A reflex is an involuntary response to a specific stimulus (a stimulus is any act or agent which causes a reaction in a particular tissue). A reflex is considered a "stereotypical" response, in that it is a mechanical response brought about by the stimulation of a particular neuron, resulting in a particular action which is not varied through the voluntary control of the individual. An example of a reflex occurs when the correct location in the knee is tapped with a physician's hammer, resulting in the contraction of the thigh muscle. This causes the lower leg to kick out. Although one normally considers such movement to be completely at the will of the individual to initiate or prevent, this reflex occurs without the involvement of the individual's brain. It is always surprising to see one's body perform such a seemingly voluntary act, with no actual effort or desire...