3 ms·
Well, first of all octopi are invertebrates, so their nervous systems are quite dramatically different from ours. If I remember right, an octopus' axons are unm
by a-priori 17y ago
Well, first of all octopi are invertebrates, so their nervous systems are quite dramatically different from ours. If I remember right, an octopus' axons are unmyelinated (I don't know much about invertebrates; I know some invertebrates have myelination, but I don't think octopi do). This means that it's quite a bit slower to transmit signals around in their bodies, so in order to react quickly they need bundles of neurons near the sources of the signals.
Perhaps as a throw-back to our invertebrate ancestors, we still have unmyelinated fibres that transmit some kinds of information, such as some forms of pain. That's why if you stub your toe, you feel two waves of pain: one sharp and quick, and the other much duller. The second one travels over unmyelinated fibres, and so takes much longer to arrive in your brain.
If all you had were unmyelinated fibres, and if control were centralized in your brain, you'd have a round-trip time of a few seconds to your feet. How could you control your legs for walking like this?
Obviously, we do have myelinated axons, so things are quicker, but it's still impractical to involve the brain in most reactions, because it's large and complex and takes a while to reach a steady-state after unexpected stimuli.
- jacquesm 17y agoThank you, that was most interesting. I knew about the myelination (sp?), but did not realize that that affected the speed of transmission so much. Very interesting about the secondary system still being active, maybe that is what allows us to measure the distance of a stimulus to the brain by using the difference between the arrival times of the pulses ? Are you working in this field ?
- a-priori 17y agoYeah, myelination really is a huge improvement. According to Wikipedia [1], the fast pain fibres I mentioned (A-delta) have a conduction speed of 3-30 m/s, and the slow fibres (C) have a speed of 0.5-2.0 m/s. As you can see on that page, there are much faster fibres than those. For some definition of "field", I suppose I do :) I did a minor in cognitive neuropsychology in university, and now I'm doing an internship for a computational neuroscience project. [1]: http://en.wikipedia.org/wiki/Axon#Types http://en.wikipedia.org/wiki/Axon#Types (Edit: I don't know why we still have slow "C" fibres. I suppose there's no pressure to get rid of them, so they've just stuck around all this time. It probably helps with learning, because it persists for a longer time, whereas the fast signal is more for the immediate reaction... just a guess.)