Nerves puzzles

Nerves carry information in just one way - through the {{electrotonic}} flow of current along their length. However, such current does not travel far because neurons are {{leaky}} structures.

Neurons can overcome this issue by reducing the {{leakage}} of current across the membrane relative to the current flow along the length of the neuron.

One way to do so is to increase the {{axon}} diameter which allows more current flow down the neuron relative to transmembrane flow, compared to a smaller {{diameter}} neuron.

A more efficient way is to provide neurons with a sheath called {{myelin}}. This fatty layer of insulation is provided by {{Schwann cells}} in the peripheral nervous system and by {{oligodendrocytes}} in the CNS.

Both mechanisms mean that neurons have to replenish less {{often}} the current lost from leakage.

Neurons replenish this current by producing an influx of current in an electrical event known as the {{Action Potential}}.

This event produces an influx of {{sodium}} ions into the cell, replenishing the current to flow further down the neuron.

However, each production of this event {{slows}} down the rate of information flow down the neuron.

An analogy would be like two people running a marathon. One person stops often to replenish their energy levels with energy drinks and the other person stops less often. No prizes for guessing who gets to the end faster!

So combining both mechanisms noted above means there is less need to produce the electrical event to replenish current flow.

Hence the fastest-conducting neurons have both myelin sheaths and {{large diameter}} axons and the slowest-conducting neurons have no myelin sheaths and have {{small diameter}} axons.