Electrical parameters of smooth muscle cells

¹ Department of Physiology and Biophysics, University of Illinois, Urbana, Illinois

Spikes in circular intestinal muscle fibers show volume-conductor shapes and conduct faster in low resistance medium than in air. Longitudinal space constant is 1.0 mm (20 cell lengths if 50% overlap); transverse space constant is 0.27 mm or 50 cell widths. The membrane time constant measured intracellularly is 31 msec, between two cells by double microelectrodes 7 msec. Cell membrane resistance is 70 megohms, between two cells 17 megohms, cell capacitance 4.5 x 10^–4µf. For a surface area of 8 x 10^–6 cm², membrane resistance is 560 ohms cm², capacitance 56 µf/cm²; because of surface folding and intercellular shunting, specific resistance is probably higher and capacitance lower. Membrane resistance in longitudinal muscle is two-thirds that in circular. Interfiber resistance rises, extracellular space increases, and velocity decreases in hypertonic solutions. Voltage-current curves show normal rectification. Depolarizing or polarizing pulses initiate 24/sec oscillations; depolarization can cause instability and spikes. Amplitude of conducted spikes is reduced by both depolarization and hyperpolarization. No change in membrane resistance occurs during a spike, probably because of parallel conducting fibers in different planes. Interfiber membranes are nonreactive, show no oscillations and no rectification, are of lower resistance than normally reactive membranes, and may represent labile zones of contact. Threshold current is about 0.2 x 10^–9 amp which corresponds to that normally crossing the relatively low-resistance interfiber junctions.

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