Regulation of the Gastrointestinal Tract

The GI tract is innervated by the sympathetic and parasympa-thetic divisions of the autonomic nervous system. As discussed in chapter 9, parasympathetic nerves in general stimulate motility and secretions of the gastrointestinal tract. The vagus nerve is the source of parasympathetic activity in the esophagus, stomach, pancreas, gallbladder, small intestine, and upper portion of the large intestine. The lower portion of the large intestine receives parasympathetic innervation from spinal nerves in the sacral region. The submucosal plexus and myenteric plexus are the sites where parasympathetic preganglionic fibers synapse with postganglionic neurons that innervate the smooth muscle of the GI tract.

Postganglionic sympathetic fibers pass through the sub-mucosal and myenteric plexuses and innervate the GI tract. The effects of the sympathetic nerves reduce peristalsis and secretory activity and stimulate the contraction of sphincter muscles along the GI tract; therefore, they are antagonistic to the effects of parasympathetic nerve stimulation.

Autonomic regulation, which is "extrinsic" to the gastrointestinal tract, is superimposed on "intrinsic" modes of regulation. The gastrointestinal tract contains intrinsic sensory neurons that have their cell bodies within the gut wall and are not part of the autonomic system. These help in the local regulation of the digestive tract by a complex neural network within the wall of the gut called the enteric nervous system, or enteric brain (discussed later in this chapter). Regulation by the enteric nervous system complements paracrine regulation by mole cules acting locally within the tissues of the GI tract, as well as hormonal regulation by hormones secreted by the mucosa.

In summary, the digestive system is regulated extrinsically by the autonomic nervous system and endocrine system, and intrinsically by the enteric nervous system and various paracrine regulators. The details of this regulation will be described in subsequent sections.

Test Yourself Before You Continue

1. Define the terms digestion and absorption, describe how molecules are digested, and indicate which molecules are absorbed.

2. Describe the structure and function of the mucosa, submucosa, and muscularis.

3. Describe the location and composition of the submucosal and myenteric plexuses and explain the actions of autonomic nerves on the gastrointestinal tract.

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Essentials of Human Physiology

Essentials of Human Physiology

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