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Hunger and the Brain

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Hunger and the Brain
303
Eating
FIGURE
8.2
–
Anabolic
The Hypothalamus and Hunger
Regions of the hypothalamus generate
signals that either increase hunger and reduce energy expenditure, called anabolic
effects, or reduce hunger and increase energy expenditure, called catabolic effects.
Brain
Catabolic
+
–
Food intake
–
+
+
Energy
expenditure
Fatness
signals
• Metabolic rate
• Physical activity
Insulin/leptin
+
–
+
Energy
balance
Fat stores
+
Source: Adapted from Schwartz et al. (2000).
far less effective in people who are obese because they eat a high-fat diet (Gura, 1999;
Heymsfield et al., 1999). In these far more common cases of obesity, the brain appears
to have become less sensitive to leptin’s satiety signals (Ahima & Flier, 2000; Lin et al.,
2000; Lustig et al., 2004).
LINKAGES
How does the brain know
when we are hungry? (a link to
Biology and Behavior)
Hunger and the Brain
Many parts of the brain contribute to the control of eating. However, research has
focused on regions of the hypothalamus that may play primary roles in detecting and
reacting to the blood’s signals about the need to eat. As shown in Figure 8.2, the hypothalamus influences both how much food is taken in and how quickly its energy is used,
or metabolized.
Some regions of the hypothalamus detect leptin and insulin; these regions generate
signals that either increase hunger and reduce energy expenditure or reduce hunger and
increase energy expenditure. There are at least twenty neurotransmitters that convey
these signals to networks in various parts of the hypothalamus and in the rest of the
brain (Cota et al., 2006; Woods et al., 1998, 2000).
Activity in a part of the network that passes through the ventromedial nucleus in the
hypothalamus tells an animal that there is no need to eat. So if a rat’s ventromedial
nucleus is stimulated, the animal will stop eating (Kent et al., 1994). However, if the
ventromedial nucleus is destroyed, the animal will eat much more than usual and maintain a much higher body weight.
In contrast, the lateral hypothalamus contains networks that tell an animal to start
eating. So when the lateral hypothalamus is stimulated, rats begin to eat huge quantities, even if they have just had a large meal (Stanley et al., 1993). When the lateral hypothalamus is destroyed, however, rats stop eating almost entirely.
Decades ago, these findings led to the suggestion that these two hypothalamic
regions interact to maintain some homeostatic level, or set point, based on food intake,
body weight, or other eating-related signals (Powley & Keesey, 1970). According to this
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