The Digestive System Sites of Secretion Digestion and

Sites of Secretion, Digestion, and Absorption

一、消化道平滑肌的特性（Physiological properties of gastrointestinal smooth muscle） n General properties – 兴奋性较低（ Low excitability） –

电生理特性（ Electrophysiological properties） n Resting potential: – between -50 and 60 mv – Ionic

慢波电位、基本电节律 (Slow wave, basic electrical rhythm, BER) The spontaneous rhythmic, subthreshold depolarizations of the

动作电位 (Spike potentials) n Action potentials • only at the peaks of slow waves

The higher the slow wave potential rises, the greater the frequency of the spike

Neurotransmitters secreted by enteric neurons（肠神经元分泌的递质） n n n Ach: Stimulatory NE: inhibitory Others: Substance

Neurotransmitters n n n Alpha-aminobutyric acid: relaxes LES Norepinephrine: decreases motility, increases contractions of

The ENS coordinates digestion, secretion, and motility to optimize nutrient absorption. Its activity is

Feedforward reflex: begin with stimuli (sight, smell), preparing the GI system for food that

四、胃肠激素-GI hormones n n The hormones synthesized by a large number of endocrine cells

Hormone Source Target Action Gastrin G cell, antrum Parietal/ ECL cell H+ secretion Secretin

第二节口腔内消化 Digestion in The Mouth n n n Mastication: chewing Deglutition Saliva –

1. moisten food ； 2. begin chemical digestion ( -amylase) 3. adjust appetite 4.

Secretion Saliva: water, ions, mucus, enzymes Acinar Cells Epithelial cells along the duct reabsorb

Control of salivary secretion Cholinergic and adrenergic neural pathways are the activators of regulated

Epiglottis blocks windpipe, prevents choking

The Esophagus （食道） n n n Long tube Connects pharynx to the stomach Peristalsis,

第三节胃内消化（Digestion in the Stomach） n n Storage capacity of ~4 cups Secretion of

盐酸分泌 Produce alkaline mucus that covers mucosa layer Synthesize and secrete the protease precursor

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The Digestive System

Sites of Secretion, Digestion, and Absorption

一、消化道平滑肌的特性（Physiological properties of gastrointestinal smooth muscle） n General properties – 兴奋性较低（ Low excitability） – 富有伸展性（High extensibility） – 紧张性收缩（Tonic contraction） –自动节律性（Autorhythmicity） – 对机械牵张、温度和化学刺激敏感（High sensitivity to temperature, stretch & chemical stimulation）

电生理特性（ Electrophysiological properties） n Resting potential: – between -50 and 60 mv – Ionic basis： selective membrane permeability to K+ –Electrogenic Na+K+ pump n n 静息电位：约-50~ -60 mv 离子机制：细胞膜主要对K+有通透性，生电性钠泵也起一定作用

慢波电位、基本电节律 (Slow wave, basic electrical rhythm, BER) The spontaneous rhythmic, subthreshold depolarizations of the cell membrane (slow wave) of the GI tract n Initiated in the interstitial cells of Cajal (ICC, pacemaker cell) n Intensity: 5~15 m. V. Frequency: 3~12 cpm, do not reach threshold with each cycle n Ionic mechanism: spontaneous rhythmic changes in Na+-K+ pump n n n 慢波或基本电节律：静息电位不稳定，缓慢周期性地自动去极化和复极化，起源于Cajial 间质细胞，产生基本电节律，经缝隙连接传至平滑肌细胞机制：钠泵活动周期性地减弱和增强

动作电位 (Spike potentials) n Action potentials • only at the peaks of slow waves • Threshold: – 40 m. V Duration: 10~20 ms –Ionic mechanism: • Depolarization: Ca 2+ influx • Repolarization: K+ efflux n n n 慢波电位不能引起平滑肌收缩，诱发收缩的是叠加在慢波上的动作电位 Ca 2+内流引起去极化， K+外流引起复极化 AP频率决定平滑肌收缩力

The higher the slow wave potential rises, the greater the frequency of the spike potentials

Enteric nervous system

Neurotransmitters secreted by enteric neurons（肠神经元分泌的递质） n n n Ach: Stimulatory NE: inhibitory Others: Substance P, Nitric oxide , Vasoactive intestinal polypeptide(VIP), Opioid peptide, serotonin, histamine, ATP… n n n Ach: 兴奋性 NE: 抑制性其它：P物质， NO， VIP，5 -HT, ATP, 组胺，阿片肽

