The knee-jerk reflex -- also know as the patellar reflex -- is a spinal reflex, meaning that the neural circuit only goes up to the spinal cord, not all the way to the brain. The advantage of a spinal reflex is that it is fast. Nerve impulses only have to go through two neurons, so the pathway is short and there is only one spot where an impulse has to jump over to another neuron. The knee-jerk reflex is tested by tapping just below the knee on the patellar tendon. The patellar tendon runs down from the quadriceps, over the patella (kneecap), down to the lower leg. Smacking the tendon stretches it a little and sensors that detect stretching of the tendon send electrical impulses back to the spinal cord. The spinal cord automatically sends a signal back along a nerve to muscles of the quadricep. The quadricep then contracts and suddenly brings the lower leg forward. This is an involuntary contraction (i.e. it occurs without the person actually *willing* it to happen). THE PURPOSE OF THE REFLEX itself is that if a sudden weight is dropped on you, the buckling of your knees would stretch your thigh muscles, automatically causing the muscles to contract harder via the spinal reflex. Thus, whenever the muscle is suddenly loaded, it reflexively contracts to support that load, so you don't collapse. Which, when you think about it, is quite useful for walking, Every time you put weight on your foot, your muscles contract to support you. Without spinal reflexes, you would have to consciously think about working your muscles to take each and every step, but the muscles would react too late to be of any use. Sources: http://www.medterms.com/script/main/art.asp?articlekey=4116 http://www.medicinenet.com/script/main/art.asp?articlekey=190 http://www.ithaca.edu/faculty/asmith/knjk.html/