How does a nerve transmit impulses?

At rest a nerve cell or fiber has more sodium (NA+) ions in the extracellular fluid and more potassium (K+) ions in the intracellular fluid. There are both NA+ and K+ ion channels in the membrane of the nerve cell and both are closed. Some of the K+ channels are a bit leaky and they let a small flow of k+ ions out of the nerve. This small flow causes the inside of the nerve to be more negative than the outside. We describe the cell as having a potential difference of -70mv. When stimulated to transmit an impulse NA+ channels in the membrane open, allowing NA+ to move into the cell down its diffusion gradient, this makes the inside of the cell less negative (more positive). This happens slowly until the cell reaches a threshold potential difference of -50mv. That is the cell has become sufficiently less negative. Once this threshold has been reached loads of NA+ channels open, allowing the ions to rush into the cell quickly causing the cell to become so much less negative it is positive, a potential difference of +40mv is reached. This positivity within the cell is known as depolarization and stimulates adjacent area of the nerve to also become positive, using the same mechanism. A wave of positivity (or depolarization) then spreads along the nerve, transmitting an impulse.