Physio_GI_Review

www.brain101.info1 GASTROINTESTINAL PHYSIOLOGY ENTERIC NERVOUS SYSTEM Smooth muscle: All smooth muscle is innervated by the autonomic nervous system. • General Properties: o Caveolae: Micro-pits allow for increased surface area on smooth muscle. o No Striations: Thin and thick filaments run through in a random order. Smooth muscle has relatively more thin filaments than thick. o Plasticity: Smooth muscle is able to stretch to a greater length and compress to a shorter length than skeletal. o Calcium supply comes more from outside the cell rather than inside (in the SR), as compared to skeletal. o Slow, Sustained contraction as compared to skeletal muscle. • Multi-unit smooth muscle: Has high innervation density. This is the type of smooth muscle found in Ciliary Muscle and Ductus Deferens. • UNITARY SMOOTH MUSCLE: The type of smooth muscle found in gut. o Sparse innervation compared to multi-unit muscle o Functional Syncytium: Gap junctions allow intercellular communication. o Shows spontaneous (basal) electrical activity even in the absence of innervation.  High basal resting potential (-57 mV -vs- -80 mV) as compared to skeletal muscle. Smooth muscle is more permeable to Na+ which accounts for spontaneous electrical activity. • SMOOTH MUSCLE CHANNELS: o Electromechanical Channels: Channels that transduce electrical activity, in one form or another, to mechanical activity of actin and myosin.  Slow-Leaking Ca+2-Channels  Ligand-Gated Channels  Voltage-Gated Na+-Channels o Pharmaco-mechanical Channels: Channels that employ a second messenger, causing contractility without a change in the cell's electrical potential. • SMOOTH MUSCLE CONTRACTION: o Ca+2 enters cell ------> Calmodulin then activates Myosin Light-Chain Kinase (MLCK) ------> MLCK then phosphorylates myosin, turning it on and enabling it to interact with actin ------> contraction occurs. o Regulatory step is binding of Ca+2 with Calmodulin. SLOW-WAVES: The basal electrical tone of smooth muscle. No contraction occurs with slow-waves. • Also called the Basal Electrical Rhythm (BER) • Magnitude of change is 5 - 15 mV, caused by entrance of Na+ into cell. No Ca+2 is associated with these waves so no contraction occurs with them. • Basal Rhythm in Different Regions: Remember these waves are only electrical -- not mechanical. o STOMACH: 3 waves per minute o DUODENUM: 12 waves per minute. In the duodenum, 30-40% of slow-waves are associated with Ca+2 as Ca+2 is added to the cells. ENTERIC NERVOUS SYSTEM: The GI nervous system is independent of the CNS. Activity can go on without any CNS input. • GI Plexes: o MYENTERIC PLEXUS: Outermost plexus located between the two layers of musculature -- between the muscularis circularis and muscularis longitudinalis. o SUBMUCOSAL PLEXUS: Located in the submucosa, just outside the Muscularis Mucosae. • EXTRINSIC REGULATORY INPUT: o Chemoreceptors and mechanoreceptors from the GI-Lumen are an important source of input. They are the origin of short reflexes (not involving the CNS) that go through the two GI plexes in the enteric NS. www.brain101.info2 o VAGO-VAGAL (long) REFLEX: Generally stimulatory (increase motility, secretomotor, vasodilatory).  The Vagus carries both afferents (70%!) and efferents. Luminal receptors send afferent signal back to the CNS via the Vagus. o INTESTINO-INTESTINAL (short) REFLEX: Generally inhibitory, involving only the Enteric NS, and completely independent of the Autonomic NS. o SYMPATHETICS are inhibitory to the GI-Tract. They work primarily by presynaptic inhibition, thus inhibiting release of ACh. In this way we get smooth muscle relaxation.  Norepinephrine binds to alpha1-Adrenoreceptors on parasympathetic nerve terminals and thereby inhibit the release of ACh. • NEUROTRANSMITTERS: o Acetylcholine increases GI-Motility when it acts on smooth muscle. o Norepinephrine decreases GI-Motility when it acts on smooth muscle. o Enkephalin (Opioid) decreases GI-motility by inhibiting the release of ACh. o VASOACTIVE INTESTINAL PEPTIDE (VIP): Acts directly on smooth muscle to cause smooth muscle relaxation.  