Reflex Arc

When a physician strikes your patellar tendon, the tendon fibers are stretched and activate the golgi tendon organ (receptor).  (This mimics what would happen to the tendon if one were to fall over backwards.) An action potential results, and is carried toward the spinal cord. Within the gray matter the sensory neuron synapses with an interneuron. The interneuron in turn synapses with one or more motor neurons, and an action potential is in turn initiated in the motor neuron(s). The motor neurons carry an action potential to various leg muscles, which contract and cause your leg to extend (to prevent falling!). 
The image on the right animated from a drawing provided by Biodidac.