The kneejerk reflex.

   Tapping the tendon stretches the quadriceps (extensor) muscle.

   Sensory  neurons  convey  the  information to the spinal cord, sending
   signals  to  motor  neurons  that supply the quadriceps, causing it to
   contract and jerk the lower leg up.

   The  sensory  neurons also communicate with interneurons which inhibit
   motor neurons that supply the hamstring (flexor) muscle, preventing it
   from contracting,

   Myelin sheaths of neuron axons give them a whitish appearance; regions
   of the CNS rich in axons are called white matter.

   Axons  that  reach  from the spinal cord to muscles in the feet may be
   over a meter long.