Current spread in the smooth muscle of the guinea-pig vas deferens.

1. The membrane polarization in response to intracellular stimulation and external stimulation, the junction potentials evoked by nerve and field stimulation and the spontaneous junction potentials were studied in the guinea-pig vas deferens.2. The responses to intracellular stimulation differed from those to external stimulation applied through a large electrode in the following ways: short time constant of the electrotonic potential; linearity of current-voltage relation; all-or-none spike only in a small proportion of the cells; high critical firing level; short latency; weak tendency for repetitive activity during depolarization; and sharp spatial decay of the response.3. The difference between intracellular and external stimulation could be explained by differences in current distribution in the tissue, if many muscle fibres were aggregated in functional bundles, with three-dimensional cell-to-cell connexions, so that the membrane near an intracellular stimulating electrode was shunted by a large area of surrounding membrane.4. The time course of the junction potentials depended on the manner by which they were produced. The junction potential evoked by hypogastric nerve stimulation was recorded in every cell with almost the same amplitude. The spontaneous junction potential decayed very sharply with distance and the time course of the falling phase was about 10 times faster than that of the evoked junction potential.The difference between the time course of the junction potentials was also explained by the difference in current distribution in the tissue.