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For an undergrad assignment I read a biology paper that mentioned the speed of nerve imuplses to be 440 km/h in myelinated fibers. However, our biology teacher told us that this reported conduction speed is not true. So my question is:

What is the actual speed of myelinated fibers?

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    $\begingroup$ It would be good if you can tell us what you did to arrive to your answer. And it would be good if you can include any relevant background information. Not everyone who answers questions on this site is a specialist in every field; but a lot of us do have good research skills, and we may be able to review your work to see if it makes sense. But the only way we can do this, is if you provide us with enough information to do so. $\endgroup$ – Bob Jun 17 '17 at 19:03
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    $\begingroup$ I saw in a few books that the speed of the myelinated fibre was 120m/s in human beings but then after sometime I saw the speed of the myelinated fibres was 440km/h in bbc documentary which is similar to 120m/s but one speed is in (m/s) and the other is in (km/h), when I convert the 120m/s into km/h so I got 432km/h therefore I needed to be sure that the error of 8km/h in 440km/h and 432km/h does not matter so I can proof it to my Teacher, I hope that you understand my question. $\endgroup$ – Inam Ullah Jun 17 '17 at 19:50
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Short answer
A conduction velocity of 440 km/h is possible in thick, myelinated fibers. However, this number is probably more representative of the upper range of conduction velocities, rather than a conservative average.

Background
First off, there are a heap of variables that affect neural conduction velocities (in myelinated fibers) in complex ways (Waxman, 1980), including but not limited to:

  • axon diameter;
  • myelin thickness;
  • internode distance;
  • temperature;
  • axonal milieu;
  • age of the subject.

Having said that, in humans myelinated, thin A-delta fibers the average conduction speed was established at 19 m/s (Gyberls et al, 1983), or 68 km/h. A range of pain-conducting fibers exist, with different diameters. As a result, they range in their conduction velocities from 0.5 m/s (2 km/h for thin C-type fibers) to 120 m/s (432 km/h for thick A-alpha type fibers).

Hence, the 440 km/h is certainly possible in thick myelinated fibers. Note I just highlighted pain-conducting fibers here as an example, and other classes of neurons may feature even faster conduction in their axons.

References
- Gybels et al., J Neurophysiol (1983); 49(1): 111-22
- Waxman, Muscle & Nerve (1980); 3(2): 141–50

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