Current has been passed through the cell membrane of muscle fibres of the isolated rabbit right ventricle with the aid of intraceUular doublebarrelled microelectrodes. Two types of muscle fibres were distinguished which are called P and V fibres. The relation between the intensity of a hyperpolarising current applied during the rising phase and the maximum amplitude of the action potential was different in these fibres. For P fibres the relation was essentially linear over most of the range of currents used. For V fibres the change in maximum action potential amplitude was either negligible or did not appear until a certain value of hyperpolarising current was reached. This behaviour of V fibres can be understood if a drop in polarisation resistance occurs during the rising phase and is of such short duration that the polarisation resistance has returned to its resting value before the crest of the action potential is reached. P fibres have an estimated mean resting polarisation resistance of (106 -4-13) K ohms, and a rheobase current strength of (0.08 ± 0.02) #a. In V fibres the resting polarisation resistance was (47 -4-29) K ohms and the rheobase current strength (0.47 4-0.28) /za.

1. The effects of polarizing current on single smooth muscle cells of the isolated guinea‐pig vas deferens have been studied by means of a bridge circuit in which the same electrode was used to pass current and record membrane potential. 2. Most cells had polarization resistances from 15 to 37 MΩ. 3. Depolarizing currents caused graded spikes up to a maximum amplitude of 75 mV. 4. Self‐regenerative spikes occurred in some cells if the membrane was depolarized by > 35 mV. 5. Currents of long duration and sufficient intensity caused a repetitive discharge. 6. Spikes arising from excitatory junction potentials were always ‘all‐or‐none’ in contrast to the graded responses to intracellular stimulation. 7. Electrotonic potentials declined much more rapidly than spontaneous excitatory junction potentials (EJPs); thus the time course of the EJP cannot be determined by the passive electrical properties of the smooth muscle cell membrane.

The solutions, n(t), of the differential equation dn/dt = alpha (1 - n) n (4 - 6n + 4n(2) - n(3)) - betan(2) (4 - 6n + 4n(2) - n(3)) in which alpha and beta are instantaneous functions of membrane potential, are shown to fit with good accuracy the time courses of the rise of potassium conductance during depolarizing steps in clamp potential, found experimentally by Hodgkin and Huxley and by Cole and Moore. The equation is derived by analysing the dynamic behaviour of a system consisting of a square array of interacting pores. The possible role of Ca(++) ions in this system is discussed.

Transmembrane potentials were recorded simultaneously from pairs of ventricular fibers in an isolated, regularly beating preparation. A double-barrelled microelectrode was used to record the potentials from, and to polarize, one fiber. A single microelectrode was used to record from a distant fiber. The existence of two systems of fibers, termed P and V, was confirmed. Histological evidence for the existence of two types of fibers is also presented. Electrotonic current spread was observed within both systems, electrotonic interaction between the two systems was rare and always weak. In the case of those pairs of fibers showing electrotonic interaction, the distance for an e-fold decrease in magnitude of the electrotonic potentials was found to be from 300 to 600 in P fibers and from 100 to 300 , in V fibers. However, no electrotonic interaction could be observed in the majority of V fiber pairs. Moreover, the magnitude of the electrotonic potential did not decay monotonically with distance in any one direction. It is concluded that the rabbit ventricle cannot be regarded as a single freely interconnected syncytium.

Intracellular microelectrodes were used to record action potentials from fibres of the isolated rabbit right ventricle and the areas of the action potentials were measured. The action potential area was found to depend in a reproducible way on the preceding pattern of stimulation. A mathematical model reproducing all the observed changes in the action potential area was developed. In the model the action potential area is taken as a linear function of the product of two time and stimulation dependent variables, M and N. The behaviour of each variable between action potentials is described by the solution of a second order differential equation. During each action potential the variables are assumed to change discontinuously, the magnitudes of the discontinuous changes being given by a set of subsidiary equations. It was found that the behaviour of all the fibres tested was described by the same set of equations, each single fibre being characterized by a set of ten independent constants.