What happens to the heart if vagus nerve was severed?

If the vagus nerve was severed, it would have several significant effects on the heart:

1. Decreased Heart Rate: The vagus nerve plays a crucial role in regulating the heart rate. It sends inhibitory signals to the heart, causing a decrease in the heart rate. If the vagus nerve is severed, this inhibitory influence is lost, resulting in an increased heart rate.

2. Reduced Contractility: The vagus nerve also modulates the contractility of the heart muscle. It releases acetylcholine, which binds to muscarinic receptors on the heart muscle cells, leading to a decrease in the force of contraction. Severing the vagus nerve reduces this inhibitory effect, resulting in increased myocardial contractility.

3. Impaired Atrioventricular Conduction: The vagus nerve influences the conduction of electrical impulses between the atria and ventricles of the heart. It slows down the conduction velocity at the atrioventricular node (AV node), which normally prevents rapid heart rates from reaching the ventricles. If the vagus nerve is severed, the AV conduction may become faster, increasing the risk of arrhythmias such as atrial fibrillation.

4. Loss of Baroreflex: The vagus nerve is involved in the baroreflex, which is a physiological mechanism that helps maintain blood pressure homeostasis. When blood pressure increases, the baroreceptors in the carotid sinus and aortic arch send signals through the vagus nerve to the brain. This triggers a decrease in heart rate and dilation of blood vessels, leading to a reduction in blood pressure. If the vagus nerve is severed, the baroreflex is impaired, compromising the body's ability to regulate blood pressure effectively.

In summary, severing the vagus nerve can lead to an increased heart rate, reduced contractility, impaired atrioventricular conduction, and disruption of the baroreflex, all of which can have significant implications for cardiac function and overall cardiovascular health.