Cardiovascular system: Sympathetic and parasympathetic.

The idea

The autonomic nervous system controls cardiac muscle without conscious input, and its two divisions have opposing effects on the heart. Sympathetic stimulation releases noradrenaline at the sinoatrial node, raising the rate at which it fires, so heart rate, stroke volume and cardiac output increase during exercise. Parasympathetic stimulation releases acetylcholine at the sinoatrial node, slowing its firing rate and reducing heart rate at rest and during recovery. The two divisions act as an antagonistic pair: at any moment the balance of their activity sets the heart rate, and the balance shifts towards sympathetic dominance as exercise intensity rises. A useful method is to trace one pathway: exercise detected, sympathetic activity increases, sinoatrial node fires faster, heart rate rises, then recovery brings parasympathetic dominance and heart rate falls.

What earns the marks

• Sympathetic stimulation increases heart rate by raising the firing rate of the sinoatrial node.
• Sympathetic stimulation increases stroke volume and therefore cardiac output during exercise.
• Parasympathetic stimulation decreases heart rate by slowing the firing rate of the sinoatrial node.
• Parasympathetic activity dominates at rest and during recovery from exercise.
• The two divisions form an antagonistic pair whose balance sets the current heart rate.
• Sympathetic activity is mediated by noradrenaline and parasympathetic activity by acetylcholine at the sinoatrial node.

Common mistakes and how to put them right

• Thinking the sympathetic division slows the heart.

  Put it right: The sympathetic division speeds the heart up, while the parasympathetic division slows it down.

• Believing the two divisions work one at a time.

  Put it right: They are antagonistic and both exert some influence, with the balance shifting with exercise intensity.

• Confusing the autonomic control of heart rate with voluntary control of skeletal muscle.

  Put it right: Cardiac muscle is controlled involuntarily through the autonomic nervous system.

• Assuming parasympathetic activity stops completely during exercise.

  Put it right: It is reduced rather than necessarily absent, and it returns to dominance during recovery.

Exam tips

• Link each division to a specific effect at the sinoatrial node rather than writing only that it speeds up or slows down the heart.
• Use the antagonistic pair idea to explain changes across exercise and recovery in one continuous account.
• Check that any heart-rate value you quote is consistent with the direction of the division you have named.