A knee jerk, or a REFLEX, is an involuntary and immediate REACTION to a stimulus. It is a type of RESPONSE that does not require conscious thought or decision-making. The term 'knee jerk' refers to the most common type of REFLEX, which is the contraction of the quadriceps muscle when the patellar tendon is tapped just below the kneecap. This REFLEX is also known as the patellar REFLEX. Other reflexes include the pupillary light REFLEX, the gag REFLEX, and the withdrawal REFLEX. A knee jerk REFLEX is an example of a REFLEX action, which is a type of REACTION that is automatic and does not require conscious thought or decision-making.