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Monday, February 20, 2017

Speed of Nerve Impulses

     The nervous system transmits impulses throughout the body. Depending on the type of fiber, impulses travels at speeds ranging from a sluggish 2 miles per hour to 200 or more miles per hour. But even this top speed is 3 million times slower than the speed of electricity through a wire! 
     Signals such as those for muscle position, travel at speeds up to 266 miles per hour. Nerve impulses such as pain signals travel slower at less than 1.5 mph. Stub your toe and you'll feel the pressure immediately because touch signals travel at 170 mph. But you won't feel the pain for another two or three seconds, because the pain signals is only moving at 1.5 mph. 
     In 1850 German physiologist Hermann von Helmholtz attached wires to a frog’s leg muscle so that when the muscle contracted it broke a circuit. He found that it took a tenth of a second for a signal to travel down the nerve to the muscle. In another experiment he applied a mild shock to people’s skin and had them gesture as soon as they felt it. It took time for signals to travel down human nerves, too. In fact, Helmholtz discovered it took longer for people to respond to a shock in the toe than to one at the base of the spine because the path to the brain was longer. 
     Helmholtz and others could manage only crude measures of the speed of thought and researchers have been trying to get more precise results ever since. Today it is clear why they have had such a hard time. Our nerves operate at many different speeds. 
     Speed also influences us in surprising ways. In one experiment for studying the speed of thought, researchers briefly show test subjects a lopsided, upside-down and then ask them which leg of the figure is longer. People with faster responses tend to score higher on intelligence tests. Some psychologists have argued that a high processing speed in the brain is a requirement for intelligence. Responses slow down when people suffer certain psychological disorders like depression. People with sluggish reaction times are more likely to die of incidents like strokes or heart attacks. 
     Three or four times a second our eyes dart in a new direction which us only about a tenth of a second to make sense of what we see in each spot. Recently, neuroscientists at MIT ran an experiment in which they briefly showed people a series of landscapes and then asked questions about the scenes. People did well on these tests even when they glimpsed each of the pictures for less than one tenth of a second. 
     A Researcher at the Max Planck Institute of Neurobiology in Germany recently extracted retinal tissue from amphibians and exposed the living tissue to a series of simple geometric patterns. Then he recorded how the nerve cells fired in response. He noticed that each neuron started firing a little earlier or a little later, depending on which picture he showed. The shifts were distinctive enough that he could predict a shape just by looking at the timing of the neural reaction. Although this test involved amphibians, researchers believe the results would hold true for human brains as well. They might not wait for all the signals from the retina to arrive before they begin building a representation of the world and might get a head start with the very first bits of information. 
     National Science Teachers Association guide to the amazing living network of nerves that interconnect your brain, muscles, and organs. Visit site.

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