Functional Types Of Movements In The Gastrointestinal Tract

Propulsion and Mixing of Food in the Alimentary Tract Functional Types Of Movements In The Gastrointestinal Tract • • • Two types of movements in the gastrointestinal tract: (1) Propulsive movements: cause food to move forward along the tract at an appropriate rate to accommodate digestion and absorption. (2) Mixing movements: keep the intestinal contents thoroughly mixed at all times. • PROPULSIVE MOVEMENTS—PERISTALSIS • Contractile ring appears around the gut and then moves forward; like putting one’s fingers around a thin distended tube. Propulsion and Mixing of Food in the Alimentary Tract Functional Types Of Movements In The Gastrointestinal Tract • PROPULSIVE MOVEMENTS—PERISTALSIS • The usual stimulus for intestinal peristalsis is: • Distention of the gut • Chemical or physical irritation of the epithelial • Strong parasympathetic nervous signals to the gut     Its occurrence is independent of the extrinsic innervation. Local stretch releases serotonin, which activates sensory neurons that activate the myenteric plexus. Cholinergic neurons in a retrograde direction activate neurons that release SP and Ach, causing smooth muscle contraction. Cholinergic neurons in an anterograde direction activate neurons that secrete NO, VIP and ATP producing the relaxation ahead of the stimulus. Propulsion and Mixing of Food in the Alimentary Tract Functional Types Of Movements In The Gastrointestinal Tract • Mixing Movements • • Mixing movements differ in different parts of the alimentary tract. In some areas, the peristaltic contractions cause most of the mixing. This is especially true when forward progression of the intestinal contents is blocked by a sphincter so that a peristaltic wave can then only churn the intestinal contents, rather than propelling them forward. At other times, local intermittent constrictive contractions occur every few centimeters in the gut wall. These constrictions usually last only 5 to 30 seconds; new constrictions then occur at other points in the gut, thus “chopping” and “shearing” the contents. Propulsion and Mixing of Food in the Alimentary Tract Ingestion Of Food • • • • • • The amount of food that a person ingests is determined by an intrinsic desire for food called hunger. The type of food that a person preferentially seeks is determined by appetite. MASTICATION (CHEWING) The anterior teeth (incisors) provide a strong cutting action (55 pounds ). Posterior teeth (molars) provide a grinding action (200 pounds ). Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve, and the chewing process is controlled by nuclei in the brain stem. Propulsion and Mixing of Food in the Alimentary Tract MASTICATION (CHEWING) • • • • • • • Stimulation of specific reticular areas in the brain stem taste centers will cause rhythmical chewing movements. Stimulation of areas in the hypothalamus, amygdala, and cerebral cortex near the sensory areas for taste and smell can cause chewing. Much of the chewing process is caused by a chewing reflex. Chewing is especially important for most fruits and raw vegetables. Digestive enzymes act only on the surfaces of food particles. Chewing mixes the food with the secretions of the salivary glands. This wetting and homogenizing action aids swallowing and digestion. The number of chews depends on the food, but usually ranges from 20 to 25. Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING • • • • SWALLOWING (DEGLUTITION) • Voluntary stage • Pharyngeal stage (involuntary) • Esophageal stage (involuntary) Nervous Initiation of the Pharyngeal Stage of Swallowing. The most sensitive tactile areas of the posterior mouth and pharynx for initiating the pharyngeal stage of swallowing lie in a ring around the pharyngeal opening, with greatest sensitivity on the tonsillar pillars. Impulses are transmitted from these areas through trigeminal and glossopharyngeal nerves into the medulla oblongata. Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING • • Pharyngeal stage The areas in the medulla and lower pons that control swallowing are collectively called the deglutition or swallowing center. • The motor impulses from the swallowing center to the pharynx and upper esophagus are transmitted by the fifth, ninth, tenth, and twelfth cranial nerves.  Effect of the Pharyngeal Stage of Swallowing on Respiration Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING • • Esophageal stage The esophagus normally exhibits two types of peristaltic movements: primary peristalsis and secondary peristalsis.  Primary peristalsis • Is simply continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal stage of swallowing (passes all the way from the pharynx to the stomach in about 8 to 10 seconds). Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING • Esophageal stage  • Secondary peristaltic The secondary peristaltic waves are initiated by: • Intrinsic neural circuits in the myenteric nervous system • Reflexes that begin in the pharynx and are then transmitted upward through vagal afferent fibers to the medulla and back again to the esophagus through glossopharyngeal and vagal efferent nerve fibers. • The musculature of the pharyngeal wall and upper third of the esophagus is striated muscle. The peristaltic waves in these regions are controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves. • In the lower two thirds of the esophagus, the musculature is smooth muscle, but this portion of the esophagus is controlled by the vagus nerves that act through connections with the esophageal myenteric nervous system. Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING   Ach from vagal endings causes contraction. NO and VIP from interneurons causes relaxation. Propulsion and Mixing of Food in the Alimentary Tract SWALLOWING  • Receptive Relaxation of the Stomach When the esophageal peristaltic wave approaches the stomach, a wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the peristalsis. Furthermore, the entire stomach and, to a lesser extent, even the duodenum become relaxed.  Function of the Lower Esophageal Sphincter • This sphincter normally remains tonically constricted. • When a peristaltic swallowing wave passes down the esophagus, “receptive relaxation” of the lower esophageal sphincter occurs ahead of the peristaltic wave. • Rarely, the sphincter does not relax satisfactorily, resulting in a condition called achalasia.