The function of vagus nerve innervating the heart is to

Options

A. maintain constant heartbeat
B. accelerate the heartbeat
C. initiate the heartbeat
D. reduce the heartbeat

Answer

### Step-by-Step Solution:

1. **Understanding the Vagus Nerve**: The vagus nerve is one of the twelve cranial nerves in the human body. It is the tenth cranial nerve and is part of the involuntary nervous system.

2. **Location and Function**: The vagus nerve extends beyond the head and neck, reaching into the thorax and abdomen. It innervates various organs including the pharynx, esophagus, larynx, lungs, stomach, and intestines.

3. **Role in Heart Function**: The vagus nerve specifically innervates the sinoatrial (SA) node of the heart. The SA node is known as the natural pacemaker of the heart, responsible for initiating the heartbeat.

4. **Effect on Heart Rate**: The primary function of the vagus nerve in relation to the heart is to reduce the heart rate. This is part of the parasympathetic nervous system's role in controlling involuntary bodily functions.

5. **Conclusion**: Based on the information regarding the function of the vagus nerve, the correct answer to the question is that the vagus nerve innervating the heart serves to reduce the heartbeat.

### Final Answer:
The function of the vagus nerve innervating the heart is to **reduce the heartbeat** (Option D).

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