It is localized with ACh in the Vagus Nerve.  VIP is in local neurons, and is released when Vagal Fibers excite these inhibitory neurons to cause relaxation: Vagus (Excitatory synapse) ------> Turn on VIP neurons (Inhibitory synapse) ---- --> Relaxation. o COLOCALIZATION: Enkephalins, VIP, NO, Serotonin, and a whole bunch of other transmitters are localized along with ACh and NorE in the autonomic nervous system. Depending on the nerve, whenever the ACh and NorE are released, so will the other substances be released. MYOGENIC CONTRACTILITY: The gut has some contractility without any nervous input whatsoever. • Luminal contents will cause basal contractility without any nervous influence at all. • Thus there is a constant inhibitory tone of VIP and NO on the gut, to prevent / slow down this contractility. PARALYTIC ILEUS: Loss of GI contractility. • It can occur chronically from overproduction of Sympathetics. • Post-Operative (Physiologic) Ileus is a very common occurrence with abdominal surgeries TYPES OF MOTILITY: • PERISTALSIS: Propulsion of material in the aboral (away from mouth) direction. o Rate of peristalsis varies in region, but peristaltic generally gets slower as we move down the tract. o Peristalsis occurs by segmental hyperpolarization followed by depolarization of muscle. o Mechanism: Bolus of food in a particular location stimulates mechanoreceptors and chemoreceptors in the GI lumen, ultimately resulting in peristalsis:  Relaxation of the muscle occurs distal to the bolus, so that the food can go forward. This is mediated by VIP / NO.  Contraction of Longitudinal Muscle layer also occurs distal to bolus, because longitudinal contraction causes widening of the GI lumen.  Contraction of the muscle occurs proximal to the bolus, in order to propel the bolus forward.  There is a basal level of VIP inhibition in the muscle, and a bolus of food turns off this inhibition: distension of lumen by a bolus will cause inhibition of release of VIP / NO ------> contraction of proximal region. • RHYTHMIC SEGMENTATION: Mixing and churning of materials without propelling them forward in the tract. o Only involved the circular muscle -- not longitudinal o Common in small and large intestine • TONIC CONTRACTION: Blocking of the passage of material, as in sphincters. o Tonic Contraction is myogenic -- it doesn't depend on innervation. www.brain101.info3 HORMONES, ENZYMES, REGULATORY SUBSTANCES AND STUFF NEUROENDOCRINE HORMONES: All of below are either exclusively endocrine (glandular secretions into bloodstream), exclusively neural (neurotransmitter) or both. All of below serve regulatory (as opposed to digestive) functions. • GASTRIN: Endocrine. o STRUCTURE: Active part of peptide is on carboxy-end. It shares the last four residues in common with CCK (Trp-Met-Asp-Phe), and it has a protective NH2 on the carboy end to help prevent degradation.  PENTAGASTRIN Drug that mimics Gastrin, containing the last four residues in gastrin, and therefore containing similar biological activity. o Distribution: Gastrin is made by G-CELLS in the ANTRUM of the Stomach. o FNXNS:  It stimulates release of HCl in Parietal Cells.  Also stimulates growth of gastric mucosa and proliferation of intestinal enterocytes.  Intestinal Resection: If you cut out part of the intestine, higher levels of Gastrin will result. o REGULATION:  Gastrin release is inhibited by acid in the stomach. Primary negative feedback mechanism.  Gastrin release is stimulated by digested proteins and by Acetylcholine. • CHOLECYSTOKININ (CCK): Endocrine and neural o STRUCTURE: Biological activity is contained in last seven residues on carboxy-end, with last four residues in common with Gastrin, and with a protective NH2 on the carboxy terminus.  