Although the nervous and endocrine systems both serve for internal communication, they are not redundant; they complement rather than duplicate each other's function (table 17.1). The systems differ in their means of communication—both electrical and chemical in the nervous system and solely chemical in the endocrine system (fig. 17.2)—yet as we shall see, they have many similarities on this point as well. They differ also in how quickly they start and stop responding to stimuli. The nervous system typically responds in just a few milliseconds, whereas hormone release may follow from several seconds to several days after the stimulus that caused it. Furthermore, when a stimulus ceases, the nervous system stops responding almost immediately, whereas some endocrine effects persist for several days or even weeks. On the other hand, under long-term stimulation, neurons soon adapt and their response declines. The endocrine system shows more persistent responses. For example, thyroid hormone secretion rises in cold weather and remains elevated as long as it remains cold. Another difference between the two systems is that an efferent nerve fiber innervates only one organ and a limited number of cells within that organ; its effects, therefore, are precisely targeted and relatively specific. Hormones, by contrast, circulate throughout the body and some of them, such as growth hormone, epinephrine, and thyroid hormone, have very widespread effects.
3endo = into; crin = to separate or secrete
But these differences should not blind us to the similarities between the two systems. Several chemicals function as both neurotransmitters and hormones, including norepinephrine, cholecystokinin, thyrotropin-releasing hormone, dopamine, and antidiuretic hormone (= vaso-pressin). Some hormones, such as oxytocin and the cate-cholamines, are secreted by neuroendocrine cells—neurons that release their secretions into the extracellular fluid. Some hormones and neurotransmitters produce overlapping effects on the same target cells. For example, norepinephrine and glucagon cause glycogen hydrolysis in the liver. The nervous and endocrine systems continually regulate each other as they coordinate the activities of other organ systems. Neurons often trigger hormone secretion, and hormones often stimulate or inhibit neurons.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.