The axon is a long filament that extends from the cell body (the soma) in nerve cells (neurons). See the simplified diagram of a neuron below.
The role of the axon is to carry nerve impulses away from the soma to the presynaptic terminals where the impulses are transmitted to other neurons or to muscles in the case of motor neurons. Axons can be very long, in some cases over a metre and carry impulses at a rate of anything up to 100 metres per second or greater. The greater the diameter of the axon, the faster the nerve impulses will travel along it. Many axons are clearly visible to the naked eye.
Axons are encased in a semi-permeable membrane (the phospholipid bilayer) which allows certain particles to pass through it but restricts others. In relation to the transmission of nerve impulses, this membrane selectively restricts the passage of charged particles (ions). The membrane contains special "gates" or "ion channels" that, when they are open, selectively let positively charged ions pass through. The two principle gates are the sodium and potassium channels. At rest, mechanisms in the neuron pump sodium ions out of the cell to create an electric potential across the membrane. This is called the resting potential. When the nerve is excited, a explosive wave of depolarising current called an action potential moves along the entire nerve through the length of axon and out into the presynaptic terminals. This is achieved by allowing the sodium ions to flood back through the sodium channels. This mechanism is discussed more fully in the section on the action potential.
Axons are sheathed in a smooth fatty protein called myelin which insulates the axon, prevents the wrong ion channels from opening and considerably increases the speed that nerve impulses travel along the axon. Without the myelin, the axons would have to be about one hundred times their volume to achieve the same speed of nerve transmissions. The myelin is wrapped around the axon in many thin layers. The myelin does not enclose the axon in one entire sheath, but has gaps at intervals called the nodes of Ranvier. The precise function of these nodes is unknown but the nodes are major sites of sodium channels and may serve to prevent the decay of nerve impulses by effectively amplifying them. They may also act to anchor the myelin sheath to the axon and to isolate each segment of myelin from its neighbours. Work on rats with genetic deformities in their nodes of Ranvier has shown that these nodes are vital to efficient transmission of nerve impulses. How the myelin sheath works is discussed more fully in the section on myelin.
In multiple sclerosis, the myelin sheath is stripped off from the neuron which considerably reduces the speed of conduction of nerve transmissions. This process is known as demyelination. The effects of this is to considerably slow down the speed of nerve transmissions along the demyelinated axons. It is known that when demyelinated axons are excited in the middle of their length, the action potential moves away from the point of excitement in both directions. When two neighbouring axons are demyelinated the action potential can "jump" from one axon into the other and generate all kinds of havoc. In the phase of the disease known as Relapsing/Remitting MS, the myelin is often laid down again in a repair process known as remyelination which can often restore near normal functioning to the axon. However, and particularly in progressive stages of the disease, the myelin can be replaced with scar tissue or the axonal body itself can be damaged.
Basic Neuron Physiology
Understanding the Neuron
Axon Guidance Molecules
Intercellular Interactions of Axons and Myelinating Glia at the Nodes of Ranvier
Neuron-Glia Interactions at the Node of Ranvier
Lesson 2 - Structure of the Neuron and Multiple Sclerosis