Saltatory Conduction

The human brain is truly a marvelous structure. When you feel sudden heat radiating from a nearby source, you immediately withdraw your hand. Have you ever thought about what prompted you? Within a fraction of a second, your brain registered the fact that there is a nearby source of heat, decided that it's not safe to be around the source, ordered the muscles of the hand to flex and take the hand away from the heat source, which the muscles registered and obeyed. Whew! That's a long chain of commands. But all of this happened without you even realizing it. This is all thanks to saltatory conduction. But before we go onto know what is saltatory conduction, we have to first understand the neuron structure. This is because the structure and function of nervous tissue is closely related.

Structure of Neuron
The neuron is the basic unit of the nervous system. A neuron or a nerve cell is made up of two parts - the head or the soma and the tail or the axon. The soma is the main cell body of the neuron, which contains the nucleus and where protein synthesis occurs. It gives out many small branches known as dendrites, which are basically cell extensions. One single long axon emerges from the soma as well. When studying the histology of nervous tissue, you'll see that the axon is a thin, fine, cable like projection which is the part where the saltatory conduction takes place. The axon, at the point where it emerges from the soma, is known as the axon hillock. From hereon, the axon is covered by myelin sheath and the neurilemma. The myelin sheath contains Schwann cells. The myelin sheath is not a continuous covering of the axon, but is interrupted at many points along its way. These interruptions along the length of the myelin sheath is known as nodes of Ranvier. The axon terminal further divides and branches out near the dendrites of the next neuron.

What is Saltatory Conduction?
The very word 'saltatory' comes from the Latin word 'saltare', which means to hop or leap. Saltatory conduction is nothing but the furtherance and propagation of the nerve's action potential along the axon by bypassing the myelin sheath and by passing directly from one node of Ranvier to another. Thus, the conduction velocity of the message increases as the message goes from one node of Ranvier to another without having to pass through the entire length of the myelin sheath. This not only betters the nervous system function by decreasing the time spent in passing messages, but also saves and reduces energy expenditure in the nerve.

How is Saltatory Conduction Possible?
An action potential is basically stimulation and passage of electrical impulses. There needs to be sufficient influx and movement of ions to bring about an action potential. In the region of the myelin sheath, there is often leakage of charge through the membrane. Thus, when there is depolarization at one of the node of Ranvier along the length of the neuron, it generates sufficient amount of voltage and potential in the adjacent node of Ranvier. Thus, in myelinated neurons, the action potential, rather passing in the regular wave pattern instead 'hops' from one node of Ranvier to another (hence the name 'saltatory' conduction). Thus, saltatory conduction is made possible by the sufficient amount of potential generated at any given node of Ranvier in a myelinated nerve cell.

This phenomenon is seen exclusively in myelinated nerves and not in all nerves of the body. There are certain pathologies associated with myelinated nerves, like when there is demyelination of nerve cells, then it leads to diseases like multiple sclerosis and optic neuritis. These diseases may be seen in the central nervous system or the peripheral nervous system.