Practice question · Multiple choice
The sympathetic and parasympathetic branches both innervate the heart continuously, pulling in opposite directions. Why is a permanent tug-of-war a better design than simply switching one branch on when it is needed?
Hints
- A resting heart rate of 70 sits between an intrinsic rate of 100 and something much lower. What does that give you?
- Ask how fast you can go up and down from a midpoint, versus from a floor.
Show the answer
D. Because a balance point can be adjusted instantly either way.
Why
Holding 70 against an intrinsic pacemaker near 100 means the vagus is applying a constant brake, and a brake can be released instantly, the rate rises before any sympathetic signal is sent. It is why heart rate variability is medically informative: the beat-to-beat wobble is the two branches adjusting against each other, and a steady heart has lost the balance.
Practise The Nervous System
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