Questions: Autonomic Nervous System: Sympathetic and Parasympathetic Physiology

Question 1

A pharmacologist administers a drug that blocks all muscarinic acetylcholine receptors. Which of the following effects would you expect?

• A. Decreased heart rate — blocking muscarinic receptors reduces sympathetic drive to the heart
• B. Increased heart rate — blocking muscarinic receptors removes tonic parasympathetic slowing of the heart
• C. No change in heart rate — muscarinic receptors are only found in skeletal muscle
• D. Decreased heart rate — the drug enhances norepinephrine release by eliminating cholinergic competition

Answer: (B)

Parasympathetic postganglionic neurons release acetylcholine at muscarinic receptors on target organs. The vagus nerve continuously delivers tonic parasympathetic slowing to the heart — without this, the intrinsic pacemaker rate is ~100 bpm. Blocking muscarinic receptors (with atropine, for example) removes this parasympathetic brake, and heart rate rises. This reveals that resting heart rate (~70 bpm) is not the heart's intrinsic rate but the result of active parasympathetic inhibition.

Question 2

What neurotransmitter do preganglionic sympathetic neurons release at the sympathetic ganglion?

• A. Norepinephrine — sympathetic neurons always use norepinephrine
• B. Acetylcholine at nicotinic receptors — both sympathetic and parasympathetic preganglionic neurons use acetylcholine
• C. Dopamine — the ganglionic synapse uses dopamine as an intermediate signal
• D. Acetylcholine at muscarinic receptors — the same receptors used by the parasympathetic postganglionic neurons

Answer: (B)

Both sympathetic and parasympathetic preganglionic neurons release acetylcholine at nicotinic receptors in their respective ganglia. The key pharmacological distinction comes AFTER the ganglion: parasympathetic postganglionic neurons release ACh at muscarinic receptors on target organs, while sympathetic postganglionic neurons release norepinephrine at adrenergic receptors. The 'sympathetic = norepinephrine' rule applies to the terminal organ synapse, not to the ganglionic synapse.

Question 3

Sympathetic postganglionic neurons release acetylcholine at target organs.

• T. True
• F. False

Answer: False

False. Sympathetic postganglionic neurons release norepinephrine (NE) at adrenergic receptors on target organs — this drives the fight-or-flight responses (increased heart rate, vasoconstriction in gut, bronchodilation, etc.). It is the parasympathetic postganglionic neurons that release acetylcholine, at muscarinic receptors. The exception is sympathetic innervation of sweat glands and some blood vessels, which do use ACh, but the dominant sympathetic terminal transmitter is norepinephrine.

Question 4

Both sympathetic and parasympathetic divisions use acetylcholine at their preganglionic synapses.

• T. True
• F. False

Answer: True

True. The two-neuron architecture is shared: preganglionic neuron (CNS) → ganglion → postganglionic neuron (organ). In both divisions, the preganglionic-to-postganglionic synapse in the ganglion uses acetylcholine at nicotinic receptors. The divisions diverge at the second synapse: parasympathetic postganglionic neurons use ACh at muscarinic receptors; sympathetic postganglionic neurons use norepinephrine at adrenergic receptors. This shared preganglionic neurotransmitter is why nicotinic blockers affect both divisions simultaneously.

Question 5

Why do atropine (a muscarinic blocker) and beta-blockers have opposite effects on heart rate, and what does this reveal about how the ANS controls the heart?

Model answer: Atropine blocks muscarinic receptors, eliminating parasympathetic signaling to the heart, so heart rate rises toward the intrinsic pacemaker rate (~100 bpm). Beta-blockers block adrenergic (β1) receptors, reducing sympathetic drive to the heart, so heart rate falls. Both drugs act on the same organ but via different receptor systems representing opposite divisions. This reveals that resting heart rate is a dynamic balance: tonic parasympathetic tone is continuously slowing the heart while sympathetic tone modulates it upward. The heart rate at any moment reflects competition between both divisions, not the activation of just one.

This question tests whether students understand the heart as a system under dual control, not simply 'sympathetic speeds it up, parasympathetic slows it down in isolation.' The pharmacology makes the underlying physiology concrete and reveals that both divisions are tonically active at rest.