Overview of the Digestive System PDF

Overview of the digestive system Oleg Osadchiy 1 Learning objectives ✓ Outline the purpose of the process of digestion. ✓ Outline the main parts of the gastrointestinal tract. ✓ Describe the four basic processes of digestion (secretion, digestion, motility, and absorption). ✓ Explain the neural, end...

Overview of the digestive system Oleg Osadchiy 1 Learning objectives ✓ Outline the purpose of the process of digestion. ✓ Outline the main parts of the gastrointestinal tract. ✓ Describe the four basic processes of digestion (secretion, digestion, motility, and absorption). ✓ Explain the neural, endocrine, and paracrine mechanisms of the control of GIT function. 2 Why do we need the digestive system? ✓ The food we eat contains a variety of nutrients that are used as building blocks for body tissues. ✓ The food is also a source of chemical energy. ✓ However, most of the food consists of molecules that are too large to be used by the cells of the body. ✓ Therefore, foods must be broken into molecules that are small enough to enter the body cells. This is accomplished during the process of digestion. 3 Components of the digestive system. Structure of the digestive system ✓ Two groups of organs compose the digestive system – (i) Gastrointestinal tract (ii) Accessory digestive organs 5 Gastrointestinal tract ✓ GIT (or alimentary canal) is a continuous tube that extends from the mouth to the anus. The length of GIT is about 4.5 meters. ✓ GIT includes – (i) Mouth (ii) Pharynx (iii) Esophagus (iv) Stomach (v) Small intestine (vi) Large intestine. 6 Accessory digestive organs ✓ The accessory digestive organs include – (i) Teeth (ii) Tongue (iii) Salivary glands (iv) Pancreas (v) Liver (vi) Gallblader. ✓ Teeth aid in physical breakdown of food, and the tongue assists in chewing ans swallowing. Other accessory digestive organs produce or store secretions that aid in the chemical breakdown of food. 7 Basic digestive processes. Basic digestive processes. Basic digestive processes (i) Secretion is the addition of fluids, enzymes and mucus into the lumen of GIT. Each day, the cells that line GIT and accessory digestive organs secrete about 7 liters of solutions containing enzymes into the lumen of GIT. 10 About 7 liters of solutions containing enzymes are secreted daily into the lumen of GIT. Different GIT enzymes exhibit maximum activity at different pH levels. Basic digestive processes (ii) Digestion – a process that ensures breakdown of the ingested food into the small molecules. Mechanical digestion – the teeth cut food, and the contractions of the smooth muscle in the stomach and small intestine churn the food. Chemical digestion – large carbohydrates, lipid, and protein molecules are split into smaller molecules by hydrolysis. This is induced by digestive enzymes secreted by the salivary glands, stomach, pancreas, and small intestine. 13 Enzymes secreted in GIT (i) Proteases (ii) Carbohydrases (iii) Lipases (iv) Nucleases The enzymes induce hydrolysis, or chemical decomposition of the nutrients supplied with food. This results in formation of small molecules (monomers) that can be absorbed from GIT lumen to the blood. 14 Overview of the chemical digestion in GIT. An example of carbohydrate hydrolysis. Disaccharide maltose is broken down into two molecules of glucose upon an action of maltase. An example of protein hydrolysis. Polypeptide is broken down into single amino acids upon an action of protease. An example of fat hydrolysis. Lipid (or triglyceride) is broken down into single fatty acids and glycerol upon an action of lipase. Sources of digestive enzymes in GIT. Basic digestive processes (iii) Motility. Contractions of smooth muscle in the wall of GIT mix food and digestive secretions, and propel them toward the anus. 20 Structure of the gastrointestinal wall. Intestinal smooth muscle layers (electron micrograph). Note circular (below) and longitudinal (above) layers of the smooth muscle that are sandwiching the neurons of the myenteric plexus (asterisk). Types of contractions of GIT smooth muscle ✓ Phasic contractions are periodic contractions followed by relaxation. They are present in all parts of GIT involved in mixing food with digestive juices, and its propulsion. This type if contraction can be found in esophagus, gastric antrum, and small intestine. ✓ Tonic contractions maintain a constant tension of the smooth muscle (tone) without regular periods of relaxation. This type of contraction can be found in the upper region of stomach, as well as in GIT sphincters (lower esophageal, ileocecal, and anal sphincters). 23 Sphincter is a bundle of circular muscle that separates two adjacent regions of GIT. Location of sphincters throughout GIT. Contractile cells Autorhythmic cells Two types of smooth muscle cells in GIT. Motility of GIT smooth muscle ✓ The smooth muscle within the GIT wall consists of two types of cells – (i) Autorhythmic cells (pacemaker cells) (ii) Contractile cells ✓ Autorhythmic cells (aka interstitial cells of Cajal) are located between the longitudinal and circular muscle layers. ✓ Autorhythmic fibers are noncontractile cells. They generate cycles of alternating depolarization and repolarization, which are called slow wave potentials or the basic electrical rhythm. 