The vagus nerve is the major parasympathetic nerve responsible for controlling the beat-to-beat regulation of heart rate. It innervates key cardiac structures including the sinoatrial (SA) node, atrioventricular (AV) node, atrial myocardium, and to a lesser extent the ventricles.

Neurotransmitter Released by the Vagus Nerve

• The vagus nerve releases the neurotransmitter acetylcholine (ACh) at its nerve endings in the heart.

• Acetylcholine binds to muscarinic M2 receptors on cardiac pacemaker cells and other myocardial tissue.

• Activation of these receptors leads to:

  • Opening of acetylcholine-sensitive potassium channels (GIRK channels), causing hyperpolarization and slowing pacemaker firing.

  • Inhibition of adenylyl cyclase activity, decreasing cAMP and reducing calcium channel opening.

  • Overall reduction in heart rate (negative chronotropic effect) and decreased conduction velocity (negative dromotropic effect).

Why Other Options are Incorrect

• Adrenaline (option 1): Sympathetic neurotransmitter primarily released by adrenal medulla and sympathetic nerve endings; increases heart rate.

• Nor-epinephrine (option 3): Main sympathetic neurotransmitter, increases heart rate and contractility.

• Dopamine (option 4): A precursor in catecholamine synthesis; not the primary neurotransmitter released by vagus nerve.

Supporting Research

• Studies in isolated animal hearts and human physiology show that acetylcholine is responsible for parasympathetic slowing of heart rate.pmc.ncbi.nlm.nih+2

• Vagal stimulation induces effects consistent with acetylcholine release, such as increased potassium channel opening and reduced calcium influx.

• Vagal tone is fundamental to maintaining the low resting heart rate and variability essential for cardiovascular health.

Summary

Conclusion

The neurotransmitter released by the vagus nerve that controls heart rate is:

(2) Acetylcholine

Understanding this fundamental physiological mechanism is crucial for grasping how the autonomic nervous system regulates the cardiovascular system.