Chapter 18, Problem 11P

BIO Axons. Neurons are the basic units of the nervous system They contain long tubular structures called axons that propagate electrical signals away from the ends of the neurons. The axon contains a solution of potassium ions K⁺ and large negative organic ions. The axon membrane prevents the large ions from leaking out, but the smaller K⁺ ions are able to penetrate the membrane to some degree. (See Figure 18.35.) This leaves an excess of negative charge on the inner surface of the axon membrane and an excess of positive charge on the outer surface, resulting in a potential difference across the membrane that prevents further K⁺ ions from leaking out Measurements show that this potential difference is typically about 70 mV. The thickness of the axon membrane itself varies from about 5 to 10 nm, so we’ll use an average of 7.5 nm. We can model the membrane as a large sheet having equal and opposite charge densities on its faces (a) Find the electric field inside the axon membrane, assuming (not too realistically) that it is filled with air. Which way does it point, into or out of the axon? (b) Which is at a higher potential, the inside surface or the outside surface of the axon membrane?

Figure 18.35

Problem 11.