Neuroendocrine Cells

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 se creted 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 Chapter 16. Here, we consider 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 ex citability, but are specialized, not to secrete neurotransmitter into a synaptic cleft, but to secrete hormones into the blood. However, not only is there a fine line between hormones and neurotransmitters, there is also little qualitative difference between se cretion 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.

Although, unsurprisingly, there is a great variety of neuroendocrine cells, they have certain characteristics that serve to unify their study. First, they are excitable and there fore have action potentials. Second, the electrical activity usually is not a simple action potential, or periodic train of action potentials (Chapter 5). Instead, the action poten tials are characterized by bursts of rapid oscillatory activity interspersed with quiescent periods during which the membrane potential changes only slowly. This behavior is called bursting. Third, bursting is often closely regulated by the intracellular Ca²⁺ con centration (Chapter 7). Thus, models of neurosecretory cells typically combine models of membrane electrical excitability and Ca²⁺ excitability, leading to a fascinating array of dynamic behaviors.