Blocking parasympathetic nervous system will typically cause which effect on heart rate?

Blocking parasympathetic nervous system will typically cause which effect on heart rate?

• A. Decrease heart rate
• B. No change
• C. Increase heart rate
• D. Irregular rhythm

Answer: (C)

Blocking parasympathetic input to the heart removes its braking effect on the heart rate. The parasympathetic system, through the vagus nerve, releasing acetylcholine onto M2 receptors slows the SA node’s firing and reduces AV conduction. When this input is blocked, the SA node fires faster and conduction is less restrained, so heart rate increases. In a healthy heart this manifests as a rise in rate rather than a decrease or no change, and rhythm typically remains regular unless another problem is present. So the expected effect is an increase in heart rate.

Blocking parasympathetic input to the heart removes its braking effect on the heart rate. The parasympathetic system, through the vagus nerve, releasing acetylcholine onto M2 receptors slows the SA node’s firing and reduces AV conduction. When this input is blocked, the SA node fires faster and conduction is less restrained, so heart rate increases. In a healthy heart this manifests as a rise in rate rather than a decrease or no change, and rhythm typically remains regular unless another problem is present. So the expected effect is an increase in heart rate.