During which phase can inspiration stimulate the SA node?

During which phase can inspiration stimulate the SA node?

• A. Parasympathetic activity
• B. Sympathetic activity
• C. Resting phase
• D. Contraction phase

Answer: (B)

Inspiration can stimulate the SA node primarily during sympathetic activity. During inspiration, the intrathoracic pressure decreases, which leads to a greater venous return to the heart. This increase in return blood flow enhances the activity of the sympathetic nervous system, which in turn can increase heart rate by stimulating the SA node. Additionally, inspiration can lead to the release of catecholamines that support this acceleration of heart rate.

When considering the other phases or activities, they do not enhance the SA node's activity in the same manner during inspiration. Parasympathetic activity usually slows heart rate through the vagus nerve and is more prominent during expiration. The resting phase does not involve active stimulation of the SA node, while the contraction phase describes the mechanical action of the heart, which is less relevant to the electrical pacing of heart rhythms influenced by respiratory patterns. Thus, sympathetic activity is the phase in which inspiration plays a significant role in stimulating the SA node.