6.2 The hypothalamus as central regulator

Research in the past 30–40 years has established that the hypothalamus, which lies below the thalamus and above the optic nerve chiasma and the pituitary gland in the brain, fulfils all of the functions listed above, at least in part. The main function of the hypothalamus is homeostasis. Factors such as blood pressure, body temperature, fluid and electrolyte balance, and body weight are held to constant values called the set-points. Although set-points can vary over time, from day to
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5.7 Summary

BMR is regulated independently of T b at least in hibernating mammals. Entry into hibernation is characterized by a gradual fall in RQ, which indicates a switch from carbohydrate to lipid metabolism for energy provision (through the phosphorylation of pyruvate dehydrogenase, the inhibitor of mitochondrial fatty acid uptake). There is evidence that some other vertebrates, such as hibernating frogs, may continue to use carbohydrate catabolism or activate gluconeogenic pathways
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5.5 Energy budgeting – the benefits of hibernation and torpor

Studies performed on ground squirrels in the wild and in the laboratory have allowed estimates to be made of energy expenditure in hibernating and euthermic animals over similar periods (Wang, 1987). The average time spent by Richardson's ground squirrel in a periodic arousal in the wild is about 10 hours and the frequency of arousal decreases during November-March, when animals are spending more than 90% of their time in torpor. Monthly total oxygen consumption in January is about 35% of tha
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5.4 Inspiratory drive

The supply of oxygen to tissues such as the heart, liver and WAT is, under euthermic conditions, invariably linked to and dependent upon local blood flow and pulmonary function. However, as we have already seen, under conditions in which blood flow is reduced to a trickle, the control of energy supply switches to local adaptations in the capillaries and tissue cells, including the oxygen affinity of erythrocyte haemoglobin, the supply and metabolism of respiratory fuels and the rate of protei
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5.3 Mitochondrial adaptations

During the winter months, whilst hibernating vertebrates maintain a very low metabolic rate, major reorganization of mitochondrial metabolism occurs. The phenomenon has been studied in some detail in frogs which, although not hibernators in the true sense, can endure very low water temperatures under the conditions of profound hypoxia that exist when they lie dormant for long periods below the surface. In contrast to normoxic conditions, the muscle mitochondria of dormant frogs depress their
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4.4 Cell survival mechanisms

Physical damage is not the only danger that faces cells recovering from low temperatures in the absence of oxygen (due to a 90% drop in blood flow to the brain) and energy supplies. A universal sign of recovery from such conditions is the production of reactive oxygen species (ROS) (Box 4). The electron transfer chain that participates in the formation of water from oxygen in mitochondrial respiration can also be used in the production of the free radical superoxide, sometimes called ‘singl
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4.2 Arresting protein synthesis

The regulation of T b in hibernators has traditionally been viewed as the fundamental physiological process in hibernation. But recently, questions have been raised about whether thermal changes initiate or simply accompany metabolic depression. Is the metabolic inactivity of animal tissues during bouts of torpor or in hibernation, the cause or the result of hypothermia? A common-sense view is that temperature directly influences metabolism by regulating enzyme activity. Evi
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3.6 Length of torpor bouts in hibernation

It is obvious that there is a very high energetic cost to arousal, and an even higher one to the periods of euthermic wakefulness prior to re-entering torpor. If an animal could simply enter torpor once, and arouse 2, 4 or 6 months later, depending on the environment, it would represent a huge energy saving. Thus, it has been assumed that either prolonged torpor is physiologically impossible, or there is some strong selective value to the species in regular arousal. In the case of some small
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2.3 Hibernators as eutherms

Hibernating endotherms are not the easiest animals to study. Thus, until the late 1960s many biologists believed that mammalian hibernation was a process in which thermoregulation was simply ‘switched off’, following the receipt of a set of ‘cues’. These cues included a declining T a, a shortening daylength, the extent of body fat and a lack of food etc. With this model, the hibernator essentially becomes an ectotherm whose T b follows the T
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2.2 Species showing torpor or deep hibernation

