L3_ Movements in GIT.pdf

Types of Movements in the Gastrointestinal Tract Two basic types of movements occur in the gastrointestinal tract: (1) Propulsive movements, which cause food to move forward along GIT for digestion and absorption. (2) Mixing movements, which mix intestinal contents. (1) Propulsive movements (Per...

Types of Movements in the Gastrointestinal Tract Two basic types of movements occur in the gastrointestinal tract: (1) Propulsive movements, which cause food to move forward along GIT for digestion and absorption. (2) Mixing movements, which mix intestinal contents. (1) Propulsive movements (Peristalsis):  Peristalsis is a reflex response initiated when GIT wall is stretched by contents of lumen.  Stretch usually initiates a circular contraction behind the stimulus and an area of relaxation in front of it which is called "receptive relaxation".  It occurs in all parts of GIT from pharynx to the rectum in an oral to caudal direction, propelling contents of lumen forward.  Depends on myenteric plexus so called myenteric reflex or peristaltic reflex.  Peristalsis is an excellent example of enteric nervous system.  Local stretch releases serotonin, which activates sensory neurons that activate myenteric plexus.  Cholinergic neurons passing into a retrograde direction in this plexus activate neurons that release substance P and acetylcholine causing smooth muscle contraction.  At the same time, cholinergic neurons passing in a antegrade direction activate neurons that secrete VIP, NO and ATP producing relaxation ahead of stimulus. (2) Mixing Movements:  Mixing movements are quite different in different parts of GIT.  In some areas, peristaltic contractions themselves cause most of mixing. If intestinal contents is blocked by a sphincter, so that a peristaltic wave can then mix intestinal contents.  At other times, local constrictive contractions occur every few centimeters in GIT wall.  Mouth & Esophagus  In mouth, food is mixed with saliva and propelled into pharynx and esophagus.  Peristaltic waves in the esophagus move the food into the stomach. Mastication (Chewing)  Anterior teeth (incisors) providing a strong cutting action and posterior teeth (molars) a grinding action.  Nerve supply: fifth cranial nerve.  Center: brain stem.  Mechanism: chewing reflex (stretch reflex):  Entry of a bolus of food to mouth causes stretch of muscles of mastication which in turn initiates a stretch reflex of jaw muscles that leads to contraction.  This automatically raises jaw to cause closure of teeth, but it also compresses bolus again against linings of mouth, which inhibits the jaw muscles once again, allowing jaw to drop another time; and this is repeated rhythmically.  Functions: Chewing breaks up large food particles and mixes food with secretions of salivary glands. This wetting homogenizing action aids subsequent digestion. Swallowing (Deglutition) In general, swallowing can be divided into: (1) Oral (voluntary) stage: which initiates swallowing. (2) Pharyngeal stage: involuntary and constitutes passage of food through pharynx into esophagus. (3) Esophageal stage: involuntary phase that promotes passage of food from esophagus to stomach. (a) Voluntary oral Stage of Swallowing:  Food is voluntarily squeezed or rolled posteriorly into pharynx by pressure of tongue upward and backward against hard palate. (b) Involuntary Pharyngeal Stage of Swallowing:  Pharyngeal stage is principally a reflex act.  Stimulus: As the bolus of food enters the pharynx.  Receptors: swallowing receptor areas around opening of the pharynx, especially on the tonsillar pillars  Center: brain stem (medulla and lower pons called deglutition or swallowing center).  Response: Initiate pharyngeal muscular contractions.  Functions: propels food into esophagus. Protection of air passages occurs during swallowing as following: 1. Soft palate is pulled upward to close the posterior nares. 2. Vocal cords of larynx are strongly approximated. Both effects prevent passage of food into trachea. (c) Involuntary Esophageal Stage of Swallowing:  Esophagus conducts food from pharynx to stomach.  2 types of peristaltic movements, primary peristalsis and secondary peristalsis. A. Primary peristalsis: a continuation of peristaltic wave that begins in pharynx and spreads into esophagus during the pharyngeal stage of swallowing. If primary peristaltic wave fails to move food into the stomach, secondary peristaltic wave occurs. B. Secondary peristaltic waves: Result from esophagus distention.  Lower Esophageal Sphincter (Gastroesophageal Sphincter) (LES)  LES is made up of 3 components. 1. Esophageal smooth muscle is more prominent at the junction with the stomach (intrinsic sphincter). 2. Fibers of crural portion of diaphragm surround esophagus (extrinsic sphincter). 3. In addition, oblique fibers of stomach wall create a flap valve that prevents regurgitation when intragastric pressure rises.  It remains tonically contracted. Tone of LES is under neural control: a. Acetylcholine released from vagal nerve endings causes contractions of LES. b. Nitric oxide (NO) and VIP from cause it to relax. Between meals normal tonic activity of the lower esophageal sphincter prevents reflux of stomach contents into the esophagus. Abnormalities of tone of LES: 1- Gastroesophageal reflux: If resting tone of LES is decreased, reflux of gastric acid content into esophagus will cause heartburn and esophagitis and can lead to ulceration and stricture of esophagus due to scarring. Tone of L.E.S. is decreased during pregnancy by high level of progesterone leading to some degree of reflux and heartburn. Also diet with high sugar and fat may lead to heartburn. 2- Achalasia: incomplete relaxation of LES which leads to accumulation of food in esophagus and its massive dilation. Myenteric plexus of esophagus is deficient at LES in this condition and release of NO and VIP is defective. It is treated by dilation of sphincter, esophageal muscle incision or by injection of botulinum toxin in the LES to inhibit acetylcholine release.