Neurotransmitters n n n Alpha-aminobutyric acid: relaxes LES Norepinephrine: decreases motility, increases contractions of sphincters, inhibits secretions Acetylcholine: increases motility, relaxes sphincters, stimulates secretions Neurotensin: inhibits release of gastric emptying and acid secretion Neuropeptide-Y: stimulates feeding behavior

The ENS coordinates digestion, secretion, and motility to optimize nutrient absorption. Its activity is modified by information from the CNS and from local chemical and mechanical sensors.

Feedforward reflex: begin with stimuli (sight, smell), preparing the GI system for food that the brain is anticipating. .

三、消化道血液循环(Gastrointestinal circulation)

四、胃肠激素-GI hormones n n The hormones synthesized by a large number of endocrine cells within the GI tract Brain-gut peptides: a number of the classical GI hormones are also synthesized in the brain n 胃肠激素：消化道粘膜内分泌细胞合成和释放的多种肽类物质胃泌素族：主要包括胃泌素和缩胆囊素; 促胰液素族：主要包括促胰液素、胰高血糖素、血管活性肠肽、抑胃肽

Hormone Source Target Action Gastrin G cell, antrum Parietal/ ECL cell H+ secretion Secretin S cell, intestine Pancreatic ducts HCO-3 , fluid stomach H+, gastrin Cholecystokinin (CCK) I cell, duodenum pancreas, acinar enzyme secretion Gallblader contraction Gastrin-releasing peptide Vagal N. ending G cell Gastric inhibitory Peptide(GIP) K cell, duodenum Cells of endo- Exocrine Insulin release Motilin Endo. Cell, Upper GI tract GI muscle GI motility MMC Peptide YY Endo. Cell, ilium Stomach pancreas H+ secretion enzyme secretion Glucagon-like Peptide 1 Endo. Cell, intestine Ghrelin X/A cell, stomach NPY N. in ARC crine pancreas Endo. pancreas Gastrin-release Insulin glucagon Food intake

第二节口腔内消化 Digestion in The Mouth n n n Mastication: chewing Deglutition Saliva – 1. 5 liters of saliva daily – Salivary amylase begins to break down starch – Salivary lipase: has minimal effect – Mucus to lubricate the food for easier swallowing – Lysozyme to kill bacteria n n n 咀嚼吞咽唾液: 约分泌1500 ml/d 唾液淀粉酶、舌脂肪酶免疫球蛋白(Ig A)、溶菌酶(lysozyme)

1. moisten food ； 2. begin chemical digestion ( -amylase) 3. adjust appetite 4. Bacteriostatic action (bacteriolysin)

Secretion Saliva: water, ions, mucus, enzymes Acinar Cells Epithelial cells along the duct reabsorb Na + and secret K+

Control of salivary secretion Cholinergic and adrenergic neural pathways are the activators of regulated secretion by acinar cells • 交感神经兴奋：酶、粘液多的唾液分泌 • 副交感神经兴奋：水量多、有机物较少的唾液分泌

Epiglottis blocks windpipe, prevents choking

The Esophagus （食道） n n n Long tube Connects pharynx to the stomach Peristalsis, muscle contraction Lower esophageal sphincter and pyloric sphincter control entry and exit from stomach Heartburn

吞咽 Deglutition

第三节胃内消化（Digestion in the Stomach） n n Storage capacity of ~4 cups Secretion of hydrochloric acid, enzymes, including a protease, gastric lipase; mucus, the hormone gastrin, and intrinsic factor (B 12) Stomach secretes about 2000 -2500 ml of gastric juice daily n n 胃体含有壁细胞、主细胞、粘液分泌细胞、肠嗜铬细胞。胃体粘膜分泌盐酸、胃蛋白酶原、内因子、碳酸氢盐、粘液和组胺。胃窦部含有主细胞和内分泌细胞(G、D cells)，G、 D细胞分别分泌胃泌素和生长抑素

盐酸分泌 Produce alkaline mucus that covers mucosa layer Synthesize and secrete the protease precursor known as pepsinogen. Synthesize and secrete the HCl acid responsible for the acidic p. H in the gastric lumen.