ABSTRACT
In a neuron, action potentials are initiated at the axon hillock because this region has the greatest density of voltage-dependent sodium channels (VDSCs) and so the lowest threshold for excitation. Thus, action potentials are undiminished in size even when conducted along peripheral axons that in humans may be up to 1 m in length. Current flow across the external surface of the axon is from the regions ahead and behind the action potential into the active zone. The function of the myelin sheath is to increase the conduction velocity substantially with relatively little increase in total axon diameter. Because the myelin sheath consists of plasma membrane, it has a high content of phospholipid with a high electrical resistance. The rate-limiting factor in determining conduction velocity is not the spread of local circuit currents, which is quite fast, but the time it takes for VDSCs to respond to depolarization.
