Which nerve of the sympathetic nervous system sends an increased rate of action potentials to the sinoatrial node (SAN) to increase heart rate?

The nerve that sends an increased rate of action potentials to the sinoatrial node (SAN) to increase heart rate is the cardiac accelerator nerve. This nerve is part of the sympathetic nervous system. Here are the steps:

1. The sympathetic nervous system is activated: This can happen due to various factors such as stress, fear, or physical activity.

2. The signal travels to the spinal cord: Once the sympathetic nervous system is activated, the signal travels down the spinal cord to the specific nerves that control the heart rate.

3. The cardiac accelerator nerves are activated: These nerves are the ones that directly control the heart rate. They are located in the thoracic region of the spinal cord.

4. The cardiac accelerator nerves send signals to the heart: These nerves send an increased rate of action potentials to the sinoatrial node (SAN) of the heart.

5. The heart rate increases: The sinoatrial node (SAN), also known as the heart's natural pacemaker, responds to these signals by increasing the heart rate.