The functional anatomy of GI smooth muscle consists of several components. The significance of gap junctions, interstitial cells of Cajal, and pacemaker cells lies in their role in the functioning of GI smooth muscle. The generation of the basic electrical rhythm (slow wave) and its regulation by chemical messengers such as hormones, paracrine, and neurotransmitters, as well as the causes of contractions associated with the slow wave, will be discussed. Additionally, the physiological events in gastric motility during the gastric phase will be examined.

In this chapter, we will delve into the physiology of the gastrointestinal (GI) tract during the gastric phase of digestion, specifically when food is in the stomach. The stomach serves as a temporary reservoir for the meal and initiates protein digestion through the secretion of acid and the enzyme precursor pepsinogen. Other functions of the stomach will also be discussed.

When food enters the stomach from the esophagus, it mechanically stimulates the gastric wall through smooth muscle distention and stretching. Additionally, chemical stimulation occurs when food, mainly oligopeptides and amino acids, is present in the gastric lumen. The regulation of gastric function during the gastric phase depends on endocrine, paracrine, and neural pathways. These pathways are activated by mechanical and chemical stimuli, which in turn activate intrinsic and extrinsic neural reflex pathways crucial for regulating gastric function. Afferent neurons passing from the GI tract to the central nervous system, primarily via the vagus nerve, respond to these stimuli and initiate parasympathetic outflow.

The endocrine pathways involve the release of gastrin, which stimulates gastric acid secretion, and somatostatin, which inhibits gastric secretion. Histamine release is an important paracrine pathway that stimulates gastric acid secretion. The activation of these pathways elicits both secretory and motor responses. Secretory responses include the secretion of acid, pepsinogen, mucus, intrinsic factor, gastrin, lipase, and HCO3−, all of which play a role in initiating protein digestion and protecting the gastric mucosa. Motor responses, on the other hand, involve changes in the activity of smooth muscle. These changes include inhibiting the motility of the proximal part of the stomach, known as receptive relaxation, and stimulating the motility of the distal part, leading to antral peristalsis. These motility changes are crucial for the storage and mixing of the meal with secretions, as well as regulating the flow of contents out of the stomach.

The stomach is anatomically divided into three regions: the cardia, the corpus (or fundus/body), and the antrum. However, for the purpose of discussing stomach physiology, it is more helpful to consider it as two functional regions: the proximal and distal parts. These two regions have distinct functions in the postprandial response to a meal, which will be addressed later. The stomach lining is covered with a folded columnar epithelium forming gastric pits, each serving as the opening of a duct where one or more gastric glands empty.
7.
What are the components of the functional anatomy of GI smooth muscle?
8.
What is the significance of gap junctions, interstitial cells of Cajal, and pacemaker cells in the functioning of GI smooth muscle?
9.
How is the basic electrical rhythm (slow wave) generated, how is it regulated by chemical messengers (hormones, paracrine, neurotransmitters), and what causes contractions associated with the slow wave to occur?
10.
What physiological events in gastric motility occur in the gastric phase?
n this chapter, gastrointestinal (GI) tract physiology when food is in the stomach (i.e., the gastric phase of digestion) will be discussed. This includes gastric function and its regulation, in addition to changes in function that occur in more distal regions of the GI tract. The main functions of the stomach are to act as a temporary reservoir for the meal and to initiate protein digestion through secretion of acid and the enzyme precursor pepsinogen. Other functions are listed in
Box 29.1
Food entering the stomach from the esophagus causes mechanical stimulation of the gastric wall via distention and stretching of smooth muscle. Food—predominantly oligopeptides and amino acids—also provides chemical stimulation when present in the gastric lumen. Regulation of gastric function during the gastric phase is dependent on endocrine, paracrine, and neural pathways. These pathways are activated by mechanical and chemical stimuli, which result in intrinsic and extrinsic neural reflex pathways that are important for regulation of gastric function. Afferent neurons that pass from the GI tract to the central nervous system (and to a lesser extent to the spinal cord) via the vagus nerve respond to these mechanical and chemical stimuli and activate parasympathetic outflow.
The endocrine pathways include the release of gastrin, which stimulates gastric acid secretion, and the release of somatostatin, which inhibits gastric secretion. Important paracrine pathways include histamine release, which stimulates gastric acid secretion. The responses elicited by activation of these pathways include both secretory and motor responses; secretory responses include secretion of acid, pepsinogen, mucus, intrinsic factor, gastrin, lipase, and HCO3 − . Overall these secretions initiate protein digestion and protect the gastric mucosa. Motor responses (changes in activity of smooth muscle) include inhibition of motility of the proximal part of the stomach (receptive relaxation) and stimulation of motility of the distal part of the stomach, which causes antral peristalsis. These changes in motility play important roles in storage and mixing of the meal with secretions and are also involved in regulating the flow of contents out of the stomach.
Functional Anatomy of the Stomach
The stomach is divided into three regions: the cardia, the corpus (also referred to as the fundus or body), and the antrum (
Fig. 29.1 ). However, when discussing the physiology of the stomach, it is helpful to think of it as subdivided into two functional regions: the proximal and distal parts of the stomach. The proximal portion of the stomach (called proximal because it is the most cranial) and the distal portion of the stomach (furthest away from the mouth) have quite different functions in the postprandial response to a meal, which will be discussed later.
The lining of the stomach is covered with a columnar epithelium folded into gastric pits; each pit is the opening of a duct into which one or more gastric glands empty (Fig.