Correct Answer: C. VIP
Anterograde dilation (receptive relaxation) of the small intestine is mediated by VIP (Vasoactive Intestinal Peptide), a non-adrenergic, non-cholinergic (NANC) neurotransmitter released from enteric neurons. When the proximal intestine receives a bolus, VIP is released from inhibitory motor neurons in the myenteric plexus, causing smooth muscle relaxation ahead of the peristaltic wave. This allows the intestinal segment to dilate and accommodate incoming chyme. VIP acts on VPAC receptors on smooth muscle cells, triggering cAMP-mediated relaxation. This mechanism is distinct from the contractile phase (driven by acetylcholine and substance P) and is essential for normal peristalsis and nutrient transit. The NANC pathway involving VIP is a key component of the enteric nervous system's intrinsic control, independent of the CNS, making it the primary mediator of receptive relaxation in the GI tract.
Why the other options are wrong
A. Serotonin — Serotonin (5-HT) is released from enterochromaffin cells and acts on enteric neurons to modulate motility, but it is NOT the primary mediator of anterograde dilation. Serotonin primarily enhances peristalsis and secretion rather than causing smooth muscle relaxation. It acts as a neuromodulator, not the direct effector of receptive relaxation. B. Substance P — Substance P is an excitatory neurotransmitter released from enteric neurons that promotes smooth muscle contraction and peristalsis. It is part of the contractile phase, not the relaxation phase. Substance P acts on NK1 receptors to cause contraction, making it antagonistic to the dilation required for receptive relaxation. D. Acetylcholine — Acetylcholine is the primary excitatory neurotransmitter of the enteric nervous system, causing smooth muscle contraction and peristalsis. While it plays a crucial role in GI motility, it mediates the contractile (retrograde) phase, not the relaxation (anterograde) phase. The receptive relaxation reflex specifically requires NANC inhibitory neurons.
High-Yield Facts
- VIP is the primary mediator of anterograde (receptive) relaxation in the small intestine via NANC inhibitory neurons.
- NANC pathway (non-adrenergic, non-cholinergic) involves VIP and nitric oxide (NO) for smooth muscle relaxation.
- Acetylcholine and substance P mediate the contractile (retrograde) phase of peristalsis, not relaxation.
- Receptive relaxation allows the proximal intestine to dilate and accommodate incoming chyme before the peristaltic wave.
- VIP acts on VPAC receptors, increasing cAMP and causing smooth muscle hyperpolarization and relaxation.
Mnemonics
VAIN for Relaxation VIP and Acetylcholine are Inhibitory/Excitatory: VIP = relaxation (anterograde), Acetylcholine = contraction. Remember: VIP = Vasodilation & Inhibition of tone. NANC = Not Acetylcholine, Not Catecholamines NANC neurons release VIP and NO for relaxation. When you see 'receptive relaxation' or 'anterograde dilation,' think NANC → VIP.
NBE Trap
NBE may pair acetylcholine with GI motility to trap students who conflate 'acetylcholine controls GI function' with 'acetylcholine causes relaxation.' The key discriminator is that acetylcholine drives the contractile phase, while VIP drives the relaxation phase of the peristaltic reflex.
Clinical Pearl
In Indian patients with achalasia or Chagas disease (rare in India but relevant for NEET PG), destruction of NANC neurons (including VIP-releasing neurons) leads to loss of receptive relaxation and impaired peristalsis. Understanding VIP's role explains why nitric oxide donors and phosphodiesterase inhibitors (which mimic VIP's cAMP pathway) can provide symptomatic relief in motility disorders.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 62 (Propulsion and Mixing of Food in the Alimentary Tract); Harrison's Principles of Internal Medicine, Ch. 297 (Disorders of Gastrointestinal Motility)_