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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’)
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(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’)
(‘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’0
(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’)
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(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)
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