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The hypothalamus regulates homeostasis. It has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure, and links the nervous system to the endocrine system.

The hypothalamus, like the rest of the brain, consists of interconnecting nerve cells ( neurons) with a rich blood supply. To understand hypothalamic function it is necessary to define the various forms of neurosecretion. First, there is neurotransmission, which occurs throughout the brain and is the process by which one nerve cell communicates with another at an intimate intermingling of projections from the two cells (a synapse). This transmission of an electrical impulse from one cell to another requires the secretion of a chemical substance from a long projection from one nerve cell body (the axon) into the synaptic space. The chemical substance that is secreted is called a neurotransmitter. The process of synthesis and secretion of neurotransmitters is similar to that shown in Figure 1 with the exception that neurosecretory granules migrate through lengths of the axon before being discharged into the synaptic space.

Figure 1: Intracellular structure of a typical endocrine cell.

Neurologists have long been aware of four classical neurotransmitters: epinephrine, norepinephrine, serotonin, and acetylcholine, but recently there have emerged a large number of additional neurotransmitters, of which an important group is the neuropeptides. While bioamines and neuropeptides function as neurotransmitters, some of them also perform the role of neuromodulators; they do not act directly as neurotransmitters but rather as inhibitors or stimulators of neurotransmission. Well-known examples are the opioids (for example, enkephalins), so named because they are the naturally occurring peptides with a strong affinity to the receptors that bind opiate drugs such as morphine and heroin. In effect, they are the body's opiates.
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