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  • Your mortal enemy has captured you and hooked you up to a bizarre experiment.

  • He's extended your nervous system with one very long neuron

  • to a target about 70 meters away.

  • At some point, he's going to fire an arrow.

  • If you can then think a thought to the target before the arrow hits it,

  • he'll let you go.

  • So who wins that race?

  • In order to answer, we have to examine the hardware of thought: neurons.

  • The human brain has about 86 billion of these cells.

  • They transmit signals down their axons by way of electrical impulses,

  • or action potentials.

  • One neuron can then pass that signal to the next at a synapse

  • by way of chemical neurotransmitters.

  • The signal is received by the next neuron's dendrites,

  • propagated down its axon, and passed further along.

  • So, the key factors that determine how quickly you think

  • include how long it takes to generate an initial action potential;

  • propagate it down the length of the axon; and transport it through the synapse.

  • We must also factor in the number of neurons involved

  • and the distance the signal has to travel.

  • Let's see what this looks like in a simple pathwayyour knee-jerk reflex.

  • A strike to your patellar tendon triggers an electrical impulse

  • that travels up a sensory neuron to your spine.

  • There the signal branches, and for the sake of simplicity,

  • we'll consider the segment that jumps into a motor neuron

  • to journey back down your leg.

  • The total length of the neurons in that pathway

  • is about 1 meter in someone who is 5 foot 5 inches,

  • and on average it takes 15 to 30 milliseconds from strike to kick.

  • Speed is distance divided by time,

  • so this signal travels somewhere between 120 to 240 kilometers per hour.

  • The initial action potential accounts for 1 to 5 milliseconds

  • and synaptic transmissions only take .1 to .5 milliseconds,

  • so the bulk of that time is spent within the axons.

  • This is consistent with research findings

  • that the average individual neuron sends signals at around 180 kilometers per hour.

  • But speeds can be boosted with myelination and increased axon diameter.

  • Myelin is a fatty sheath that insulates an axon,

  • preventing electrical currents from leaking out.

  • Meanwhile, axons with larger diameters offer less internal resistance.

  • These compounded factors can raise the speed of an action potential

  • as high as 432 kilometers per hour.

  • There's plenty of variation: some people think faster than others,

  • and your own speed of thought changes throughout your lifetime.

  • In particular, as you reach old age,

  • the myelin sheath covering your axons wears down,

  • and other neuronal structures degrade.

  • Back to the dastardly experiment.

  • Arrows shot from recurve bows fly, on average,

  • around 240 kilometers per hour.

  • Which means that given a sufficiently long, myelinated or large-diameter neuron,

  • your thoughts actually could win the race.

  • Butthere's a wrinkle.

  • The arrow and thought don't leave the gate at the same time;

  • first the arrow fires, then once you perceive it,

  • your signal can start down its path.

  • Processing images or music, participating in inner speech,

  • and recalling memories all require complicated neural pathways

  • that are nowhere close to the linearity of the knee-jerk reflex.

  • The speed at which these thoughts occur is mostly consistent,

  • with variations based on myelination and axon diameter.

  • But the duration of a thought will vary significantly depending on its routes,

  • pitstops, and destination.

  • In this case, when you perceive a threatening stimulus,

  • you'll invoke a fear startle response.

  • Similar to the knee-jerk response,

  • a startle can be involuntary and quite fast.

  • If the string twangs loud enough,

  • you might react in less than 65 milliseconds.

  • More likely though, your startle reaction will be based on sight.

  • Our eyes can process an image as quickly as 13 milliseconds,

  • but computation of what you're seeing and determining the danger it poses

  • can take as long as 180 to 200 milliseconds.

  • In that time the arrow will have gained a head start of about 13 meters.

  • The target is far enough away

  • that you've got just enough of a chance to catch up,

  • if you can quickly, and quite literally, think your way out.

Your mortal enemy has captured you and hooked you up to a bizarre experiment.

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B2 neuron axon signal arrow action potential speed

How fast is the speed of thought? - Seena Mathew

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    林宜悉 posted on 2020/11/16
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