FIGURE 22-16 Conduction of action potentials in myelinated axons. Because the myelin layer renders the axon impermeable to ion movement across its membrane and because voltage-gated Na⁺ channels are found only on axonal membrane at the nodes of Ranvier, the influx of Na⁺ ions associated with an action potential can occur only at nodes. When an action potential is generated at one node (step 1), the excess positive ions in the cytosol, which cannot move outward across the sheath, diffuse rapidly down the axon, causing sufficient depolarization at the next node (step 2) to induce an action potential at that node (step 3). By this mechanism the action potential jumps from node to node along the axon.