Biological basis of motivation and emotion

Motivation and emotion feel psychological, but both are rooted in the body: hunger is signalled by stomach and blood chemistry, thirst by fluid balance, sleep by brain circuits and hormones, and emotion by the limbic system and stress hormones. For example, a fall in blood glucose and a rise in the hormone ghrelin together push an animal to seek food, while the amygdala flags a threatening face before conscious thought catches up. This chapter covers the biology of hunger, thirst, sleep and sex, and the limbic and hormonal basis of emotion.