The strength of ventricular contraction increases when SAN is stimulated by

Options

A. Vagus nerve
B. Parasympathetic nerve
C. Sympathetic nerve
D. All of these

Answer

To answer the question "The strength of ventricular contraction increases when SAN is stimulated by", we need to analyze the role of different nervous systems and their effects on the heart.

### Step-by-Step Solution:

1. **Understanding the SA Node**: The Sinoatrial (SA) node is known as the natural pacemaker of the heart. It generates electrical impulses that initiate the heartbeat and regulate the heart rate.

2. **Identifying the Nervous Systems**: There are two main divisions of the autonomic nervous system that influence heart function: the sympathetic nervous system and the parasympathetic nervous system. The vagus nerve is a part of the parasympathetic nervous system.

3. **Effects of the Vagus Nerve**: The vagus nerve primarily has a calming effect on the heart. When stimulated, it decreases the heart rate and can lead to a reduction in the strength of ventricular contractions. Therefore, it does not increase the strength of ventricular contraction.

4. **Effects of the Parasympathetic Nervous System**: Similar to the vagus nerve, the parasympathetic nervous system is responsible for promoting relaxation and reducing heart activity. Thus, it does not contribute to increasing the strength of ventricular contractions.

5. **Effects of the Sympathetic Nervous System**: The sympathetic nervous system, on the other hand, has the opposite effect. It stimulates the heart and increases both the heart rate and the strength of ventricular contractions. This is achieved through the release of hormones like adrenaline and epinephrine during stress or physical activity.

6. **Conclusion**: Given the roles of these nervous systems, the correct answer to the question is that the strength of ventricular contraction increases when the SA node is stimulated by the sympathetic nervous system.

### Final Answer:
The strength of ventricular contraction increases when the SA node is stimulated by the **sympathetic nervous system**.

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