Saltatory Conduction

In a myelinated neurone, the myelin sheath insulates the axon membrane so ions cannot move across it, except at the nodes of Ranvier. Depolarisation can therefore only happen at the nodes.When an action potential occurs at one node, the Na⁺ entering the axon sets up local currents that flow inside the axon, under the myelin, to the next node. These currents depolarise the membrane at the next node to threshold, and an action potential is generated there. The impulse appears to jump from node to node.This is saltatory conduction, from the Latin saltare, to jump.Saltatory conduction has two advantages over conduction along an unmyelinated axon:• Speed. The action potential only has to be regenerated at the nodes, roughly every 1 mm, instead of at every point on the membrane. Impulses in myelinated axons can travel at up to about 120 m s⁻¹, compared with a few m s⁻¹ in unmyelinated axons of similar diameter.
• Energy efficiency. Ions only cross the membrane at the nodes, so far fewer Na⁺ and K⁺ ions have to be moved back by the sodium-potassium pump, and less ATP is used per impulse.Myelination allows vertebrates to have thin axons that are still fast. To reach the same speed without myelin, an axon would need to be much wider, like the squid giant axon, and far fewer axons would fit into a nerve.Worked example: nodes are 1.2 mm apart and the action potential takes 0.02 ms to pass from one node to the next.speed = 0.0012 m ÷ 0.00002 s = 60 m s⁻¹