Among the birds, torpor occurs in a number of species in the orders Apodiformes (hummingbirds and swifts), Caprimulgiformes (nightjars, nighthawks, goatsuckers and poor wills) and Coliiformes (mousebirds). In all of the hummingbirds (family Trochilidae) studied to date, torpor, if it occurs, takes place on a daily (or more usually nightly) basis. They are able to re-warm themselves independently of T a and show an increased thermogenesis if T a falls below
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2.1 Degrees of torpor

Adaptive hypothermia occurs in at least six distantly related mammalian orders (Table 1) and in several orders of birds. There is a spectrum running from those species which can tolerate a drop in T b by 2° C for a few hours, to the seasonal deep hibernators which maintain a T b as low as 4° C for weeks on end.

Table 1 Groups o
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5.4.1 Summary of Section 5

Several anatomical and biochemical adaptations to living in very cold water have evolved in polar fish, particularly those of the southern oceans, which have evolved in isolation for many millions of years. Cold, turbulent water is rich in oxygen. One family of fairly large fish lacks blood pigments but its blood is less viscous and it has additional respiratory surfaces. Many fish have cryoprotectants in the blood and other body fluids, and the muscles of some contain numerous mitochondria a
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3.1 Introduction

It is clear from Sections 1 and 2 that seasonal or irregular periods of fasting are an integral part of living at high latitudes, especially for large animals. When people (and many tropical and temperate-zone mammals) lose weight, either because they are eating less or because they are suffering from a digestive or metabolic disorder, protein is broken down in substantial quantities long before the lipid stores are exhausted. Even frequent and vigorous exercise cannot prevent the breakdown o
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2.4.1 Summary of Sections 1 and 2

Large seasonal changes in temperature and sunlight dominate primary plant production and hence the food supply. Food intake is regulated by the endogenous seasonal control of appetite, fattening and activity, as well as by food availability. Energetically demanding activities such as breeding and migration are only feasible during a brief period and must be tightly synchronized to season. Greater accessibility of food suitable for chicks makes long-distance migration to and from high arctic r
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References

Blakemore, C. and Cooper, A. (1970) Development of the brain depends on visual environment, Nature, 228, pp. 477–8.
Caspi, A., McClay, J., Moffitt, T. E., Mill, J., Martin, J., Craig, I. W., Taylor, A. and Poulton, R. (2002) Role of genotype in the cycle of violence in maltreated children, Science, 297, pp. 851–4.
Caspi, A., Sugden, K., Moffitt, T. E., Taylor
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End of unit questions

Question 1

Explain your reasons for agreeing or disagreeing with each of the following sentences.

(a) If a disease has a genetic basis and someone has the (abnormal) alleles for the disease, then that person will have the symptoms of the d
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10 Unit summary

The unit began by considering what factors contribute to individual differences. The case was made, with the spiders, and later with genetic diseases, that the genome was very important. Subsequent sections revealed that the external environment (e.g. maternal care, the presence of light) and the internal environment (e.g. hormones and drugs) were very important and that they can both shape and determine the development of the organism. Environmental factors, in the form of hormones and drugs
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9.4 Summary of Section 9

This section has illustrated what has to be done, by way of a long-term study, to yield meaningful information on the relationship between genes and development and the behaviour of the organism. It also illustrates the hugely complex nature of the relationship between genes and development and the behaviour of the organism. Yet this complexity is not the exception, it is the rule.


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9.2 Antisocial behaviour disease

The psychological arena is hugely complex because there are additional issues of responsibility and treatment. Briefly, society takes a more lenient attitude towards the behaviour of someone who is ill (diseased) compared to someone who is well. The diseased person is not fully responsible for their actions (‘They can't help it’). Therefore any individual with antisocial (aggressive) behaviour who is diagnosed as having a disease is largely absolved of blame. Having a disease, means, at l
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8.4 Fragile X syndrome

Fragile X syndrome is the final example of a genetic disease considered here.

Activity 21

What does the term ‘genetic disease’ mean?

Answer

Genetic disea
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