Voluntary muscle activity such as

  Premature infants may be unable to flex to cold stimuli as mature infants can do to minimize their exposed surface area.  The premature infant has poor muscle tone and primarily lies with his extremities extended.  These infants have a very large surface area-to-body-mass ratio, in other words, a large area is exposed to the environment which can result in rapid heat loss.  

Shivering--

Shivering is a specialized muscular response that increases muscle metabolic rate and increases oxygen consumption.  Since premature infants don't shiver, neonates must therefore depend on another method of heat production. 

Chemical thermogenesis

This process is initiated in the hypothalamus by way of the sympathetic nervous system.  The brown fat, located primarily in the subscapular, axillary, adrenal, and mediastinal regions, is very limited in premature infants--remember, the more premature the infant the less brown fat he/she has stored.  Norepinephrine is released at the brown fat site as a result of cold stress.  When Norepinephrine is released it causes the release of free fatty acids (FFA), which undergo combustion in the mitochondria of the brown fat cells—releasing HEAT.  One consequence with the release of the FFA is the FFA will compete with bilirubin for albumin binding sites, increasing the risk of hyperbilirubinemia and possibly kernicterus.  Kernicterus refers to yellowish staining of bilirubin in nuclear centers of the central nervous system, particularly in the basal ganglia, cerebellum, and hippocampus.  Prognosis for kernicterus is poor if left untreated.   

            Of the three methods of heat production (voluntary muscle activity, shivering, and chemical thermogenesis) chemical thermogenesis is the one most utilized by the neonate.