How a nerve impulse travels
The membrane sits near -70 mV until sodium rushes in, depolarising it, and myelin makes the impulse jump between nodes.
A worked example
A myelinated fibre conducting at 80 m/s carries a signal 0.40 m from elbow to spinal cord in 0.40 / 80 = 0.005 s, or 5 ms.
The mistake people actually make
Sodium entering makes the inside less negative; potassium leaving afterwards restores it. Do not reverse the pump either, which moves 3 Na+ out for every 2 K+ in.