Magnetic stimulation of ferret papillary muscle.

1. Ferret papillary muscles were magnetically stimulated by the discharge of a capacitor through an inductance. 2. An induced current density of 39 mA cm-2 averaged over 280 microseconds (= 11 microC cm-2) corresponded to threshold. 3. A short galvanic current pulse was found to have a comparable charge threshold. 4. The time-varying magnetic field or the magnetically induced electrical field did not result in excitation unless current was allowed to flow. 5. Threshold conditions could be reached by adding subthreshold magnetically induced current to subthreshold galvanic current. A suprathreshold galvanic current could be prevented from exciting by simultaneously applying induced current of opposite direction. 6. It is concluded that outward current through the membrane capacity is the stimulus, independent of the origin of the current.