Saltatory conduction

From The School of Biomedical Sciences Wiki
Jump to: navigation, search

Due to the myelination of neurones within mammalian nervous systems, action potentials may only occur at the Nodes of Ranvier. Myelin is made up of insulating cells which means depolarisation cannot occur in myelinated regions. Between these cells however, there are gaps known as the Nodes of Ranvier which are unmyelinated. As depolarisation cannot occur at the cells making up the myelin sheath, the wave of depolarisation can only occur at the Nodes of Ranvier. Thus, action potentials appear to jump from node to node when travelling down an axon.

This phenomenon is known as saltatory conduction, and serves as a means of increasing the rate of propagation of an action potential [1] (200m/s as opposed to 2m/s)[2]

Not only does saltatory conduction increase the speed of impulse transmission by causing the depolarization process to jump from one node to the next, it also conserves energy for the axon as depolarization only occurs at the nodes and not along the whole length of the nerve fibre, as in unmyelinated fibres. This leads to up to 100 times less movement of ions than would otherwise be necessary, therefore conserving the energy required to re-establish the Na+ and K+ concentration differences across the membranes following a series of action potentials being propogated along the fibre. [3]

Ion Channels and Action Potentials

The influx of Na+ ions and efflux of K+ ions produce the action potential. In the resting state, voltage sensitive Na+ and K+ ion channels are closed. The simultaneous activation of many sodium channels in the membrane of an axon causes an influx of Na+ ions. This influx of positive charges causes the membrane potential (of a neurone) to become more positive, producing a gradual depolarization of the membrane. Once the threshold value of the membrane potential has been reached (-45mV) a series of events is triggered leading to the initiation and generation of an action potential. At threshold level of the membrane potential, more voltage sensitive sodium channels are activated, resulting in a greater influx of Na+ ions. The influx of positive charges depolarises the membrane further. 

At the peak of the Action potential the membrane is much more permeable to Na+ than K+; consequently the value of the membrane potential is closer to the Na+ equilibrium potential than to the K+ equilibrium potential. After the peak has been reached the inactivation channels close, Na+ influx is blocked, and the membrane potential begins to repolarise. As sodium channels become inavactivated, the potassium channels begin to become activated. This increase in potassium conductance causes the membrane potential to become more negative and contributes to the repolarisation phase of the action potential. Finally, the prolonged opening of K+ channels causes a continued efflux of K+ ions. This removal of positive charges from the cell in turn causes the membrane potential to remain  briefly before returning to the resting level[4].

Non myelinated neurones

References

  1. Alberts, B (2008). Molecular Biology of the Cell. New York: Garland Science. 680
  2. OMICS international. 2014. saltatory conduction. [ONLINE] Available at: http://research.omicsgroup.org/index.php/Saltatory_conduction#cite_ref-AP_1-0. [Accessed 17 November 15].
  3. Linden, R., Ward, J. (2013) Physiology at a Glance, 3rd edition, Oxford: Wiley-Blackwell
  4. Human Physiology by Rhoades and Pflanzer - 3rd edition 1996
Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox