Serosa

Mucosa. The innermost layer of the wall is the mucosa, which consists of a mucous membrane, the lamina propria, and the muscularis mucosa. The mucous membrane provides important protective and absorptive functions for the digestive tract. The nature of the epithelial cells lining the tract varies from one region to the next. Rapidly dividing stem cells continually produce new cells to replace worn out epithelial cells. The average life span of these epithelial cells is only a few days. The lamina propria is a thin middle layer of connective tissue. This region contains the capillaries and small lymphatic vessels that take up the digested nutrient molecules. The muscularis mucosa is a thin layer of smooth muscle. Contraction of this muscle may alter the effective surface area for absorption in the lumen.

Submucosa. The submucosa is a thick middle layer of connective tissue. This tissue provides the digestive tract wall with its distensibility and elasticity as nutrient materials move through the system.

Muscularis externa. The outer layer of the wall is the muscularis externa. In most regions of the tract, it consists of two layers of muscle: an inner circular layer and an outer longitudinal layer. Contraction of the circular layer narrows the lumen of the tube. Contraction of the longitudinal layer causes the tube to shorten.

The muscle of the digestive tract consists of single-unit smooth muscle. Within each layer, the muscle cells are connected by gap junctions forming a syncytium. Action potentials generated at a given site travel along the muscle layer. Furthermore, this muscle is self-excitable; it undergoes slow but continuous electrical activity producing rhythmic contractions of the digestive tract wall. The cycles of depolarization and repolarization in the smooth muscle are referred to as slow-wave potentials. These potentials do not reach threshold during each cycle, so contraction does not necessarily occur with each depolarization. Smooth muscle contraction will take place only when the slow wave actually depolarizes all the way to threshold. At this point, voltage-gated Ca++ channels open, Ca++ ions enter the cell and one or more action potentials are generated. These action potentials result in phasic contractions. The force and duration of muscle contraction is determined by the number of action potentials generated. Typically, phasic contractions last only a few seconds.

Muscular activity, or gastrointestinal motility, is enhanced by stretching the muscle, as occurs with the presence of food materials and distension of the digestive tract wall. It is also enhanced by parasympathetic nervous stimulation and by several specific gastrointestinal hormones. Motility is inhibited by sympathetic nervous stimulation and by circulating epineph-rine. The two basic forms of gastrointestinal motility are:

• Segmentation

• Peristalsis

The contents of the digestive tract must be constantly churned and mixed so that materials are exposed to digestive enzymes and come into contact with the wall of the tract for absorption. This mixing is carried out by segmentation, or stationary muscular contractions. This form of motility divides some portion of the tract into alternating constricted and uncon-stricted regions. Segmentation contractions move back and forth so that a previously constricted region relaxes and a previously relaxed region contracts. This activity results in thorough mixing of the contents with digestive enzymes and other secretions. This is the more important form of motility in the small intestine, where most digestion and absorption take place.

The contents of the tract must also be continually moved along so that it can be acted upon by the sequential regions of the tract. Peristalsis is a muscular contraction that produces a ring of contraction that moves along the length of the tract. This wave-like contraction causes propulsion and forces the contents forward. Peristalsis is more important in the esophagus and stomach.

Gastrointestinal sphincters are formed where the circular layer of smooth muscle is thickened. Sphincters occur at several points along the tract. Their function is to limit the movement of food materials from one region to another. For example, the pyloric sphincter found between the stomach and duodenum of the small intestine plays an important role in limiting the rate of gastric emptying. Sphincters undergo tonic contractions that may be sustained for minutes or hours.

Serosa. The connective tissue membrane that surrounds the wall of the digestive tract is the serosa. This membrane secretes a watery fluid that provides lubrication and prevents friction between the digestive organs as they move about in the abdomen. The serosa is continuous with the peritoneum, which is the serous membrane lining the abdominal cavity. The peritoneum also forms sheets of tissue, or mesentery, that suspend the digestive organs from the wall of the abdomen. The mesentery acts as a sling that, while offering structural support for the organs, also provides for the range of movement needed during the digestive process.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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