A new method is described which permits the measurement of membrane currents of thick muscle fibres (diameter 300 μm or more) ofAstacus fluviatilis orBalanus balanus under voltage clamp conditions.
The potential difference across a small patch of membrane (60–100 μm in diameter) is controlled by connecting a voltage source across it with two external electrodes. One of them is connected to the fluid bathing the muscle fibre. The other, tubular one is in touch with the test area. The current flowing through the electrodes represents the sum of the membrane current flowing across the test area and the leak current flowing in the external fluid between the electrodes. In the first version of the method the leak current is limited by a circular sucrose gap around the test area. In the second, more elaborate method, the leak current is eliminated by a system of two concentric sucrose rings with a guard ring electrode between them. This method permits in addition the measurement of full sized action potentials in the test area.