Located in the dorsomedial portion of the hypothalamus near the wall of the third ventricle is an area called the primary motor center for shivering. This area is normally inhibited by signals from the heat center in the anterior hypothalamic-preoptic area but is excited by cold signals from the skin and spinal cord. Therefore, this center becomes activated when the body temperature falls even a fraction of a degree below a critical temperature level.
Increased muscular activity results in wasted heat - here the heat is utilized for warmth.
Shivering can also appear after surgery. This is known as post-surgical shivering.