One definition of reaction time is the interval of time between some signal (one that demands a response from you) and the initiation of your response. It takes time for nerve signals to reach the brain and be processed. If you stub your toe, you know it should hurt but is does not, at least for a second. Then it hurts, it hurts a lot.
Signals from different sensory systems get to your brain at different times. The fastest are signals that relay muscle position. Next are the signals that relay the sensations of touch and vibration, followed by signals relay the sensations of pain and temperature. The evolutionary significance of this variation is unclear, but one theory holds that position and touch arrive earliest because they coordinate movement.
Limb position signals, ones that originate inside skeletal muscles, race toward the brain at up to 390 feet per second. To see this in action, close your eyes and wave your arms in the air. You know instantly where your arms are in space, although you cannot see them.
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