Review Question - QID 100529

The conduction velocity of the structures of the heart are in the following order: Purkinje fibers > atrial muscle > ventricular muscle > AV node. Once placed in order based upon the sequence of conduction, the correct answer is 1.1 m/s, 0.05 m/s, 2.2 m/s, 0.3 m/s.

The conduction velocity of cardiac tissue is determined by a wide variety of factors, including the number and identity of various ion channels, the number and type of gap junctions, and the size and composition of muscle fibers. Conduction through the Purkinje system is the fastest within the heart, allowing for coordinated contraction of the ventricles. Atrial muscle conducts faster than ventricular muscle. Conduction through the AV node is the slowest, to allow the ventricles enough time to fill with blood.

For test-taking purposes, it is less important to remember the specific rates and more important to recognize the relative velocities of each structure. As mentioned, from fastest to slowest: Purkinje fibers > atrial muscle > ventricular muscle > AV node. That means that Purkinje fibers have the highest velocity (2.2 m/s), while the AV node has the lowest velocity (0.05 m/s). The final step is to put the values in the correct order corresponding to the sequence of conduction within the the heart.

Illustration A depicts the cardiac conduction system. The electrical impulse originates in the SA node, travels through conduction pathways to the AV node - the only conducting tissue between the atria and ventricles - then distributes into the ventricles through the bundle of His and Purkinje fibers.

Incorrect Answers:
Answers 1, 3-5: These choices do not correspond to the correct conduction velocities in each part of the cardiac conduction system.

Gap junctions, connections between cells that allow for signal transduction, have been shown to influence the conduction velocity within cardiac tissues. They are comprised of connexins. Various isoforms of connexins are associated with different cardiac structures, as well as abnormal cardiac rhythms (1).