Activity on PARIETAL CELLS: CCK in the stomach can bind to Gastrin receptors to BLOCK the effects of Gastrin. o Distribution: CCK is made from I-CELLS o FNXNS:  Stimulates contraction of the gall bladder  Stimulates secretion of pancreatic enzymes.  Inhibits gastric emptying as part of the Entero-Gastric Reflex. The presence of CCK indicates that the duodenum is currently full and gastric emptying should be slowed. o REGULATION:  CCK-release is stimulated by the presence of peptides in the duodenum. • SECRETIN: Endocrine and neural o Distribution: Secretin comes from S-CELLS in the duodenum. o FNXNS:  It inhibits stomach motility when released in Duodenum bia the Entero-Gastric Reflex. o REGULATION:  Secretin-release is stimulated by acid in the Duodenum. • SOMATOSTATIN: The universal inhibitory substance. It acts in endocrine, neural, and paracrine fashion. o Distribution: Somatostatin is all over the place. • GASTRIC INHIBITORY PEPTIDE (GIP): Endocrine. o FNXNS:  Inhibits the release of Gastrin by a pharmacological mechanism. Thus the effect is dose- dependent, and a large (non-physiological) dose is required to elicit a response.  Dr. Greenwald thinks this effect is secondary importance because it is only pharmacological.  Major fnxn = GIP stimulates release of Insulin from Pancreas o DISTRIBUTION: Antrum of stomach + duodenum. • VASOACTIVE INTESTINAL PEPTIDE (VIP): Primarily neural • MOTILIN: Endocrine. o FNXN: It elicits the Migrating Motor Complex in the small intestine, to propel bacteria aborally. • GASTRIC RELEASING PEPTIDE (GRP) (Bombesin): Neural. Involved in the release of Gastrin. Its release is Non-Adrenergic Non-Cholinergic. o REGULATION: Its release stimulated during the Cephalic Phase of gastric secretion. • ENKEPHALIN (an Opioid): o FNXN: Decreases GI-motility by inhibiting the release of ACh. www.brain101.info4 PREGNANCY: • Pregnant women tend to gain weight because they have increased levels of CCK (higher fat and protein absorption) and lower levels of Somatostatin. o Higher CCK is especially marked during first trimester. • INFANTS have very high levels of Gastrin to accompany their very high calorie-per-body-weight intake. Gastrin interacts with hypothalamus to somehow promote anabolic growth in infants. MOTILITY THE ESOPHAGUS: • Anatomy and Pressures: o Upper Esophageal Sphincter (UES): Skeletal muscle, essentially comprising the cricopharyngeus muscle.  Resting pressure = 50-60 mm Hg to prevent swallowing of air.  Muscle tone is neurogenic and depends on CNS neural input from swallowing center to remain active. o Body: Combination of skeletal and smooth muscle.  Resting pressure = -5 mm Hg o Lower Esophageal Sphincter: Smooth muscle, normally closed in order to prevent gastric reflux.  Resting pressure = 30 mm Hg  LES contractility is myogenic. The way we relax the LES is by putting tonal amounts of VIP / NO on the sphincter.  VIP inhibition of LES is Non-Adrenergic, Non-Cholinergic (NANC). We know this because Atropine does not prevent the inhibition:  Give atropine, and the LES will still relax because VIP is not stopped.  Give a VIP-Antibody and the LES will no longer relax because inhibition has been removed. • SWALLOWING REFLEX: Can be studied with Manometry (esophageal pressure) studies. o Oral Phase: 1 second, voluntary. o Pharyngeal Phase: 1 second, involuntary. It is stimulated by the presence of the slightest food or liquid (saliva) in the back of the throat.  You cannot swallow if your mouth is absolutely dry.  Aspiration of food is prevented:  Respiration is inhibited from this point forward.  Epiglottis is NOT important in preventing aspiration. Rather it is adduction vocal cords that prevents food getting into trachea.. o Esophageal Phase: 8-10 second, involuntary Esophageal Peristalsis  Esophageal Peristalsis is a Vago-Vagal (CNS mediated) Reflex.  RELAXATION of Lower Esophageal Sphincter occurs early in the swallowing reflex -- before the end of peristalsis of the esophagus.  