What happens to the heart if the vagus nerve was severed?

Answer

The most immediate effect would be an increase in heart rate. In addition, regulation of heart rate, contractile state, and conduction of electrical impulses through the heart would be impaired.

The reason for the observed effects are that the vagus nerve carries important information to the heart from a part of the central nervous system called the parasympathetic nervous system. The parasympathetic nervous system is often considered the "rest and digest" system. It decreases heart rate, decreases how strongly the heart contracts with each beat, and decreases the rate at which electrical impulses are conducted through the heart. This is in contrast to the effects of the sympathetic nervous system on the heart; the sympathetic nervous system raises heart rate, force of contraction, and rate of electrical conduction in the heart.

A normal heart does not require information from the parasympathetic and sympathetic nervous systems in order to beat. The heart has an intrinsic pacemaker that allows it to generate heart beats on its own. The heart's pacemaker generates a pace at a rate of about 100 cycles per second, which means that if the heart was to take matters into its own hands (ie, have no innervation), then it would beat at about 100 beats per minute (bpm).

If that's the case, then why do normal adult hearts beat around 70 beats per minute and not 100?

As was hinted at above, normal hearts are innervated by the parasympathetic and sympathetic nervous systems. And both act on the heart at the same time. While you might think that the systems would cancel each other out, it turns out that the heart receives more sympathetic stimulation than parasympathetic stimulation at rest. The result is that the heart is effectively told to reduce the heart rate from the normal 100 bpm to something closer to 70 bpm.

Thus if you take a normal heart at rest that's innervated by both sympathetic and parasympathetic systems, and then sever the vagus nerve, you lose the parasympathetic input to the heart, while leaving the sympathetic input intact. The result is that the resting heart rate increases, to something closer to 100 bpm.

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