Single-unit (visceral) smooth muscle differs from multi-unit smooth muscle in its electrical coupling and contractile behavior. Which feature is responsible for the spontaneous rhythmicity and phasic contractions of single-unit smooth muscle (e.g., intestinal)?

• A Presence of gap junctions (connexin 43 and 40) enabling electrical syncytium formation, plus intrinsic pacemaker cells (interstitial cells of Cajal) generating slow waves ✓
• B Absence of gap junctions, ensuring independent activation of individual cells
• C High density of L-type Ca²⁺ channels making each cell spontaneously excitable without pacemaker cells
• D Tonic activation by circulating epinephrine maintaining persistent low-level membrane depolarization

Explanation

Single-unit smooth muscle (intestine, ureter, uterus) behaves as an electrical syncytium because cells are connected by gap junctions (mainly connexin 43), allowing action potentials to spread from cell to cell. The intrinsic pacemaker cells (interstitial cells of Cajal, ICCs) at the myenteric plexus level generate rhythmic slow waves (pacemaker potentials). When slow waves reach threshold, action potentials and phasic contractions occur. Multi-unit smooth muscle (iris, vas deferens, large airways) lacks gap junctions and is predominantly neurogenically controlled. ICC loss or dysfunction is associated with gastroparesis and intestinal pseudo-obstruction.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.