Fast inspiratory heart rate

We all know that the human heart is located in the left atrium, but there is also a small part of the heart in the right ventricle. Only when the left atrium and the right ventricle work together can the heart beat normally and the heart and blood vessels circulate normally. Some people say that they feel their heart beats slowly when they exhale, but their heart rate beats too fast when they inhale. Why is this?

In addition to its automatic rhythmicity, the electrical activity of the sinoatrial node is also controlled by the cardiac autonomic nervous system. The neurotransmitter released by the postganglionic fibers of the cardiac sympathetic nerves is norepinephrine, which binds to the β-adrenergic receptors on the myocardial cell membrane.

It binds to the body and changes the permeability of ion pores on the cell membrane in some way, thereby affecting the electrical activity of the heart. The excitability of the sympathetic nerves increases, the calcium influx into the sinoatrial node pacemaker cells accelerates, the rising speed of phase 4 increases, the autonomy increases, and the heart rate increases.

The neurotransmitter released by the postganglionic fibers of the vagus nerve is acetylcholine, which binds to the M-type choline receptors on the myocardial cell membrane and exerts a regulatory effect on the heart. Increased vagal nerve tone can slow down the rise rate of phase 4 depolarization of sinoatrial node pacemaker cells, reduce autonomy, and slow down heart rate.

The regulatory effects of the sympathetic nerve and vagus nerve on the sinoatrial node are both opposing and unified, thus maintaining the normal electrophysiological activity of the sinoatrial node. The regulatory effect of the sympathetic nerve on the sinoatrial node is slow and can only be reflected after 20 seconds. The vagus nerve's regulatory effect on the sinoatrial node is rapid and can be seen in the next cardiac cycle. Therefore, short-term regulation of the sinoatrial node is mainly achieved through the vagus nerve. When inhaling, the vagus nerve tone decreases, the sinoatrial node automaticity increases, and the heart rate increases; when exhaling, the vagus nerve tone increases, the sinoatrial node automaticity decreases, and the heart rate slows down.