Conduction Velocity and Amplitude of the Action Potential as Related to Circumference in the Isolated Fibre of Frog Muscle

Summary. 1. Conduction velocity and amplitude of the action potential and fibre circumference were measured in isolated frog muscle fibres in a large volume of Ringer's solution. 2. Conduction velocity was found to be a linear function of fibre circumference. 3. The velocity in a fibre of 100 μ diameter (circumference 314 μ ) at 20°C was 2.44 & 0.05 m/sec. for spring frogs and 2.05 & 0.07 m/sec. for winter frogs. The total range in eonduction velocity at 20°C was 1.2–3.1 m/sec. for spring frogs and 1.0–2.76 m/sec. for winter frogs. 4. The amplitude of the action potential, both at constant distance from the fibre surface and at 500 μ from the midpoint of the fibre, was relatively greater in fibres with large than in fibres with small circumference. 5. The amplitude decreased steeply with electrode distance from the fibre up to 200 μ, and reached one tenth of the maximally measured value at a distance of about 600 μ. At equal distance the amplitude was 20 per cent smaller when the electrode faced the rounded side of an elliptical fibre, than when it faced the flat surface (large diameter twice the small). 6. The findings with respect to conduction velocity and amplitude as a function of circumference are interpreted to indicate that the specific electrical properties of the membrane differ according to fibre size.