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University of Toronto St. George
Cell and Systems Biology
Richard Stephenson

low grade matter, less energy in grass. dont get sleep not ime coz need to eat to maintain enough e nergy. hypotheszed, rel bw mr and body size. implied slee p had sth to with mr or energy met. LECTURE 9: Regular sleep itself doesnt involve much of a redu ction in MR but really depends on the perspective. Hamster- nocturnal Polyphasic. In this hamster, overnight, typical ul trasian periodcity in MR asstd with ultradian osci llation in body temps. Based on what we know from other studies, these ultradian rhythms in oscillat ion of MR were asstd with oscillations in activity so the dips in metabolism were asstd with periods in time when the animals were mainly asleep and t hese peaks in metabolism asstd with activity. Hams ters pretty active when awake but also we can see even when theyre asleep MR is fairly high and the saving ast with sleep of MR is not all that great . In little animals like this, the savings are mor e important in large animals such as ourselves. Th e bigger the animal, the less important sleep is f or energy conservation; mainly as a result of the fact that large animals have greater thermal inert ia and are much less stressed as a result of that by the thermoregulatory system. So depends on pers pective. One area where its become obvious that energy cons ervation is important is a situation in which ener gy availability andor the energy cost of living a sstd with env conditions such as cold gives rise t o a behaviour appearing to be exaggertaion on NREM sleep, this is an example of an animal that under takes daily torpor such as daytime when these are asleep most of the time. If you place animals in c www.notesolution.comonditions where food is limited and ambient temp i s low, 12c for ex, and esp if you simulate winter conditions by having rel short day which influence s mel release and infl overall endocrine function, then that elicits that torpor behaviour. Many sma ll rodents, birds do this. Under stress conditions like this, many small animals like this find it h ard to survive under these conditions for many hou rs in one go wo eating if they maintain a normal MR. Hummingbrids in a rel cool env basically starv e to death if they have to go thru the entire nigh t at a normal MR. They have such a high mr anyways bc of their small body size and so this places t hem on the edge of survival. So they evolved this torpid condition, they enter here everyday. We can see unlike the situation where they go in to slee p in the night, where their MR falls a bit, and th eir body temp falls a bit, in the torpid state bod y temp falls precipitously to rel low levels and t heir MR follows suit. Enormous savings. So the que stion is is this an extension of sleep or this a d ifferent situation altogether? Does it tell us any thing about sleep at all? Other species that dont necessarily have dialy torpor will sometime instea d have a seasonal behaviour referred to hibernatio n. More intense that torpor and the torpic bouts t end to last more than just a few hours. During hib ernation, these bouts are seasonal. As seen, these hibernation bouts last for many days. More extrem e in that body temp tends to fall further than it does during torpor. MR also falls to very low leve ls. Overall savings greater than daily torpor. The se animals have periodic increases in their body temp (euthermic intervals) or arousals where bod y temp comes back up to the normal levels and MR i ncreases as a result of that each time. Theres a big overshoot in metabolism that follows and thats the cost that occurs in association with rewarmin g these animals. It cost them a fortune! Its a mys tery, then, as to why theyd do this. It was assum ed they do this in order to eat and pee etc but as later seen, most of them actually spend their tim
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