27 Autorhythmic cell Contractile cell Motility of GIT smooth muscle ✓ The depolarizing phase of the slow wave is caused by the opening of Ca2+ channels (inward current). ✓ The repolarizing phase of the slow wave is caused by opening of K+ channels (outward current). ✓ If the slow wave depolarization reaches the threshold, an action potential (aka spike potential) is generated, which causes the smooth muscle to contract. 29 The amplitude of GIT smooth muscle contraction increases upon an increase in the frequency of spike potentials. The frequency of slow wave potentials varies in different parts of GIT, ranging from 3 to 12 per minute. ✓ The amplitude of the slow wave potentials is influenced by the ENS, ANS, and hormones. ✓ Acetylcholine increases the frequency of generation of spike potentials by autorhythmic cells, and stimulates the GIT motility. ✓ Adrenaline and noradrenaline produce the opposite effects. Contractile smooth muscle cells ✓ The smooth muscle cells are interconnected by gap junctions through which the action potentials can spread. So the smooth muscle behaves like a functional syncytium. ✓ When action potential is generated in one muscle fiber, it spreads to other fibers in the syncytium, so they contract in unison. 33 Basic digestive processes. Basic digestive processes (iv) Absorption – is the movement of the products of digestion from the lumen of GIT into the blood or lymph. Once absorbed, the nutrients circulate along the vascular system and can be used by the body cells. 35 Splanchnic circulation. Paracellular pathway Intestinal lumen Enterocyte Blood Transcellular pathway Tight junction Two pathways for absorption of nutrients in the small intestine. Passive transport ✓ Along the gradient. ✓ No energy is required. Passive transport ✓ Simple diffusion ✓ Facilitated diffusion Active transport ✓ Against the gradient. ✓ Requires energy (ATP). ✓ Involves transporters. Active transport ✓ Primary active. ✓ Secondary active. Mechanisms of absorption of nutrients in the small intestine. Sites of absorption of major nutrients. Most of absorption takes place in the small intestine. Control of GIT function. Three types of mechanisms that control GIT function. Neural control of GIT function. Neural mechanisms in GIT control ✓ GIT is innervated by – (i) an intrinsic set of nerves known as the enteric nervous system; (ii) an extrinsic set of nerves which are part of the autonomic nervous system. 43 Submucosal and myenteric plexuses in GIT wall Innervation of GIT. Structure of the gastrointestinal wall. Enteric nervous system ✓ ENS is the “brain of the gut”. It consists of 100 million neurons localized in the GIT wall. ✓ The neurons of ENS are arranged into two plexuses – the myenteric plexus and the submucosal plexus. ✓ The myenteric plexus, or plexus of Auerbach, is located between the longitudinal and circular smooth muscle layers of the muscularis externa. ✓ The submucosal plexus, or plexus of Meissner, is found within the submucosa. 46 Neural plexuses in GIT wall. Organization of the enteric nervous system. Enteric nervous system ✓ The plexuses of ENS consist of motor neurons, interneurons, and sensory neurons. ✓ Motor neurons of the myenteric plexus supply the longitudinal and circlar smooth muscle layers of the muscularis externa, so they control GIT motility. ✓ Motor neurons of the submucosal plexus supply exocrine and endocrine cells of GIT, so they control GIT secretion and absorption of nutrients. 49 Enteric nervous system ✓ Interneurons of ENS interconnect the neurons of the myenteric and submucosal plexuses. ✓ Sensory neurons in the submucosal plexus supply the mucosal epithelium, and receive information from chemoreceptors (respond to certain chemicals in the GIT lumen) and mechanoreceptors (activated by stretch of the GIT wall). 50 Submucosal and myenteric plexuses in GIT wall Innervation of GIT. Parasympathetic innervation of GIT. Parasympathetic control of GIT ✓ Although the neurons of ENS can function independently, they receive inputs from the parasympathetic and sympathetic divisions of the ANS. ✓ Parasympathetic innervation of GIT – (i) VIIth (facial) and IXth (glossopharyngeal) cranial nerves innervate salivary glands (ii) Most of GIT is supplied by the vagus nerve (X). (iii) Distal part of the large instestine is innervated by pelvic nerves from the sacral region of the spinal cord. 53 Postganglionic Preganglionic neurons of the vagus nerve and pelvic nerves synapse with parasympathetic postganglionic neurons located in the myenteric and submucosal plexuses. Parasympathetic control of GIT ✓ Postganglionic parasympathetic neurons can either (i) synapse with neurons of the ENS, or (ii) directly innervate smooth muscle and secretory glands within the wall of GIT. Stimulation of the parasympathetic nerves causes an increase in GIT secretion and motility. 