Fig. 3-26. Local circuit current flow during propagation of a spike. A. Spike frozen in time in a region of membrane as it was travelling from right to left. B. Magnitudes and directions of total membrane current, I_m, capacitative current, i_c, and ionic current, i_i, along the membrane. As usual, downward deflections indicate inward current. C. The direction and density of membrane currents are indicated by arrowhead orientations and density of red lines. D. Current flow directins and magnitudes in membrane equivalent circuits shown at five points (indicated by vertical dashed lines) along the membrane at which the spike is just beginning, 1; in its upstroke phase, 2; at its peak overshoot, 3; in its downstroke phase, 4; in its hyperpolarizing after potential, 5; and at rest, 6 (Noble, Physiol Rev 46:1-50, 1966; Brinley, Excitation and conduction in nerve fibers, in Mountcastle, Medical Physiology, 13th ed. Vol I, St. Louis: Mosby, 1974).