Motility function of the gastrointestinal system

The 3 major patterns of the digestive tract motility Peristalsis Rhythmic segmentation Tonic contraction

It requires intact enteric nervous system but can Usually the stimulus for peristalsis is distention It requires intact enteric nervous system but can be modulated by the autonomic nervous system

Esophageal Function Esophagus is a collapsible muscular tube. It transports food to stomach Secretes mucus with no digestive or absorptive function. Upper oesophageal sphincter (UES) UES closes in between swallowing to: Prevent esophageal contents reflux into pharynx Prevents air entry into the esophagus Diaphragm Lower oesophageal sphincter (LES) LES : Closed in between swallowing Prevents stomach content reflux into esophagus

Swallowing or Deglutition Swallowing is the movement of food from mouth into the Stomach It is facilitated by secretion of saliva and mucus It involves mouth, pharynx and esophagus It is coordinated by the swallowing center in medulla oblongata and lower pons It involves 3 phases or stages (oral, pharyngeal & esophageal)

Phases of Swallowing Oral phase: Voluntary The swallowing starts when the food bolus is forced to the back of the oral cavity and into the pharynx by the movement of the tongue upward and backward against the palate

Phases of Swallowing 2. Pharyngeal phase (Involuntary) Tongue sweeps backward → bolus into the oro-pharynx Elevation of soft palate and contraction of the upper constrictor muscle of the pharynx to close nasopharynx. Epiglotis and vocal cords shut off the larynx Reflex apnea Relaxation of UES Peristatic contraction begins in the upper constrictor muscle → middle & inferior constrictor muscle

Phases of Swallowing 3. Esophageal Phase (Involuntary) Begins once the food bolus enters the esophagus Peristalsis pushes the bolus onwards.

Esophageal Phase cont… At the lower of the oesophagus LES (Cardiac sphincter) relaxes to allow the bolus to enter the stomach It is usually closed to prevent gastric reflux

Functions of the stomach Storage of ingested food until it is emptied into small intestine. Secretion of HCl and enzymes that begin protein digestion. Stomach mixing movements: Ingested food is pulverized and mixed with gastric secretion  Chyme Chyme is emptied into the duodenum (gastric emptying) 10

Four Aspects of Gastric Motility: Stomach Motility Function Four Aspects of Gastric Motility: Gastric Filling Gastric Storage Gastric Mixing Gastric Emptying 11

Gastric Filling and storage: The stomach is able to accommodate such a 20-fold change in its volume Question: HOW? Answer: Receptive Relaxation During a meal stomach relaxes slightly with each mouthful RECEPTIVE RELAXATION Receptive relaxation enhances the stomach ability to accommodate with extra volume of food.

Gastric emptying & mixing: 1 1. Peristaltic contraction (PC) originates in the upper fundus → the pyloric sphincter 2. The PC becomes more vigorous as it reaches the antrum. 3 4 2 3. The strong PC propels the chyme forward 4. A small portion of the chyme is pushed through the “partially” open sphincter into the duodenum 5 5. When PC reaches the pyloric sphincter, the sphincter closes tightly → No further emptying 6 6. The chyme that was not delivered in to the duodenum is forced backward into the stomach for further mixing 13

Regulation of Gastric emptying Distension of the stomach by the presence of food Secretion of gastrin and increase parasympathetic impulses Stimulate contraction of lower esophageal sphincter, increase motility of the stomach Relax of the pyloric sphincter Gastric emptying

Carbohydrates empty > Proteins > fats Distension of the duodenum by chyme, fatty acids Enterogastric reflex parasympathetic stimulation Sympathetic stimulation CCK Gastric motility Carbohydrates empty > Proteins > fats Inhibition of gastric emptying

Small Intestine Site of digestion and absorption Divided into 3 regions: duodenum, jejunum, ileum Joins the large intestine at ileocecal sphincter Stomach chyme must be transported from duodenum down the full length of the small intestine for digestion and absorption to take place efficiently. Chyme is chopped, churned and mixed with small intestine secretions

Small intestine Motility Two types of SI movement 1. Segmentations Mix contents with the digestive juices and bring the particles of food into contact with the mucosa for absorption They do not push the intestinal contents along the tract. 2. Migrating motility complexes (MMC) It is type of peristalsis It starts in the lower stomach and pushes chyme forward along the small intestine.

Ileocecal sphincter Ileocecal juncture prevents contamination of the small intestine by large intestine contents. Normally remain partially closed. After a meal gastroileal reflex intensifies ileum peristalsis and forces chyme into caecum. Gastrin cause sphincter relaxation. When caecum distended the contraction of ileocaecal sphincter intensifies

Large Intestine Figure 23.29a

Motility of Large Intestine Haustration (segmentation in colon) Slow segmenting movements that move the contents of the colon (occur every 30 min) Controlled by the intrinsic plexuses Stimulated by distension Movement are slow → absorptive and storage functions Mass movements (propulsive movements) ↑↑↑ motility in ascending and transverse colon occurring after a meal Drive the colonic contents to the distal portion of large intestine Triggered by: ─ Gastrocolic and duodenocolic reflexes ─ Irritation ─ Intense parasympathetic stimulation 20

Defecation Reflex Two anal sphincters: Parasympathetic nerve Fibers (pelvic nerve) Two anal sphincters: Internal anal sphincter: smooth muscle & NOT under voluntary control. External anal sphincter: striated muscle, under voluntary control & is innervated by the pudendal nerve Skeletal motor nerve (pudendal nerve)

Defecation Reflex Mass movement Rectum is distended Activation of stretch receptors Signals to sacral spinal cord Gives desire to defecate Through pelvic nerve (parasympthatic), relaxation of internal anal sphincter If the conditions are right: Voluntarily relaxation of external anal sphincter allows defecation. Otherwise, the reflex subsides until the rectum is filled again