Malcolm is studying alone in his room late at night when he hears a loud noise downstairs. his heartbeat increases significantly and his breathing becomes shallow. he wonders if a burglar has entered the house and decides to investigate. when he gets downstairs he discovers his cat has knocked over a plant stand. his body begins to relax and return to normal. which part of his nervous system is responsible for returning malcolm to a normal state

Parasympathetic nervous system 

The parasympathetic nervous system (PNS) is one of the three divisions of the autonomic nervous system. The PNS rests and relaxes the body after it is at a heightened state of arousal (usually due to the fight and flight system being activated). The PNS is responsible for returning heart beat back to a normal rate, slowing down rapid breathing, relaxing muscles, constricting pupils and relaxing the digestive and excretory system so that they return to normal functioning.