Nervous System

Why does your knee jerk when the doctor taps it?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. When a doctor takes a rubber hammer and taps your knee, a sensory neuron takes a signal to your spinal cord and then to the motor neuron controlling the muscles of your leg. Nerve impulses move down this motor neuron stimulating the muscles to contract. This "knee-jerk" reaction is such a quick muscular response, so simple relative to other kinds of responses, that your brain is not even involved. It occurs through a monosynaptic reflex pathway, which is the simplest type of neural pathway.

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