Which of the following is used to study current flow across a single channel:

Patch clamp: measures ionic currents through a single ion channel. It has four configurations: Voltage clamp: whole cell voltage clamp measures currents across cell membranes. In using this apparatus, two electrodes are inseed into the nerve fiber. One of these electrodes is used to measure the voltage of the membrane potential, and the other is used to conduct electrical current into or out of the nerve fiber. Iontophoresis: a technique of introducing ionic medicinal compounds into the body through the skin by applying a local electric current.