Conduction in smooth muscles: comparative electrical properties

Abstract

Electrical and morphological properties were correlated in a variety of mammalian smooth muscles. In the sucrose gap electrode the gut muscles (esophagus muscularis mucosae, intestinal muscles and taenia coli) showed small slow waves (5 mv) and spikes (25–45 mv), while dog retractor penis and rat ureter gave spikes up to 110 mv. Membrane depolarization with K₂SO₄ was 50–72 mv in both groups of muscle; with KCl it was one-third less. Conduction velocity ranged from 15 cm/sec. for esophagus muscle to 4.1 for intestinal muscles and 1.6 cm/sec. for retractor penis. Resistance of 0.5-mm strips in the sucrose gap was approximately 1 megohm for propagating visceral muscles, 5–12 megohms for nonpropagating blood vessel muscles. Fast conduction is correlated with low resistance and high capacitance (in sucrose), brief spikes, short relative refractory periods, low threshold stimulus duration, long fibers and low interfiber spaces (close packing of cells). Conduction required many parallel fibers; it failed in strips less than about 100 µ diameter and conduction distance varied with excitability. The electrical unit is the individual fiber, but the conducting unit involves many fibers. Conduction appears to depend on relatively low resistance paths of parallel fibers which interact electrically. It also depends on excitability of individual cells which is kept high by stretch and chemical agents.