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Shivering (also called shuddering) is a bodily function in response to early hypothermia or just feeling cold in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Skeletal muscles begin to shake in small movements, creating warmth by expending energy. Shivering can also be a response to a fever, as a person may feel cold. During fever the hypothalamic set point for temperature is raised. The increased set point causes the body temperature to rise (pyrexia), but also makes the patient feel cold until the new set point is reached. Severe chills with violent shivering are called rigors. Rigors occur because the patient's body is shivering in a physiological attempt to increase body temperature to the new set point.
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. Therefore, this center becomes activated when the body temperature falls even a fraction of a degree below a critical temperature level.
Cold-defensive and febrile shivering responses require activation of rostral medullary raphe neurons, especially those located near the midline in the region of the raphe pallidus nucleus between 2.3 and 3.5 mm caudal to the interaural line, corresponding to an antero-posterior level between 400 μm caudal to 800 μm rostral to the caudal border of the facial nucleus, and are modulated by activation of local 5-HT1A receptors; the central command pathway for shivering parallels that for sympathetically regulated non-shivering thermogenesis in brown adipose tissue (discussed below): Cutaneous cold afferent-triggered activation of neurons in the dorsomedial hypothalamus and GABAergic transmission from the median preoptic nucleus to the medial preoptic area mediates the shivering response as well as the brown adipose tissue non-shivering thermogenic and the tachycardic responses to environmental cooling.
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 postanesthetic shivering.
Newborn 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 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. Chemically, in mitochondria the proton gradient producing the proton electromotive force that is ordinarily used to synthesize ATP is instead bypassed to produce heat directly.