Which structures are innervated by both the parasympathetic and sympathetic nervous systems?

Which structures are innervated by both the parasympathetic and sympathetic nervous systems?

• A. SAN and AVN
• B. SAN only
• C. AVN only
• D. AV bundle and Purkinje fibers

Answer: (A)

The key idea is that the heart’s pace and the speed of conduction through the nodal system are finely controlled by both branches of the autonomic nervous system, but not all parts receive dual input equally. The SA node sets the heart’s natural rate, and the AV node controls how quickly impulses pass to the ventricles. Both of these nodal tissues receive parasympathetic input from the vagus nerve and sympathetic input from cardiac nerves. Parasympathetic activity releases acetylcholine, acting on muscarinic receptors to slow the intrinsic pacemaker rate and slow AV nodal conduction. Sympathetic activity releases norepinephrine, acting on beta-1 receptors to increase the rate and accelerate conduction through the nodes. This dual innervation lets the heart speed up or slow down in response to changing needs. In contrast, the AV bundle (Bundle of His) and Purkinje fibers in the ventricles are mainly influenced by sympathetic tone; parasympathetic innervation to these ventricular conduction pathways is minimal. So they aren’t innervated by both divisions, which is why the structures that are innervated by both are the SA node and the AV node.

The key idea is that the heart’s pace and the speed of conduction through the nodal system are finely controlled by both branches of the autonomic nervous system, but not all parts receive dual input equally. The SA node sets the heart’s natural rate, and the AV node controls how quickly impulses pass to the ventricles. Both of these nodal tissues receive parasympathetic input from the vagus nerve and sympathetic input from cardiac nerves. Parasympathetic activity releases acetylcholine, acting on muscarinic receptors to slow the intrinsic pacemaker rate and slow AV nodal conduction. Sympathetic activity releases norepinephrine, acting on beta-1 receptors to increase the rate and accelerate conduction through the nodes. This dual innervation lets the heart speed up or slow down in response to changing needs.

In contrast, the AV bundle (Bundle of His) and Purkinje fibers in the ventricles are mainly influenced by sympathetic tone; parasympathetic innervation to these ventricular conduction pathways is minimal. So they aren’t innervated by both divisions, which is why the structures that are innervated by both are the SA node and the AV node.