Description: 

Travel into the brain to see how its network of neurons transmit your thoughts and what factors determine how quickly you think. 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? Seena Mathew examines the speed of thought.

Transcription: 

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 pathway— your 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. But… there’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.

Questions: 

1. What neurons would have the fastest signal transmission? 

2. How much cells do human brain have?

3. What is a knee-jerk reflex?