Gastrointestinal physiology II.

Transcription

1 Gastrointestinal physiology II. 62. Functions of the upper GI tract: chewing, salivation, swallowing. 63. Motor functions of the stomach. Vomiting (emesis). 1 Motor functions of the mouth and the oral cavity Food intake: Sucking(brainstem motor reflex in the newborn) Biting and initiationof chewingis VOLUNTARY, buttheprocessitselfis quite automated. CHEWING (MASTICATION) Chewing deficit does not compromise digestion but the process isgreatly prolonged and large particles may hurt the gastric mucosa. Chewing is a unilateral motor reflex: Opening the mouth activates a stretch reflex and the jaw rises. Stimulation of mechanoreceptors in the mucosa inhibits stretch reflex and the jaw drops. Then the cycle starts over Movements of the tongue keep the food between the teeth! 2 1

2 SWALLOWING (DEGLUTITION) a vital BRAINSTEM reflex 1. From the mouth to the pharynx...voluntary phase 2. Pharyngeal stage 3. Esophageal stage }...reflex phases The tongue pushes back the bolus into the pharynx (initiation) All passages that are otherwise open must be closed (airways): soft palate closes nasal cavity, larynx is elevated, glottis closes, breathing stops. The muscles of the pharynx and upper third of the esophagus arestriated muscle: peristaltic activity in the upper portion of the alimentary canal is controlled by somatomotor innervation. The peristaltic wave starts from the pharynx, the UES relaxes. Here the peristalsis is caused by SEQUENTIAL FIRING of motor units The esophageal smooth muscles are controlled by parasympathethic activity of the vagus nerve: smooth muscle is actively RELAXED followed by rebound CONTRACTION 3 SWALLOWING (DEGLUTITION) 4 2

3 Primary peristalsis: Peristaltic wave initiated from the pharynx. ESOPHAGUS Neurogenic tone inhibited! upper esophageal sphincter. Myogenic tone inhibited! Secondary peristalsis: residual bolus left in the esophagus initiates new peristalsis. (e.g., during vomiting) lower esophageal sphincter 5 PRESSURE WAVES IN THE ESOPHAGUS DURING SWALLOWING(esophagometry) Different mechanisms still smooth peristaltic wave! Boron, Boulpaep, Medicap Physiology, Elsevier Saunders,

4 ACHALASIA: when the LES does not open 7 FUNCTIONS OF THE SALIVA Digestion (amylase, lipase) Excretion Protection of the mucosa Bacteriostatic action Lubrication, solvent for taste Alkalic ph (fresh saliva) preserves teeth Articulated speech 8 4

5 9 Saliva secretion (1,5 liters daily) A cooperation of acinus and ductal cells Acinus cell acinus: primary secretion ISOTONIC ductus intercalaris ductus: secondary secretion HYPOTONIC Myoepithel cell 10 5

6 acinus primary saliva secondary active transport Na + and water follow passively duct resorption secretion final saliva 11 Transport in the salivary ducts 12 6

7 COMPOSITION OF THE SALIVA VARIES WITH FLOW SALIVA PLASMA Concentration (mm/l) Flow (ml/min) Organic components: enzymes (lipase, amylase), mucus, IgA, lysozime, lactoferrin, EGF (epidermal growth factor) 13 Parasympathetic: large volume, serous Sympathetic: small volume, viscous 14 7

8 FUNCTIONAL DIVISONS OF THE STOMACH MOTILITY Proximal (orad) portion SECRETION Oxyntic (HCl producing) portion Distal (caudad) portion Gastrin producing portion 15 FUNCTIONS OF THE STOMACH Storage and emptying in small portions Grinding, milling Production of intrinsic factor--- VITAL FUNCTION!!! HCl promotes absorption of iron(keeping it as Fe 2+ ) Bacteriostatic function Trophic influence on the rest of the GI tract (gastrin promotes growth and development of the mucosal membrane) Digestion of proteins 16 8

9 MOTILITY OF THE PROXIMAL PORTION OF THE STOMACH During the filling phase: receptive relaxation Mecanisms: vago-vagal reflex, local reflex, stress relaxation During emptying: Gradual tonic contraction The proximal portion does not participatein peristalsis. X stretching 17 Mechanisms of receptive relaxation in the proximal stomach Feeding (mechanical, taste, visual, olfactory stimuli) stimulates vagal efferents activating ENS neurons that INHIBIT smooth muscle contraction (using NO, VIP) cephalic phase Food in the stomach will activate somatosensory STRETCH receptors that deliver the signal to the brainstem evoking activation of the same parasympathethic nerves (long vago-vagal reflex) Also ENS sensory neurons are activated that can also elicit LOCAL reflexes to dilate the stomach Finally myogenic stress relaxation (in response to stretch) can also be demonstrated These three are responsible for the gastric phase MOST important! 18 9

10 Intestinal phase(least important): Hormones of the small intestine dilate proximal stomach to promote storage (to slow emptying) 19 MOTILITY OF THE DISTAL PORTION OF THE STOMACH 1. Empty stomach: MMC 2. Filled state: PERISTALSIS pacemaker region The major function of the stomach is storage and grinding. The pyloric region contracts before the peristaltic wave reaches the pylorus. The pyloric canal serves as a filter allowing only small parts <2mm! to pass Only 2-4 ml liquid chyme enters the duodenumin each cycle

11 No BER proximal part of the stomach distalis parts gyomor of the stomach pacemaker BER! t 21 3 cycles/min Most material is mixed, and ground against the pyloric muscle Small injection of acidic cntent into the duodenum Boron, Boulpaep, Medical Physiology, Elsevier Saunders,

12 contraction ring 23 Control of gastric peristalsis and emptying Gastric phase (stimulation): (1) neural control, stretch and contents of the stomach stimulate ENS neurons promoting gastric motility. (2) Hormonal control: Gastrin produced in response to neuronal and chemical stimuli also promotes motility. Intestinal phase inhibition: acid, nutrients in the gut evoke (1) Hormonal inhibition secretin, CCK, GIP etc, and (2) ENS local enterogastric reflex 24 12

13 CONTROL OF GASTRIC EMPTYING Intestinal mechanisms: INHIBIT EMPTYING Humoral: CCK Secretin GIP Protein Lipid Acid Neural: Hyperosmosis Acid Distention Gastrin Gastric mechanisms: PROMOTE EMPTYING Distention Protein Neurotensin Ileum pyloric sphincter: intrinsic neurons (enkephalin, VIP) extrinsic: vagus, sympathicus 25 Vomitus: A complex brainstem protective reflex to empty the stomach and often proximal small intestinal contents. Multiple brainstem areas and spinal cord segments are involved. 14th century illustrations of vomiting 26 13

14 VOMITUS Evoking stimuli: GI tract irritation Visceral pain Kinetosis cerebellum Drugs Visual, olphactory stimuli emotions stim. pregnancy irradiation Nausea: symptoms preceding vomitus chemosensitive trigger zone (Area postrema) medullary center Reverse filling of the stomach is caused by giant retrograde intestinal contractions Müller maneuver(inspiration effort, closed glottis) + opening of the LES and contraction of the abdominal muscles (content enters the esophagus) Forceful expiration (Valsalva) and opening of UES EXPULSION throwing up Remnants delivered back to stomach by a secondary peristalsis new cycle may start 27 14