We are all the time reacting on variety of things. The whole system of ours has been compared to a complicated reflex. Whether it's a sentence someone said or a knock on the shoulder, everything comes down to reflexes. Although generally speaking reflex means quick and time after time equally happening action. The comparison is therefore kind of misleading because for example: an insult said to another can be turn into action after a week for instance.
I thought of the patellar reflex today and tried to work out how it works. Basicly it means striking the patellar tendon just below the patella which stretches the quadriceps muscles and the muscle spindles in the thigh making the leg kick. It just couldn't get into my head how can the muscle spindles stretch if we strike the tendon. After a while I just simply came to a mindblowing conclusion: they just do :D I guess the stretching doesn't have to be that massive after all so that the quick and snappy strike to the tendon makes the muscle stretch a tiny bit.