Action Potentials Recorded Intra‐ and Extra‐ cellularly from the Isolated Frog Muscle Fibre in Ringer's Solution and in Air

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Summary.: Action potentials were recorded inside and outside isolated frog muscle fibres in Ringer's solution and in moist air in order to correlate the different potentials and to study volume conductionThe extracellular action potential recorded from isolated frog muscle fibres in Ringer's solution was diphasic with a first positive and a second negative phase.The duration of the potential measured at a distance of 30 μ from the surface of the fibre was 2.67 ± 0.14 msec (21.2° C). It increased slightly with increasing fibre circumference from 2.5 msec in a fibre of 150 μ circumference to 3.2 msec in a fibre of 350 μ, circumference.The amplitude increased proportionally to the square of the fibre circumference. The peak to peak amplitude was 0.8 to 2.9 m V 30 μ from the surface of the fibre. Near the fibre the amplitude decreased linearly with the logarithm of the distance from the axis of the fibre. At distances (d) exceeding 0.15 mm from the axis of the fibre the amplitude decreased as d‐1,3.The peak of the intracellularly measured potential coincided with the negative peak of the extracellular potential with the fibre in Ringer's solution. This indicates that the extracellular action potential can be accounted for by means of the second derivative of the intracellular potential. A third positive phase of the volume conducted potential which is present in nerve disappears within the noise level for the muscle fibre in Ringer's solution because of the more protracted course of repolarization.The spread of the action potential along the fibre determined from the intracellularly recorded action potential was 8.8 ±0.9 mm (conduction velocity × duration of the potential from 50 per cent rise to 50 per cent fall).From the action potentials recorded at different distances from the fibre equipotential lines of the travelling potential field were constructed. By means of this field the charge passing the membrane during depolarization was determined to 1.4× 10‐6 coulomb per cm2.The surface action potential from a fibre surrounded by a moist gas mixture had an amplitude of the same order of magnitude as the intracellularly measured action potential with the fibre surrounded by Ringer's solution. The time for rise from 20 to 100 per cent of crest was about half the rise time of the intracellularly recorded potential. The duration of the reconstructed monophasic potential from 50 per cent rise to 50 per cent full was slightly shorter than the intracellular action potential.The conduction velocity of the potential with the fibre in air was half that measured in Ringer's solution (2.41 ± 0.12 m per sec in Ringer's solution; 1.05 ± 0.05 m per sec in air at 21° C). From this difference in propagation velocity the impedance of the Ringer's film surrounding the fibre in air was estimated to 12 megohms per cm. The thickness of the Ringer's layer was about two μ.

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