Myelinated Motor Neurone

Action Potential

The Myelin Sheath

Myelinated Motor Neurone

Here is the structure of a myelinated motor neurone:

Labelled “insulating sheath”, the myelin sheath is responsible for protecting the electrical impulses that run across the neurone.

But first, what happens in a resting state where no impulses are being sent?

This is the resting potential where the membrane permeability differentiates between sodium (Na⁺) and potassium (K⁺) ions so that at any given time there are more Na⁺ ions outside than inside and more K⁺ ions inside than outside.

According to these electrochemical gradients, Na⁺ ions should move back inside to balance out their concentration (equilibrate) while K⁺ ions should move back outside the membrane until the concentrations are equal inside and out. This clearly isn’t the case, so what gives?

Found on the membrane there are Na⁺/K⁺ pumps which carry out active transport against the electrochemical gradient of these ions. The resting potential of the membrane is negative on the inside and positive on the outside – but how? Aren’t both sodium and potassium ions positively charged? This is achieved by the pump transferring 3 Na⁺ ions out while taking only 2 K⁺ ions in. This is where the difference comes from….