Saturday, October 16, 2010

Myelination allows the Action Potential to jump down the axon.

The myelin sheath provides insulation for the neuron and increases the speed of the action potential transmission. For neurons with no sheath the speed of the action potential is proportional to the axon diameter (you may remember this from Krassen’s lectures in Biph2000).

The region of the axon covered in the sheath does not have any channels present in the membrane, they are instead localised to the nodes between the sheaths. In this way the potential can jump to each node with practically no loss of signal. This type of transmission is called “salutatory conduction”.

Information taken from: Molecular Cell Biology, Lodish, 6th edn, Chapter 23.
Image taken from:


  1. Also, these gaps in the myelin sheath are called Nodes of Ranvier.

    I wonder how the concentration of voltage gated ion channels would compare in the Nodes of Ranvier to the concentration in the membrane of a normal cell.

    I also wonder how far a part these nodes can be before they stop speeding up the signal, and start inhibiting it.

  2. Interesting questions.

    At first I thought the concentration would have to be higher for the nodes. But the action potential always reaches the same voltage peak.
    Which would surely mean that the concentrations are always the same. The distance between the nodes would therefore have to account for the decay in signal as it jumps.

  3. Another really important point to make is the danger presented to people when these myelin sheaths start to decay or be destroyed. As the myelin sheaths are lost, the nerves become less insulated, which is a hallmark of neurodegenerative diseases, most prominantely known being MS.
    As the myelin degrades, what happens is that the signals along the axon can be impaired or lost,and as a result the nerve eventually withers.