Which automaticity foci are not inhibited by cardiac parasympathetic activity?

Which automaticity foci are not inhibited by cardiac parasympathetic activity?

• A. Atrial foci.
• B. Junctional foci.
• C. Ventricular foci.
• D. None of the above.

Answer: (C)

Ventricular foci are unique in that they are not inhibited by the parasympathetic nervous system. While the atrial and junctional foci are influenced by vagal tone, which can suppress their automaticity, ventricular foci operate independently of such parasympathetic input.

This is primarily because the purkinje fibers and myocardial cells within the ventricles have a built-in capacity for automaticity that is less affected by autonomic regulation. As a result, when the heart muscle is under stress or when other pacemakers are suppressed (for example, due to increased vagal tone), the ventricles can still generate impulses autonomously. This characteristic makes ventricular foci critical in maintaining a cardiac rhythm when higher pacemakers (like those in the atrial or junctional areas) are not firing correctly or are being inhibited.

In summary, understanding the behavior of these foci in relation to autonomic control is foundational for interpreting EKGs effectively, particularly in clinical scenarios involving arrhythmias where the role of parasympathetic activity may be a factor.