A.3.5. Saltatory Propagation
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Introduction: ‘Saltatory’ propagation is a special type of propagation for the action potential. It only occurs along the ‘very’ long axons of nerve cells. The purpose of this type of propagation is to increase the speed of propagation.
A. Structural and physiological components required:
A cell membrane, along an axon, with Na+ and K+ ion channels located in this membrane.
A myelin sheath wrapped around the axon. This sheath is interrupted by nodes of Ranvier. The myelin sheath is like an insulator that isolates the axon membrane from the extracellular fluid.
The nodes of Ranvier are the locations where the cellular membrane of the axon is in contact with the extracellular fluid.
The Na+ and K+ ion channels are only located at those Nodes of Ranvier (spoken as ‘ranveer’)
The potential distribution in a myelinated axon is the same as in a ‘naked’ axon. Inside is negative and outside is positive (see panel below).
This means that at that location, the inside of the cell is then positive and the outside is negative. This occurs at the height of the action potential, during the overshoot.
In contrast to normal electrical propagation however, the next available channels are located further away, at the next node. The myelin sheath between these two nodes works as an insulator to the extracellular current flow.
So, the flux of positive K+ ions inside the membrane will decrease the negative resting potential while the removal of positive Na+ ions outside the membrane will decrease the positive potential.
When the depolarization of the resting membrane in the neighboring nodes reaches threshold, a new action potential is generated at these nodes.
Then, this new action potential will influence the next patch of resting membrane in the next node of Ranvier and the whole story starts all over again (panel 3).
A local action potential, by influencing the flow of ions in its neighborhood, will effectively induce a new action potential in the next node of Ranvier. The action potential propagates therefore in lager steps from one node of membrane to the next and so on.
The big difference between normal propagation and saltatory propagation is speed. In normal propagation, the action potential is making small steps and that takes time. In saltatory propagation, these small steps are avoided by the myelin isolation that forces the action potential to make bigger steps.
Saltatory propagation is much faster than normal propagation because most of the distance is covered by current flow of ions, which is very fast. In normal propagation, this flow is each time interrupted by having to induce a new action potential, which has to reach threshold, and then to depolarize and reach the overshoot, before the next current flow starts etc.
The term ‘saltatory’ means ‘jumping’ (salto = a leap), as in ‘Salto Mortale’ (= a deadly or daring jump!)
In primitive animals, such as the squid, there are also nerve cells and axons but these axons are not myelinated.
You only see myelinated axons in more advanced animals such as vertebrates (although some invertebrates, like the shrimp, do have myelin-like structures).
The theory is that during evolution, the nervous system became more complicated and needed faster propagation to control the body.
There are several diseases, such as multiple sclerosis, in which the myelin cells are destroyed.
