Gastrointestinal system Xiang-Yao LI, PhD Department of Physiology Zhejiang University School of Medicine Email: lixiangy@zju.edu.cn

Accessory Digestive Organs Alimentary Canal Continuous, muscular digestive tube winding throughout the body Digests and absorbs food particles Contains the following organs: Mouth, Pharynx, Esophagus, Stomach, Small Intestine Large Intestine Accessory Digestive Organs Teeth, Tongue, Salivary Glands Liver, Gallbladder, Pancreas

General properties of gastrointestinal smooth muscle Low excitability High distensibility Tonic contraction Autorhythmicity High sensitivity to temperature, stretch and chemical stimulation

Electrophysiological properties of gastrointestinal smooth muscle Resting membrane potential -40~-80 mV Ionic basis Em (selective membrane permeability to K+, Na+, Cl- and Ca2+) Electrogenic Na+-K+ pump

Slow wave (basic electrical rhythm) The spontaneous rhythmic, subthreshold depolarizations of the cell membrane (slow wave) of the gastrointestinal tract that characterizes the underlying electrical activity of the bowel Initiated in the interstitial cells of Cajal (ICC) (pacemaker cell)

Santiago Ramon Y Cajal He and Camillo Golgi received the Nobel Prize in 1906 for introduction of the silver-chromate stain

Slow wave (basic electrical rhythm) Intensity: 10~15 mV Frequency: 3~12 cpm Ionic mechanism spontaneous rhythmic changes in Na+-K+ pump activity

Normal BER frequencies in the gastrointestinal system

Spike potential (Action potential) Duration: 10~20 ms Ionic mechanism: Depolarization: Ca2+ influx Repolarization: K+ efflux

The digestion and absorption Carbohydrate About 250-300g/day: Plant polysaccharides starch (2/3), Disaccharides: sucrose, lactose (1/3) Mouth(5%) + Small intestine (95%) Amylase Brush border enzymes Cellulose (fiber) Large intestine Bacteria

Proteins

Proteins

Fat

Fat

Neural control of gastrointestinal function Enteric nervous system (intrinsic) Autonomic nervous system (extrinsic)

Enteric (Intrinsic) nervous system Myenteric plexus (Auerbach’s plexus) Submucosal plexus (Meissner’s plexus) Neurotransmitters secreted by enteric neurons Ach, NE, ATP, serotonin, dopamine, cholecystokinin, substance P, vasoactive intestinal polypeptide, somatostatin, leu-enkephalin, met-enkephalin, bombesin, etc.

Autonomic nervous system Parasympathetic nerve Mainly ACh Stimulatory (+) Sympathetic nerve NE Inhibitory (-)

Afferent sensory nerve fiber from the gut Sensory fibers with their cell bodies in the ENS terminate in the ENS Sensory fibers with their cell bodies in the ENS send axons upward through the ANS to terminate in the prevertebral sympathetic ganglia Sensory fibers with their cell bodies in the dorsal root ganglia or in the cranial nerve ganglia send axons to multiple area of the spinal cord or brain stem

Gastrointestinal reflexes Three types Reflexes that are integrated entirely within the enteric nervous system Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract

Gastrointestinal hormones The hormones synthesized by a large number of endocrine cells within the gastrointestinal tract Physiological functions Control of the digestive function Control of the release of other hormones Trophic action

Gastrointestinal hormones Four main types Gastrin Secretin Cholecystokinin (CCK) Gastric inhibitory peptide (GIP)

Digestion in the stomach

Difficult to see food bolus (raw liver Difficult to see food bolus (raw liver?)– can a brighter color be used (blueberry yogurt)? The swallowing reflex is coordinated by the medulla oblongata, which stimulates the appropriate sequence of contraction and relaxation in the participating skeletal muscle, sphincters, and smooth muscle groups.

The coordinated sequence of contraction and relaxation in the upper esophageal sphincter, the esophagus, and the lower esophageal sphincter is necessary to deliver swallowed food to the stomach.

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