At the end of swallowing the LES should tighten up again to prevent reflux of gastric contents. • Types of peristalsis: o PRIMARY PERISTALSIS: The initial peristalsis, initiated by the swallowing reflex. o SECONDARY PERISTALSIS: Any subsequent peristalsis, to get any remaining food out of the esophagus. It is initiated by distension of esophagus and mechanoreceptors on smooth muscle.  The UES does NOT open with secondary peristalsis. It doesn't need to open. • ACHALASIA: Tonic high pressure at the LES, making it difficult to swallow. Failure of LES to relax due to lack of VIP or because enteric system has been knocked out. o ETIOLOGY: Could be caused by sympathetic over expression (Sympathetics will cause relaxation via stimulation of VIP neurons) or by VIP under expression. o SYMPTOMS:  Distended esophagus because food can't easily get to stomach.  Lacking or uncoordinated peristalsis; or no peristalsis at all.  Spastic uncoordinated contractions following meal. www.brain101.info5 • GASTRO-ESOPHAGEAL REFLUX DISEASE (GERD): Having an incompetent or over-relaxed LES. Heartburn. o Newborn babies don't have a competent LES, hence they burp up food a lot. o Secondary peristalsis can help alleviate the symptoms by pushing unwanted chyme back into the stomach. o Esophagitis and Esophageal Cancer can result from chronic cases. o Lying down after a meal (i.e. lack of gravity) worsens the reflux. o PROPULSID = drug that causes contractions of the LES, hence a treatment for GERD. It acts on ACh receptors to amplify the effect of ACh. THE STOMACH: • FNXNS: o RECEPTIVE RELAXATION: The stomach is a reservoir of food and can accommodate large changes in volume. Pressure increase with more food is gradual.  Mechanism: Vago-Vagal. More food ------> distend stomach wall and activate mechanoreceptors ------> more VIP on stomach wall ------> relaxation. o It converts food to chyme. o It controls the rate of Gastric Emptying, so duodenum doesn't get overloaded with bolus. • BASAL ELECTRIC RATE: Stomach BER is about 3 events per minute. o This number represents the maximum number of contractions that can occur per minute. • GASTRIC EMPTYING: The rate of movement of food from the antrum of the stomach, through the Pyloric Sphincter (a true sphincter), and into the duodenum. o General Properties:  Retropulsion: Stomach contractions originating at antrum and going backward, to prevent too rapid of gastric emptying.  Liquids empty before solids.  Fats are slowest emptying of all substances. CHO's and proteins empty first.  Isotonic contents empty before hypotonic contents.  Stomach acid impedes the rate of gastric emptying. o ENTERO-GASTRIC REFLEX: Negative feedback from duodenum will slow down the rate of gastric emptying, by multiple mechanisms. Basically, whenever there is food in the duodenum, gastric emptying will be down-regulated.  Acid in duodenum ------> stimulate Secretin release ------> inhibit stomach motility via Gastrin inhibition  Fats in duodenum ------> stimulate CCK and GIP ------> inhibit stomach motility  Hypertonicity in duodenum ------> (unknown hormone) ------> inhibit gastric emptying. o ABNORMAL EMPTYING: The major role of the duodenum is to restore isotonicity.  Dumping (Gastric Emptying) Syndrome = TOO RAPID emptying, which can result from resection of part of the stomach  SYMPTOM: Too rapid emptying ------> hypertonic bolus in duodenum ------> pull fluid in from circulation ------> Severe cardiac problems and hypovolemia.  