Many regulatory molecules produced throughout the body act within the organs that produce them.These molecules may regulate different cells within one tissue, or they may be produced within one tissue and regulate a different tissue within the same organ.
Thus far in this text, two types of regulatory molecules have been considered—neurotransmitters in chapter 7 and hormones in the present chapter. These two classes of regulatory molecules cannot be defined simply by differences in chemical structure, since on this basis the same molecule (such as norepi-nephrine) could be included in both categories; rather, they must
Chapter Eleven be defined by function. Neurotransmitters are released by axons, travel across a narrow synaptic cleft, and affect a postsynaptic cell. Hormones are secreted into the blood by an endocrine gland and, through transport in the blood, influence the activities of one or more target organs.
There are yet other classes of regulatory molecules. These molecules are distinguished by the fact that they are produced in many different organs and are active within the organ in which they are produced. Molecules of this type are called autocrine regulators if they are produced and act within the same tissue of an organ. They are called paracrine regulators if they are produced within one tissue and regulate a different tissue of the same organ (table 11.9). In the following discussion, for the sake of simplicity and because the same chemical can function as an autocrine or a paracrine regulator, the term autocrine will be used in a generic sense to refer to both types of local regulation.
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