Once this initial action potential is initiated, principally at the axon hillock, it propagates down the length of the axon. Under normal conditions, the action potential would attenuate very quickly due to the porous nature of the cell membrane. To ensure faster and more efficient propagation of action potentials, the axon is myelinated. Myelin, a derivative of cholesterol, acts as an insulating sheath and ensures that the signal cannot escape through the ion or leak channels. There are, nevertheless, gaps in the insulation (nodes of ranvier), which boost the signal strength. As the action potential reaches a node of Ranvier, it depolarises the cell membrane. As the cell membrane is depolarised, the voltage-gated sodium channels open and sodium rushes in, triggering a fresh new action potential.