Nerve impulses can travel at up to 119m/s. To put that into perspective, it is faster than a Bugatti Veyron travelling at its full speed (254mph).

How is this possible?

It comes down to the distance that ions from one action potential need to be in order to trigger the next action potential. To understand action potentials and nerves in general (which could be useful in understand this article), check out my previous article here.

The distance that between action potentials that can be triggered is known as the length constant. The slowest part of an action potential is the time it takes for ions to cross the cell membrane and cause depolarisation. The time taken for one action potential to actually trigger the next action potential is extremely fast. This means that the longer the length constant is, the fewer action potentials are required to reach the end of a neurone, and thus the electrical impulse travels along the neurone much faster.

figure 1. Structure of a nerve cell

There are two ways to increase length constant, and thus increase the speed of an impulse:

1. Increase axon diameter
2. Myelin sheath

How would increasing the diameter of the axon increase speed?

This is very simple. Increasing the diameter of the axon will decrease the resistance against the flow of ions. This means that the ions that enter during an action potential will be able to move further and trigger an action potential further away, the length constant is increased. This will increase the speed of conduction.

However, the most effective way to increase conduction speed is through myelin sheath.

What is myelin sheath?

This is a cell that surrounds the nerve cell axon and insulates it. It mainly consists of lipids and this ultimately means that there are no channels available on parts of myelination. Action potentials cannot occur in areas that are covered in myelin. Myelin sheath is shown in both figure 1 and figure 2.

figure 2. Myelin sheath

Myelin sheath prevents action potentials occurring, isn’t that counterintuitive to increasing speed of conduction?
It may seem this way at first glance, but it is certainly not the case. Since no channels are available for action potentials in the insulation of the myelin sheath, they can only occur in the non-insulated portions of the nerve cell known as nodes of Ranvier.

figure 3. Normal vs Saltatory conduction

This results in action potentials almost jumping from node to node (as seen in the animation at the start of this article, and in figure 3). This is because the insulation of the myelin sheath dramatically increases the length constant of the nerve cell, resulting in action potentials only occurring in the nodes of Ranvier. The process of action potentials jumping along nodes of Ranvier is known as salutatory conduction. It is the mechanism that allows action potentials to travel so quickly in the human body. Myelination is by far the most effective way to increase the speed of nerve conduction and this makes it extremely important in human physiology.

What happens if the myelin sheath breaks down?

Nerve speed will slow down. It is as simple as that! A loss of myelination will cause a decrease in the function and speed of nerve cells, and this can cause a many number of diseases. Demyelination can cause symptoms such as muscle weakness and paralysis, loss of vision, loss of sensation, incoordination and many others.

The most well known of these is multiple sclerosis (MS) which is caused by demyelination in the central nervous system. Other diseases caused by demyelination are Guillain-Barre syndrome or Charcot-Marie-Tooth disease, however both of these occur due to demyelination in the peripheral nervous system.

References:
https://hypertextbook.com/facts/2002/DavidParizh.shtml
https://biologydictionary.net/myelin-sheath/
https://en.wikipedia.org/wiki/Demyelinating_disease