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Explores what it is that arouses an animal or human from an inactive state to a condition of activity, and what the biological mechanisms are for this change to happen. This work focuses on a reproductive behaviour typical of many female animals. Sensory stimuli from the male trigger responses in a well-defined circuit of nerve cells. At the top of the circuit, certain nerve cells receive and retain sex hormones such as estrogens and progesterone. As a result, specific genes in these nerve cells are turned on at specific times, affecting in turn the rest of the neural circuit and causing a state of sexual responsiveness. According to Donald Pfaff, the biological bases for the most primitive human drives are largely explained by mechanisms uncevered in animal brains that have not changed in their fundamental properties for over millions of years of evolution. Focusing on a single instinctive behaviour, in this case the sex drive, is a step toward understanding the biological reasons for the change from unmotivated to motivated animal behaviour.
