammalian Cardiac Conduction Pathway
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Topics

Circulatory and Transport Systems in Animals

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