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19.5.12

What is the longest axon?

This is a fascinating question I got over on Quora.

Short answer: probably the dorsal root ganglion in the blue whale.

Initially I thought it would be a motor axon in the sciatic nerve, but after consideration I'm pretty sure that the dorsal root ganglion (DRG) has a longer axon than the motor information carried in the sciatic nerve (which is the longest nerve in the body, but not axon).

The DRG is a weird neuron because it's unipolar, so it's got a loooong axon, where one end has receptors in the skin and the other end enters the spinal cord, ascends in the fasciculus gracilis all the way up to the medulla in the brainstem, and synapses in the nucleus gracilis before then continuing to send information up to the thalamus and, finally the primary somatosensory cortex for "conscious" perception.


Note that for the sensations in the toes, this means that the axon goes all the way from the toe to the medulla, which is at about the same height as the mouth. This can be more than 2 meters long in tall people. That's a long axon!

Here's my logic for this answer:

  • The DRG is the longest axon in the human body.
  • Humans are mammals.
  • The (confirmed) largest and longest animal to have ever lived is the blue whale.
  • Blue whales are also mammals and thus have nervous systems roughly equivalent to humans.
  • Therefore the longest axon in the blue whale, which is itself the longest animal, is probably the DRG.


When trying to confirm my answer, however, I learned a lot of crazy stuff. For example....

The largest blue whales are around 30 m long. This would suggest a DRG axon of at least 25 m, or 75 feet, long. Here's where it gets nuts and things stop making sense to me...

Axons typically conduct signals between a wide range of speeds: 0.5 to 100 m/s.

This means that if I were to flick a whale's tail (as one tends to do), it could take anywhere from a third of a second (a long time in brain time!) to more than SIX SECONDS to reach the whales' "conscious" perception (assuming they have consciousness).

Even more nuts, according to this paper:

...blue whale spinal axons growing at 3 cm/day represent an increase in volume that is likely more than double the volume of the entire neuron cell body—each day. This rapid volume increase for neurons is akin to the peak cellular growth rate observed for rapidly dividing cancerous cells.

(bold emphasis mine)

Basically, these axons are growing faster than cancerous cells and the speed at which they stretch should cause them to tear or rupture.

What?

Man, brains are crazy.

ResearchBlogging.org
Smith, D. (2009). Stretch growth of integrated axon tracts: Extremes and exploitations Progress in Neurobiology, 89 (3), 231-239 DOI: 10.1016/j.pneurobio.2009.07.006

10 comments:

  1. Another estimate suggests sauropod dinosaurs beat that.

    "(N)eurons may have spanned the distance from the end of the tail to the brainstem, as in all extant vertebrates. In the longest sauropods these neurons may have been 40–50 meters long, probably the longest cells in the history of life."

    Wedel MJ. A monument of inefficiency: the presumed course of the recurrent laryngeal nerve in sauropod dinosaurs.
    Acta Palaeontologica Polonica
    : in press.
    doi: http://dx.doi.org/10.4202/app.2011.0019

    I'm skeptical of these estimates, though. Has anyone actually done the tracing to show these are truly single neurons? Even if they single cells in small vertebrates, it is possible that large vertebrates were organized differently.

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    1. Yeah, I saw that, but the size of the sauropod that may be larger than the blue whale is based on a few vertebrae. That's why in the post I used the word "confirmed" largest animal... we know the blue whale has a DRG that is continuous; presuming that the nervous system layout of a dinosaur that may or may not have been as large as assumed based on the size of some fossilized vertebrae seemed like too many levels of supposition for me.

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    2. Although the exact lengths are subject a lot of uncertainty, there is very good evidence for sauropods longer than the longest blue what. Much less of case for sauropods heavier than blue whales, but even that amazingly is not completely ruled out. See SV-POW! showdown: sauropods vs whales.

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    3. Mike, that may very well be the case, but it's then a big leap to make the presumption that their nervous systems are anything like those we see in mammals. Maybe their sensory neurons aren't unipolar? Maybe they synapse in the spinal cord first before ascending to the brain? Just too manny unknowns, so I stuck with the known.

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    4. Are you suggesting that the dorsal root ganglion only has an elongate axon only in mammals? Unless you know something I don't then the very reasoning you used in your article:

      * Humans are mammals.
      * Blue whales are also mammals and thus have nervous systems roughly equivalent to humans.

      Works just as well if you substitute "amniote" for "mammal" and "Amphicoelias fragillimus" for "blue whale".

      Anyway, see Matt's lengthy but approachable discussion the paper in question at The world’s longest cells? Speculations on the nervous systems of sauropods -- if you know something we don't about non-mammalian nervous system, Matt will want to be corrected, so please do leave a comment!

      (Disclosure: I pre-submission reviewed the paper in question.)

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    5. We're certainly getting fair afield from my expertise, I was just playing it safe and going with what we 100% know. Certainly reptiles have the equivalent of dorsal column nuclei, but because I don't know any more than that I was hedging my scientific bets.

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    6. Gaaah, coming _very_ late to this. The architecture of having neurons in the dorsal root ganglia with axons stretching from the most distal extremity to the brainstem is shared in everything from hagfish on up. I cited some sources on this in the paper mentioned above. So we can safely infer that dinosaurs had the same long nerves as humans and whales.

      Anyway, nice writeup. To my chagrin, I wasn't aware of the Smith (2009) paper until I read your post. It would have been a great thing to discuss in my own paper.

      I'm fascinated that we know so little about how these long axons are supported metabolically. AFAIK we have no actual data on axoplasmic streaming from anything larger than a lab rat. So even the physiology of these long axons in humans is not well understood.

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    7. Matt: Cool! Thanks for weighing in and clarifying. Funny how these kinds of posts draw people in long after they're written. It's even cooler when the primary researchers comment. So thank you, I appreciate it!

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  2. Thanks Brad! This was a random thought during our neuroscience retreat, so I threw it out to Twitter then Quora and I'm impressed you got a bit of data so fast!

    The propagation issue is really interesting... Apparently Robert Wadlow (world's tallest person ever at 8'11") had problems with sensation in his feet which ultimately caused his death: http://voices.yahoo.com/biography-robert-wadlow-alton-illinois-gentleman-428116.html?cat=49 I wonder if spike propagation issues contributed?

    Its probably REALLY hard to do an DRG trace in a blue whale for ethical and accessibility reasons... let alone try to see if there are compensatory biophysical processes to support faster spike propagation. You'd be limited to working on the rare "putrefying carcass" that might wash up on a beach: http://articles.latimes.com/2007/sep/15/local/me-whale15

    I was also just speculating with some of my fellow grad student about whether or not one could synthetically grow an absurdly long neuron in culture... if you give a cell the right grown factors and axon guidance cues, could you just make it keep going?

    I smell a Guinness Record...

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    1. In that review I linked to the author discusses experiments that artificially stretched neurons... so it's definitely a possible experiment!

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