A behavioural pattern is considered to be innate when it is essential for survival and already present at birth, as it is predetermined by the genetic make-up of the organism. A reflex is the simplest form of an innate behaviour. It is a programmed reaction to an outside stimulus that is carried out unconsciously. For example, the eyelids close automatically as soon as a draft of air stimulates the surface of the eye and the pupils of a cat will contract as soon as it looks into bright light. These are reflexes that an organism does not have to learn; they are referred to as unconditioned reflexes. An unconditioned reflex is always an unconscious response, and therefore it is impossible to suppress it at will. Such a reflex always requires a stimulus that triggers a certain behaviour. Many unconditioned reflexes exist in order to protect the organism, for example coughing, nausea, or the draw back reflex of the body part that touches a hot object. Anatomically, a reflex is based on a chain of stimulus and reaction, which is referred to as a reflex arc. A well-known example is the knee jerk or patellar reflex in humans, which is triggered by a light hit to the patellar tendon in the knee. The knee jerk reflex is often used in medicine to test the function of the spinal cord and associated nerves. The real purpose of this reflex is to protect humans from injury when tripping.