Thermogenesis
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Thermogenesis is the process of
Types
Depending on whether or not they are initiated through locomotion and intentional movement of the muscles, thermogenic processes can be classified as one of the following:
- Exercise-associated thermogenesis (EAT)
- Non-exercise activity thermogenesis (NEAT), energy expended for everything that is not sleeping, eating or sports-like exercise.[2]
- Diet-induced thermogenesis(DIT)
Shivering
One method to raise temperature is through shivering. It produces heat because the conversion of the chemical energy of ATP into kinetic energy causes almost all of the energy to show up as heat. Shivering is the process by which the body temperature of hibernating mammals (such as some bats and ground squirrels) is raised as these animals emerge from hibernation.
Non-shivering
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Non-shivering thermogenesis occurs in
In this process, substances such as free
Acetylcholine stimulates muscle to raise metabolic rate.[14]
The low demands of thermogenesis mean that free fatty acids draw, for the most part, on lipolysis as the method of energy production.
A comprehensive list of human and mouse genes regulating cold-induced thermogenesis (CIT) in living animals (in vivo) or tissue samples (ex vivo) has been assembled[15] and is available in CITGeneDB.[16]
Evolutionary history
In avians and eutherians
The biological processes which allow for thermogenesis in animals did not evolve from a singular, common ancestor.[17] Rather, avian (birds) and eutherian (placental mammalian) lineages developed the ability to perform thermogenesis independently through separate evolutionary processes.[17] The fact that the same evolutionary character evolved independently in two different lineages after their last known common ancestor means that thermogenic processes are classified as an example of convergent evolution. However, while both clades are capable of performing thermogenesis, the biological processes involved are different. The reason that both avians and eutherians both developed the capacity to perform thermogenesis is a subject of ongoing study by evolutionary biologists, and two competing explanations have been proposed to explain why this character appears in both lineages.[17]
One explanation for the convergence is the “aerobic capacity” model. This theory suggests that
The second explanation is the “parental care” model. This theory proposes that the convergent evolution of thermogenesis in birds and eutherians is based on shared behavioral traits. Specifically, birds and eutherians both provide high levels of parental care to young offspring. This high level of care is theorized to give new born or hatched animals the opportunity to mature more rapidly because they have to expend less energy to satisfy their food, shelter, and temperature needs.[17] The “parental care” model thus proposes that higher aerobic capacity was selected for in parents as a means of meeting the needs of their offspring.[18] While the “parental care” model does differ from the “aerobic capacity” model, it shares some similarities in that both explanations for the rise of thermogenesis rest on natural selection favoring individuals with higher aerobic capacities for one reason or another. The primary difference between the two theories is that the “parental care” model proposes that a specific biological function (childcare) resulted in selective pressure for higher metabolic rates.
Despite both relying on similar explanations for the process by which organisms gained the capacity to perform non-shivering thermogenesis, neither of these explanations has secured a large enough consensus to be considered completely authoritative on convergent evolution of NST in birds and mammals, and scientists continue to conduct studies which support both positions.[19][17][18]
Non-shivering thermogenesis
Evolution of Skeletal-Muscle Non-Shivering Thermogenesis
The second form of NST occurs in skeletal muscle. While eutherians use both BAT and skeletal muscle NST for thermogenesis, birds only use the latter form. This process has also been shown to occur in rare instances in fish.[17] In skeletal muscle NST, Calcium ions slip across muscle cells to generate heat.[17] Even though BAT NST was originally thought to be the only process by which animals could maintain endothermy, scientists now suspect that skeletal muscle NST was the original form of the process and that BAT NST developed later.[17] Though scientists once also believed that only birds maintained their body temperatures using skeletal muscle NST, research in the late 2010s showed that mammals and other eutherians also use this process when they do not have adequate stores of brown adipose tissue in their bodies.[22]
Skeletal muscle NST might also be used to maintain body temperature in heterothermic mammals during states of torpor or hibernation.[17] Given that early eutherians and the reptiles which later evolved into avian lineages were either heterothermic or ectothermic, both forms of NST are thought not to have developed fully until after the K-pg extinction roughly 66 million years ago.[23] However, some estimates place the evolution of these characters earlier, at roughly 100 mya.[24] It is most likely that the process of evolving the capacity for thermogenesis as it currently exists was a process which began prior to the K-pg extinction and ended well after. The fact that skeletal muscle NST is common among eutherians during periods of torpor and hibernation further supports the theory that this form of thermogenesis is older than BAT NST. This is because early eutherians would not have had the capacity for non-shivering thermogenesis as it currently exists, so they more frequently used torpor and hibernation as means of thermal regulation, relying on systems which, in theory, predate BAT NST. However, there remains no consensus among evolutionary biologists on the order in which the two processes evolved, nor an exact timeframe for their evolution.
Regulation
Non-shivering thermogenesis is regulated mainly by
See also
References
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- PMID 12468415.
- ^ Fox SI (2011). Human Physiology (Twelfth ed.). McGraw Hill. p. 667.
- PMID 16933999.
- .
- doi:10.1139/z92-025.
- S2CID 14289041.
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- S2CID 259002752.
- PMID 12547935.
- S2CID 18266296.
- PMID 22290535.
- PMID 25976513.
- PMID 29688375.
- PMID 29688375.
- ^ PMID 29170642.
- ^ PMID 10737405.
- ^ PMID 493968.
- PMID 16933999.
- ^ PMID 19128480.
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- ISBN 978-3-642-28677-3.
- ISBN 9780444641304.
External links
- Thermogenesis at the U.S. National Library of Medicine Medical Subject Headings (MeSH)