Smooth muscle in the intestinal wall undergoes slow-wave spontaneous depolarizations. Under which condition do these slow waves generate action potentials and produce contraction?
- A Slow waves always produce action potentials; contractile force varies only with neural input
- B When the slow-wave depolarization reaches threshold (~−40 mV), L-type Ca²⁺ channels open, action potentials occur, and the resulting Ca²⁺ influx triggers contraction ✓
- C Only when parasympathetic acetylcholine causes IPSP-like hyperpolarization that rebounds above threshold
- D Slow waves produce contraction directly without action potentials via IP3-mediated Ca²⁺ release
Correct answer: B. When the slow-wave depolarization reaches threshold (~−40 mV), L-type Ca²⁺ channels open, action potentials occur, and the resulting Ca²⁺ influx triggers contraction
Explanation
Intestinal interstitial cells of Cajal generate rhythmic slow waves (basic electrical rhythm) at ~3/min in stomach and ~12/min in duodenum. These oscillations alone do not guarantee contraction; they only trigger Ca²⁺-dependent action potentials when the membrane reaches threshold (around −40 mV). Excitatory factors (acetylcholine, stretch, substance P) raise resting membrane potential toward threshold, while inhibitory factors (VIP, NO) hyperpolarize the membrane and prevent action potential generation despite ongoing slow waves.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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