Membrane potential of nerve axons

The measurement of Em (A y) is also difficult.179 Three methods have been used (1) measurement with microelectrodes (2) observation of fluorescent probes (3) distribution of permeant ions. Microelectrodes inserted into mitochondria182 have failed to detect a significant value for Em. Fluorescent probes are not very reliable,179 183 leaving the distribution of permeant ions the method of choice. In this method a mitochondrial suspension is exposed to an ion that can cross the membrane but which is not pumped or subject to other influences that would affect its distribution. Under such conditions the ion will be distributed according to Eq. 18-11. The most commonly used ions are K+, the same ion that is thought to reflect the membrane potential of nerve axons (Chapter 30), or Rb+. To make the inner mitochondrial membrane permeable to K+, valinomycin (Fig. 8-22) is added. The membrane potential, with n = 1 in Eq. 9-1, becomes ... 

---