Action potentials travel down membranes at about 1m/s which isn't fast enough. Myelin sheaths allow the potential to travel up to 100 times faster.
It has gaps/nodes roughly 1µm long every 100µm. The membrane only contacts the extracelluar fluid at these points so the voltage gated Na+ channels are also only found here. This allows the potential to jump from one node to the next.
Multiple sclerosis (MS) is caused by loss of myelin sheath in some areas of the brain and spinal cord.
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