An axon is a thin spur of a neuron, and an essential component for the transmission of information. The beginning of the axon is found in the axon hillock, a bump on the cell body. The axon ends at the synapse, where communication with another cell is possible through the transfer of neurotransmitters. Sometimes many axons have to go to the same area in the brain, and they bundle together. These bundles of axons are known as nerves, except in the brain, where we call a bundle of axons a tract.
The main function of the axon is to conduct an action potential. This is an electrical pulse that is moved by charged ions entering and leaving the axon. In this way, an axon can transport signals over a very long distance. Some axons are more than a metre long. To transmit an action potential quickly over such a long distance, neurons work together with glial cells. These cells coat the axon with a substance called myelin, which provides insulation. This prevents the electrical charge in the axon from leaking out. To make it possible for the ions to enter the axon anyway, the myelin sheath is sometimes interrupted. These interruptions are called Ranvier's nodes.
At the nodes of Ranvier, positive ions can flow into the cell and continue an action potential. The resulting peak voltage then spreads through the myelin-wrapped section of the axon until it reaches the next node. There the whole story begins again. An action potential thus jumps, as it were, from one opening in the myelin sheath to the next. Due to this system, an action potential can reach a speed of 150 metres per second.
Author: Myrthe Princen (translated by Thomas von Rein )
Image: Marcel Loeffen