Organization of the Endocrine System

Abstract

Multicellular organisms are endowed with coordinating systems that regulate and integrate the function of the different cells composing these organisms. Two main interacting systems perform this critical function: the nervous system and the endocrine system. The former employs electrochemical signals to convey regulatory inputs to peripheral organs and to receive information from them; the latter produces chemical agents that, in general, are transported systemically by the bloodstream to the target organs. The two systems are closely interconnected. The most conspicuous connection is that of the hypothalamus and the pituitary gland. Hypothalamic neurosecretory cells produce substances that are delivered to the portal blood vessels (see Chapter 5) and transported to the anterior pituitary (adenohypophysis), where they regulate the secretion of adenohypophyseal hormones. Other hypothalamic neurons send their axons to the posterior pituitary, from which they release their neurosecretory products directly into the bloodstream. The nervous system also innervates most, if not all, endocrine glands, including the gonads, the thyroid, and the adrenals. The nerves control not only blood flow but also the secretion of hormones. In turn, the endocrine system regulates the function of the nervous system. For example, gonadal and adrenocortical hormones act directly on the central nervous system to either inhibit or to stimulate the secretion of neuropeptides involved in the control of the pituitary-gonadal and pituitary-adrenal axes, respectively (i.e., gonadotrophin-releasing hormone [GnRH], also known as luteinizing hormone-releasing hormone [LHRH], and corticotropin-releasing hormone [CRH]; see Chapter 5). Although conventional definitions of the nervous and endocrine systems emphasize their differences, the two systems also display similarities. For instance, the nervous system produces not only substances that act across synapses, but it also releases signaling molecules that reach distant target cells via the bloodstream.