An attempt has been made to decide upon the most reasonable equivalent circuit that will describe the passive linear properties of the Purkinje strands of sheep heart muscle. In order to do this, measurements were made of the phase angle of the characteristic admittance as well as the longitudinal impedance, both as functions of frequency. The results of both types of experiments indicate the presence of a longitudinally oriented capacity with a time constant of about 60–70 µsec. It is suspected that this capacity and time constant represent the connection between the cells, both radially and longitudinally. The plasma membrane contains another capacity with a time constant of about 15 msec.

Membrane characteristics were studied in isolated muscle strands from auricles of frogs using the "square pulse" technique. Changes in the time course and spatial spread of subthreshold electrotonic potentials were measured. If acetylcholine is applied in concentrations which cause slowing or stoppage of the heart beat, the following changes are produced: ( a ) the length constant (λ) of the membrane is reduced, ( b ) the time constant is shortened. The effects are reversible and increase with acetylcholine concentration. The membrane changes caused by acetylcholine dimmish with time. It is concluded that during acetylcholine inhibition, as well as during vagal inhibition, the conductance of the muscle membrane is increased. Appreciable changes in the resting membrane potential need not accompany inhibition.