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.