Q. Explain the sequence of steps by which an action potential propagates without loss. Illustrate a diagram of an axon indicating the site where the peak action-potential currents flow across the membrane. Show the lines of current flow to unexcited parts of the cable. Elucidate how these local currents cause the action potential to propagate.

Sketch the Na+ and K+ currents which flow during an action potential. Look up how gNa and gK vary with time during the action potential -- given in the class notes -- and then recall that INa =gNa(V - VNa) , IK = gK(V-VK), and that the voltage V(t) is varying according to the action-potential shape. You could do this qualitatively, but justify your reply. A common mistake is to confuse the currents that flow throughout the action potential the voltage-clamp currents flow in response to a depolarizing pulse.