Vagus Nerves: Understanding Cardiac Effects

The Vagus nerve, also known as the 10th cranial nerve (CN X), is a crucial part of the parasympathetic nervous system. It plays a significant role in regulating various bodily functions, including heart activity. The parasympathetic nervous system generally promotes "rest and digest" responses, which means it tends to slow down and conserve energy in the body, especially in the heart.

Cardiac Regulation by Vagus Nerves

When the Vagus nerves are stimulated, they release acetylcholine, a neurotransmitter that acts on the heart to reduce its activity. Let's analyze the given options to determine which effect is not caused by the Vagus nerves:

• Heart rate is slowed down.

  This is a well-known effect of Vagus nerve stimulation. The Vagus nerve reduces the firing rate of the sinoatrial (SA) node, which is the natural pacemaker of the heart. This action is known as a negative chronotropic effect, leading to a decrease in heart rate.

• Conductivity of the bundle is reduced.

  The Vagus nerve also reduces the speed at which electrical impulses travel through the atrioventricular (AV) node and the bundles of His. This effect, known as a negative dromotropic effect, can slow down the conduction of electrical signals from the atria to the ventricles.

• Force of contraction is reduced.

  The Vagus nerve has a negative inotropic effect, meaning it reduces the force of contraction of the heart muscle. While its effect is more pronounced on atrial contraction, it can also indirectly affect ventricular contraction by reducing heart rate and conduction, thereby reducing overall cardiac output.

• The duration of systole is increased.

  Systole is the phase of the cardiac cycle when the heart ventricles contract and pump blood out. The Vagus nerve's primary role is to decrease cardiac activity, including heart rate and the force of contraction. An increase in the duration of systole would imply a prolonged and potentially stronger contraction, which is contrary to the inhibitory effects of the Vagus nerve. In fact, by reducing the force and speed of contraction, the Vagus nerve might slightly shorten the duration of ejection or have minimal impact on overall systole duration for a given stroke volume. Increasing the duration of systole is typically associated with conditions that require more time for ejection, or in cases of sympathetic stimulation which enhances contractility and often leads to a more forceful, but not necessarily longer, ejection phase in healthy hearts. Therefore, this effect is not caused by the Vagus nerves.

Summary of Vagus Nerve Actions

In summary, the Vagus nerve decreases heart rate (negative chronotropy), reduces conduction velocity (negative dromotropy), and lessens the force of cardiac contraction (negative inotropy). Its overall action is to reduce the heart's workload and activity.