Nodes of Ranvier

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Nodes of Ranvier are microscopic gaps found within myelinated axons. Their function is to speed up propagation of action potentials along the axon via saltatory conduction[1].

The Nodes of Ranvier are the gaps between the myelin insulation of Schwann cells which insulate the axon of neuron.

The Node of Ranvier is the 1-2 micrometre gap between the glial cells of the myelin sheath. These glial cells are called Schwann cells, and they help to electrically insulate the neuron. The Nodes of Ranvier are only present when the axon of a neuron is myelinated. Myelination allows for an increased rate of action potential transmission due to action potentials "jumping" between Node of Ranvier, this is called saltatory conduction.

The movement of sodium ions to depolarize the membrane can only occur at the Node of Ranvier, as the sodium voltage-gated channels are found only at the nodes of Ranvier[2]. The Schwann cells of the myelin sheath block the movement of sodium ions elsewhere along the axon.

However, in multiple sclerosis (MS) the myelin sheath is degraded which leads to demyelination. This allows for action potentials to move as current loops instead of by saltatory conduction, which slows the transmission of action potentials and therefore a decrease in reaction time.

The interruptions in the myelin sheath were first discovered in 1878 by a French histologist and pathologist called Louis-Antoine Ranvier, who first described the nodes as constrictions[3].

The Nodes of Ranvier are pivotal in the process of saltatory conduction. The Nodes themselves are approximately one micrometre in length apart, and they are the physical gaps between the myelin sheath cells themselves[4]. The action potential can jump from node to node along the axon, causing the transmission speed to increase and reach around 120 metres per secondl. The myelination sheath produced by the Schwann Cells increases the membrane resistance (Rm), which, along with an increased diameter (D) of the axon, will increase the velocity of the action potential. This is outlined by using the equation:

velocity ∝ √(Rm x D)[5]

The gaps are rich in ion channels, such as sodium and calcium ion channels, resulting in maximised speed at which ion mediators can be released into tissues and adjacent neurones. This is particularly handy at the synaptic cleft (the area between a pre and a postsynaptic neurone), because the quicker that sodium ions are released into the cell, the quicker that depolarisation can occur and the faster a response is produced as a result of the action potential that is caused[6].

Overall, myelination is a highly specialised property of axons and ensures that impulses travel at sufficiently high speed around the autonomic nervous system so that the body can produce a successful response.

References

  1. Alberts et al. (2008). Molecular Biology of The Cell. 5th edition. New York: Garland Science. p680
  2. Jeff Hardin, Gregory Bertoni, Lewis J. Kleinsmith. (2006). Becker's World of the Cell. p380
  3. http://www.britannica.com/EBchecked/topic/417103/node-of-Ranvier
  4. https://psych.athabascau.ca/html/Psych289/Biotutorials/1/nodes.shtml?
  5. Cell Biology Lecture 10, Dr.T.Cheek
  6. http://www.nature.com/nrn/journal/v4/n12/full/nrn1253.html
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