DELAYED EMPTYING can occur from diabetic neuropathy. It can cause nausea, heartburn, and reflux. SMALL INTESTINE: • Basal Electrical Rate: o Fastest is in duodenum (12 cycles / min). It gets increasingly slower as you move through intestine. o LAW OF THE INTESTINE: The decreasing electrical rate as you move through tract is ultimately responsible for the movement of food in an aboral (i.e. forward) direction. The general movement of food aborally is a result of the basal electrical rate. o MYOGENIC: Small intestinal motility is myogenic. If you give tetrodotoxin to kill all the nerves, you still get motility. • ILEOCECAL SPHINCTER: Smooth muscle sphincter which acts by short (intestino-intestinal) reflexes. Atropine has no effect on it. o Distension on ileal side of sphincter ------> sphincteral relaxation ------> bolus can pass through. www.brain101.info6 o Distension on colonic side of sphincter ------> sphincteral contraction ------> bolus is prevented from moving backward. COLON: • Basal Electrical Rate: The colon has the slowest of all BER's. • HAUSTRATIONS: Slow segmental movements that move food very slowly through colon. This movement is going on continually. • Mass Movements result from GASTRO-COLIC REFLEX: Food entering into stomach can cause much more rapid and forceful peristalsis in colon, ultimately resulting in defecation. o This phenomenon will esp. happen in the morning. • ANAL SPHINCTER: Internal Anal Sphincter is smooth and external anal sphincter is skeletal. o As you increase pressure in rectum (distend it), two things happen:  The Internal Anal Sphincter relaxes to accommodate the fecal matter.  The External Anal Sphincter contracts to prevent defecation.  Pooping is voluntary (usually). • FIBER is food that is not digested nor digestible. A large constituent of fiber is cellulose which human can't digest. o Fiber lowers bowel transit time, especially through the colon. • MIGRATING MOTOR COMPLEX (MMC): Housekeeping function throughout the small intestine, to sweep bacteria aborally. o The MMC occurs post-prandially, after a meal. o MMC is caused by Motilin and Acetylcholine -- it is blocked by atropine. VOMITING: • VOMITING (EMETIC) CENTER: In the medulla. The following receptors feed into the vomiting center. o Chemical Trigger Zone: Floor of the fourth ventricle, controlled by higher centers.  Apomorphine is a drug that stimulates the chemical trigger zone. o Labyrinthine Receptors in the inner ear, effected by balance. o Touch Receptors in throat (as in gagging reflex) o Mechanoreceptors and Chemoreceptors in stomach and duodenum. o RETCHING AREA: That region of the brain responsible for the act of retching (reverse peristalsis). • OUTPUT: The vomiting act. Four muscle groups are stimulated in synchronous order to produce vomiting. o Four groups of muscles are stimulated: this is the process of retching, which is reverse peristalsis accompanied by relaxation of esophageal sphincters.  Inspiratory Muscles: Deep inspiration ------> negative thoracic pressure to facilitate upchucking.  Abdominal Muscles ------> positive intraabdominal pressure to facilitate upchucking.  Esophageal, gastric, and duodenal muscles all undergo reverse peristalsis.  Esophageal sphincter (LES and UES) must relax for vomiting to occur. o Massive Autonomic Discharge occurs with vomiting. combined sympathetic / parasympathetic on salivary glands causes hypersalivation SECRETIONS and ABSORPTION SALIVARY SECRETIONS: Average about 1500 mL a day. • Secreted Substances: Major function of saliva is protection and digestion. o Salivary Amylase: Secreted primarily by Parotid gland.  Amylase normally operates at pH 7-8 and is therefore inactivated once in the stomach. However, if it is inside a bolus of food and protected on all sides then it can still be active even in stomach. o Mucus: Secreted by the other glands (Mandibular and sublingual).  FNXN: Lubrication of food and it serves as a buffer. o Lactoferrin: Binds Fe in mouth, preventing bacteria from getting it. It thereby serves as an antibacterial role. www.brain101.info7 o Lingual Lipase: Released from tongue itself, allows easy movement of fats on the tongue.  It can serve a backup function in case pancreatic lipase is lacking. o Secretory IgA: Antibacterial secretions. o Lysozymes: Antibacterial secretions. • XEROSTOMIA: Dry mouth. It can lead to caries (cavities) because the anti-bacterial salivary