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.