Shivering (also called shuddering) is a bodily function in response to early hypothermia in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Muscle groups around the vital organs begin to shake in small movements in an attempt to create warmth by expending energy. Shivering can also be a response to a fever, as a person may feel cold, though their core temperature is already elevated.
Located in the posterior hypothalamus near the wall of the third ventricle is an area called the primary motor centre for shivering. This area is normally inhibited by signals from the heat centre in the anterior hypothalamic-preoptic area but is excited by cold signals from the skin and spinal cord. Therefore, this centre becomes activated when the body temperature falls even a fraction of a degree below a critical temperature level.
Increased muscular activity results in the generation of heat as a byproduct. Most often, when the purpose of the muscle activity is to produce motion, the heat is wasted energy. In shivering, the heat is the main intended product and is utilized for warmth.
Shivering can also appear after surgery. This is known as post-surgical shivering.
New born babies, infants, and young children experience a greater (net) heat loss than adults because they cannot shiver to maintain body heat. They rely on non-shivering thermogenesis. Children have an increased amount of Brown Fat (increased vascular supply, and high mitochondrial density), and, when cold-stressed, will have greater oxygen consumption and will release norepinephrine (NE). NE will react with lipases in brown fat to break down fat into Triglycerides (TG). TG 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. (See also Brown Fat, which describes a mechanism by which in Mitochondria a proton gradient producing a proton electromotive force (PEM) that is ordinarily used to synthesize ATP is instead bypassed to produce heat directly instead.)