The GI tract is a tube that is divided into different regions which serve various functions related to digestion and absorption. It is lined with three layers: mucosal, submucosal, and muscle layers. The control mechanisms for GI function include hormonal, paracrine, and neurocrine pathways. The innervation of the GI tract is composed of two components: extrinsic and intrinsic. Extrinsic innervation consists of the parasympathetic and sympathetic subdivisions, with an important sensory component. The intrinsic or enteric nervous system can act independently of extrinsic neural innervation. When a meal is in different regions of the GI tract, sensory mechanisms detect the presence of nutrients and initiate appropriate physiological responses mediated by endocrine, paracrine, and neurocrine pathways.
This introductory chapter provides a broad overview of the anatomy and regulatory mechanisms in the GI tract. In the following chapters there will be discussion of the integrated response to a meal to provide the details of GI physiology. The response to a meal is classically divided into phases: cephalic, oral, esophageal, gastric, duodenal, and intestinal. In each phase the meal presents certain stimuli (e.g., chemical, mechanical, and osmotic) that activate different pathways (e.g., neural, paracrine, and humoral reflexes) that result in changes in effector function (e.g., secretion and motility). There is considerable crosstalk between the regulatory mechanisms that have been outlined, and this will be discussed in the next chapters. As with maintenance of homeostasis in other systems of the body, control of GI function requires complex regulatory mechanisms to sense and act in a dynamic fashion.
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The GI tract is a tube subdivided into regions that subserve different functions associated with digestion and absorption.
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The lining of the GI tract is subdivided into layers— the mucosal, submucosal, and muscle layers.
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There are three major control mechanisms: hormonal, paracrine, and neurocrine.
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The innervation of the GI tract is particularly interesting because it consists of two interacting components, extrinsic and intrinsic.
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Extrinsic innervation (cell bodies outside the wall of the GI tract) consists of the two subdivisions of the
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ANS: parasympathetic and sympathetic. Both have an important sensory (afferent) component.
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The intrinsic or enteric nervous system (cell bodies in the wall of the GI tract) can act independently of extrinsic neural innervation.
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When a meal is in different regions of the tract, sensory mechanisms detect the presence of the nutrients and mount appropriate physiological responses in that region of the tract, as well as in more distal regions. These responses are mediated by endocrine, paracrine, and neurocrine pathways.
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