A micropipette filled with a physiologically appropriate electrolyte solution (e.g. potassium chloride), whose tip can be inserted through the plasma membrane into cells without major disturbance to cellular function. It can be used to detect changes in electrical potential, for example during the passage of an action potential along a nerve cell. A second electrode is immersed in the extracellular fluid, and both electrodes are connected via an amplifier to an oscilloscope.