Located in the posterior
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 and becomes activated when the body temperature falls even a fraction of a degree below a critical temperature level. The center then causes rapid contractions of skeletal muscles, producing heat as a byproduct. Newborn babies, infants, and young children experience a greater (net) heat loss than adults because of greater
surface-area-to-volume ratio. As they cannot shiver to maintain body heat, they rely on
non-shivering thermogenesis. Children have an increased amount of
brown adipose tissue (increased vascular supply, and high mitochondrial density), and, when cold-stressed, will have greater oxygen consumption and will release
norepinephrine. Norepinephrine will react with lipases in
brown fat to break down fat into
triglycerides. Triglycerides are then metabolized to
glycerol and non-esterified fatty acids. These are then further degraded in the needed heat-generating process to form CO2 and water. Shivering can also appear after surgery. This is known as
postanesthetic shivering. In humans, shivering can also be caused by cognition. This is known as
psychogenic shivering. ==Shivering and the elderly==