Hockenbury Discovering Psychology 5th txtbk

Biological Motivation325whose brains were continuously infused with ghrelin ate voraciouslyand gained weight. When ghrelin receptors were blocked,their eating behavior subsided (Nakazato & others, 2001; Tschöp& others, 2000).What about people? Researcher Donald Cummings and hiscolleagues (2002, 2006) showed that ghrelin is involved in theshort-term regulation of eating behavior. As you can see inFigure 8.3, blood levels of ghrelin rise sharply before and fallabruptly after meals. Cummings also found that ghrelin seemsto be involved in the long-term regulation of energy balanceand weight. When participants in one of his studies lost weightby dieting, their overall plasma levels of ghrelin—and feelings ofhunger—increased.Two other internal factors are correlated with meal onset—body temperature and metabolism rate. Prior to eating, body temperatureincreases and metabolism decreases. As the meal is consumed,this internal physiological pattern reverses: Bodytemperature decreases and metabolism increases (De Vries & others,8007006005004003002001000Breakfast1993; LeBlanc, 2000). This slight post-eating decrease in body temperaturemight explain why some people, including about a third of our students, occasionallyfeel cold after eating a meal.Psychological Factors That Trigger EatingIn Chapter 5, we described classical conditioning and operant conditioning. Bothforms of conditioning can affect your eating behavior. For example, much asPavlov’s dogs were conditioned to salivate at the sound of a bell, your eating behaviorhas probably been influenced by years of classical conditioning. The time of dayat which you normally eat (the conditioned stimulus) elicits reflexive internal physiologicalchanges (the conditioned response), such as the changes in blood levels ofinsulin, glucose, and ghrelin, increased body temperature, and decreased metabolism.In turn, these internal physiological changes increase your sense of hunger.Other stimuli, such as the setting in which you normally eat or just the sight of foodutensils, can also become associated with the anticipation of eating (Davidson,2000; Nederkoorn & others, 2000).Operant conditioning and positive reinforcement play a role in eating, too. Voluntaryeating behaviors are followed by a reinforcing stimulus—the taste of food.Granted, not all foods are equally reinforcing. Because of prior reinforcementexperiences, people develop preferences for certain tastes, especially sweet, salty,and fatty tastes. In other words, foods with one of these tastes hold greater positiveincentive value for some people. Hence, your motivation to eat is influencedby prior learning experiences that have shaped your expectations, especiallythe anticipated pleasure of eating certain foods.Satiation SignalsSensing When to Stop EatingThe feeling of fullness and diminished desire to eat that accompanies eating a meal istermed satiation. Several signals combine to help trigger satiation. One satiation signalinvolves stretch receptors in the stomach that communicate sensory information to thebrainstem. The sensitivity of the stomach stretch receptors is increased by a hormonecalled cholecystokinin, thankfully abbreviated CCK. During meals, cholecystokinin issecreted by the small intestines and enters the bloodstream. In the brain, CCK acts asa neurotransmitter. Many studies have shown that CCK promotes satiation and reducesor stops eating (Smith & Gibbs, 1998). CCK also magnifies the satiety-producing effectsof food in the stomach by slowing the rate at which the stomach empties.Psychological factors play a role in satiation, too. As you eat a meal, there is adecline in the positive incentive value of any available food, but especially the specificPlasma ghrelin level (pg/ml)6 a.m.8 a.m.10 a.m.LunchDinnerNormal-weight individualsNoon2 p.m.4 p.m.6 p.m.8 p.m.10 p.m.Midnight2 a.m.4 a.m.Figure 8.3 Ghrelin: The Hunger HormoneThe recently discovered hormonenamed ghrelin is manufactured primarilyby the stomach and stimulates appetite(Nakazato & others, 2001). The graphshows the average plasma levels of ghrelinfor 10 normal-weight individuals over thecourse of 24 hours. Ghrelin levels riseshortly before eating, precipitating feelingsof hunger. During and shortly afterthe meal, ghrelin levels, and feelings ofhunger, fall (Cummings & others, 2002).So, if your stomach is ghrelin, you must behungry!energy homeostasisThe long-term matching of food intake toenergy expenditure.ghrelin(GRELL-in) Hormone manufactured primarilyby the stomach that stimulates appetite andthe secretion of growth hormone by thepituitary gland.positive incentive valueIn eating behavior, the anticipated pleasureof consuming a particular food; in general,the expectation of pleasure or satisfaction inperforming a particular behavior.satiation(say-she-AY-shun) In eating behavior, thefeeling of fullness and diminished desire toeat that accompanies eating a meal; in general,the sensation of having an appetite ordesire fully or excessively satisfied.cholecystokinin (CCK)(kola-sis-tow-KINE-in) Hormone secretedprimarily by the small intestine thatpromotes satiation; also found in the brain.