Because hormones are so powerful, an animal’s body must strictly regulate the levels of these molecules in the bloodstream. This tight control often occurs by negative feedback interactions. Recall from chapter 24 that in negative feedback, an action counters an existing condition. For example, eating a candy bar can cause blood sugar to rise, which prompts the pancreas to release a hormone called insulin. In response, the body’s cells take up sugar from the blood. As blood sugar drops, insulin production slows. Similar feedback loops ensure that endocrine glands adjust the secretion of all hormones as required to maintain homeostasis. homeostasis, p. 520 B. The Nervous and Endocrine Systems Work Together Although the nervous and endocrine systems both specialize in communication, they differ in many ways. First, neurons use action potentials and neurotransmitters to send messages, whereas the endocrine system employs hormones. Second, each neuron influences only a few cells at a time, whereas hormones circulate throughout the body in the blood and may affect many different cells. Third, the endocrine system communicates much more slowly than the nervous system. A nervous impulse is virtually instantaneous, and its effects disappear as soon as the stimulus vanishes. Hormones take minutes, hours, or even days to exert their effects, which are generally more prolonged. Despite these differences, the nervous and endocrine systems are tightly integrated — so much so that some biologists refer to them together as the “neuroendocrine system.” The most obvious connection is a physical one: the hypothalamus. This region of the brain is clearly part of the central nervous system. Yet the hypothalamus contains neurons called neurosecretory cells that release hormones directly into the bloodstream. Moreover, the hypothalamus directly or indirectly controls the action of many endocrine glands. The nervous and endocrine systems also share chemical links. For example, some chemicals, such as epinephrine and norepinephrine, can act as neurotransmitters (if released from a neuron) and hormones (if released from an endocrine gland). In addition, neurotransmitters and hormones share some target cells. Endocrine System Animal physiologists are still learning how hormones and neurons interact to oversee growth and development, influence appetite, regulate the concentrations of vital nutrients in the blood, and ready the body to confront stress. This chapter therefore cannot paint a complete picture of endocrine action. Instead, the objective is to explain some of the best understood hormonal effects—beginning with section 27.2, which describes how target cells respond to hormones. 27.1 Mastering Concepts
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