Questions: Autonomic Nervous System Organization and Organ Effects

Question 1

A researcher administers a drug that blocks all nicotinic acetylcholine receptors at autonomic ganglia. Which ANS division is preferentially impaired?

• A. Parasympathetic only, because it relies on acetylcholine throughout
• B. Sympathetic only, because postganglionic sympathetic fibers use norepinephrine
• C. Both divisions equally, because both use acetylcholine at the preganglionic synapse
• D. Neither division, because ganglionic transmission uses different receptors than organ-level transmission

Answer: (C)

Both sympathetic and parasympathetic divisions use acetylcholine at the preganglionic synapse (on nicotinic receptors in the ganglion). The difference between divisions is only at the postganglionic–organ junction: sympathetic postganglionic fibers release norepinephrine, parasympathetic release acetylcholine. Blocking nicotinic ganglionic receptors therefore disrupts both divisions. The common misconception is that because 'parasympathetic = acetylcholine,' only parasympathetic would be affected — but that only applies to the end-organ synapse.

Question 2

An athlete has a resting heart rate of 48 bpm. Which best explains this?

• A. High sympathetic tone accelerates cardiac conduction, which paradoxically lowers resting rate
• B. High parasympathetic (vagal) tone dominates cardiac pacemaker activity at rest, slowing the SA node
• C. Low norepinephrine release reduces β₁ stimulation, indicating sympathetic hypoactivity
• D. The SA node in trained athletes is intrinsically slower due to structural remodeling alone

Answer: (B)

At rest, parasympathetic (vagal) tone dominates cardiac control — the SA node is continuously slowed by acetylcholine acting on muscarinic receptors. Endurance training increases vagal tone, so athletes have a slower resting rate. Option C is partially true (less sympathetic input contributes) but the primary driver is increased parasympathetic dominance, not just sympathetic hypoactivity. This illustrates how the ANS is a continuously modulated dial, not a binary switch.

Question 3

The sympathetic and parasympathetic divisions use mostly different neurotransmitters at most synapse in their respective pathways.

• T. True
• F. False

Answer: False

Both divisions use acetylcholine at the preganglionic synapse (acting on nicotinic receptors in the ganglion). The distinction in neurotransmitter only applies postganglionic: sympathetic fibers release norepinephrine onto target organs; parasympathetic fibers release acetylcholine (acting on muscarinic receptors). Knowing this matters pharmacologically — ganglionic blockers affect both divisions, while muscarinic blockers or adrenergic blockers selectively target one division's end-organ effects.

Question 4

Dual innervation of an organ allows the nervous system to fine-tune organ function beyond what a single division could achieve alone.

• T. True
• F. False

Answer: True

Most organs receive opposing input from both divisions, enabling continuous bidirectional modulation rather than simple on/off control. The heart, for example, can be sped up by withdrawing parasympathetic tone, increased sympathetic input, or both simultaneously. This reciprocal arrangement also means both overactivation and underactivation of either division can cause distinct pathologies — loss of parasympathetic GI innervation causes ileus; excess sympathetic tone raises cardiovascular risk.

Question 5

Why does loss of parasympathetic innervation to the GI tract cause ileus (bowel paralysis), and what does this reveal about the resting state of parasympathetic tone?

Model answer: Parasympathetic tone actively drives GI motility at rest — it promotes peristalsis and secretion via acetylcholine on muscarinic receptors. Removing that input doesn't return the bowel to a neutral state; it removes the drive that keeps it moving. This reveals that parasympathetic activity is not optional or only situational but is the dominant resting input to the GI system. The ANS is always 'on' at some level in both divisions.

This illustrates a key principle: the 'rest-and-digest' label doesn't mean parasympathetic activity is only active during relaxation. It is tonically active in the GI tract under normal conditions. The sympathetic system inhibits digestion during stress by suppressing this ongoing parasympathetic activity — not by imposing paralysis directly. Understanding tonic versus phasic ANS activity is essential for interpreting both normal physiology and drug effects.