There are many hormones that circulate through the body, controlling a diverse array of functions, from appetite to body temperature to blood pH . These hormones are secreted from specialized cells in various glands, such as the hypothalamus and pituitary , the pancreas , or the thyroid . Models of hormone physiology at the level of the entire body are discussed in Chap. 16 . This present chapter instead considers models of the cells that secrete the hormones, the neuroendocrine cells. They are called neuroendocrine (or sometimes neurosecretory) as they have many of the hallmarks of neurons, such as membrane excitability, but are specialized, not to secrete neurotransmitter into a synaptic cleft but to secrete hormones into the bloodstream. However, not only is there a fine line between hormones and neurotransmitters, but there is also little qualitative difference between secretion into a synaptic cleft and secretion into the bloodstream. Thus, it does not pay to draw too rigid a distinction between neurons and neuroendocrine cells.