55 Submucosal and myenteric plexuses in GIT wall Innervation of GIT. Sympathetic innervation of GIT. Sympathetic control of GIT ✓ Sympathetic nerves that supply GIT originate from the thoracic and upper lumbar regions of the spinal cord. That is where the preganglionic sympathetic neurons are localized. ✓ Preganglionic neurons send axons that synapse on postganglionic neurons, localized in prevertebral and paravertebral sympathetic ganglia. ✓ The neurons of the superior cervical sympathetic ganglion (paravertebral) innervate salivary glands and the upper part of esophagus. 58 Prevertebral sympathetic ganglia (i) Celiac ganglion innervates the lower part of esophagus and the stomach. (ii) Superior mesenteric ganglion innervates small intestine and the proximal part of the large intestine. (iii) Inferior mesenteric ganglion innervates the distal part of the large intestine. 59 Sympathetic control of GIT ✓ Like parasympathetic nerves, the postganglionic sympathetic fibers may (i) Synapse on the neurons of ENS located in the myenteric plexus and submucosal plexus, or (ii) Directly innervate digestive glands or smooth muscle. Stimulation of sympathetic nerves results in reduced motility and secretion of GIT. ✓ That is why emotions such as anger, fear, and anxiety may slow digestion. 60 GIT reflexes. GIT reflex is a response that is triggered by chemical or mechanical stimulation of receptors in GIT wall by the food (chyme), and results in changes of GIT motility or secretion. GIT reflex pathways ✓ The initial components of the GIT reflex pathway are the chemoreceptors and mechanoreceptors in GIT wall. ✓ The receptors send information to the sensory neurons within the ENS. ✓ Axons of the sensory neurons can synapse with other neurons located in ENS, CNS, or ANS, in order to send information about the content of GIT lumen and the degree of stretch of GIT wall. ✓ Neurons of ENS, CNS, or ANS then activate or inhibit the GIT glands and smooth muscle. 63 Long and short GIT reflexes. GIT reflex pathways ✓ If the reflex pathway is confined entirely within the GIT wall, then it is called a short reflex. ✓ If the reflex pathway involves not only GIT wall but also the CNS and autonomic nerves, then it is referred to as a long reflex. 65 Three types of mechanisms that control GIT function. Endocrine control of GIT. Structure of the gastrointestinal wall. Endocrine cell Endocrine effects in GIT. GIT hormones ✓ GIT mucosa contains endocrine cells which can secrete certain hormones. ✓ The hormones are released into the blood vessels in GIT wall, enter the portal circulation, pass the liver, and then enter the systemic circulation. ✓ The systemic circulation then brings a hormone to the target cell, which can be located in GIT (e.g. digestive glands, or GIT smooth muscle), or elsewhere in the body. 70 Four GIT hormones (i) Gastrin – produced by G-cells in the stomach. Stimulates gastric acid secretion. (ii) Secretin – produced by S-cells in the duodenum. Stimulates secretion of bicarbonates by pancreatic cells. (iii) Cholecystokinin – produced by I cells in duodenum and jejunum. Stimulates secretion of pancreatic enzymes and contraction of the gallbladder. (iv) Glucose-dependent insulinotropic peptide (GIP) – produced by endocrine cells in duodenum and jejunum. Increases insulin secretion by pancreatic cells, and inhibits gastric acid secretion. 71 Acid from stomach stimulates secretin release from endocrine cells in the duodenum Secretin is released into the bloodstream Secretin stimulates secretion of bicarbonates by pancreatic glands Hormonal action of secretin. Hormonal action of cholecystokinin. Three types of mechanisms that control GIT function. Paracrine control of GIT. Paracrine effects in GIT. Paracrine control in GIT ✓ Like hormones, paracrine agents are secreted by endocrine cells. ✓ However, in contrast to hormones, paracrine agents do not enter the systemic circulation, and act locally, i.e. within the same tissue that secretes them. ✓ Paracrines reach their target cells by diffusing short distances through interstitial fluid. Hence the site of their secretion is within a short distance from the site of their action. 77 Examples of paracrine agents in GIT (i) Somatostatin – secreted by D cells of the GIT mucosa. Inhibits secretion of GIT hormones, and inhibits gastric seceretion. (ii) Histamine – secreted by endocrine cells in the stomach mucosa. Stimulates gastric acid secretion. (iii) Prostaglandins – secreted by subepithelial myofibroblasts. Inhibit gastric acid secretion. (iv) Adenosine - secreted by various types of cells. Produces vasodilation (increases local blood flow). 78 After this lecture, you are able to ✓ Outline the purpose of the process of digestion. ✓ Outline the main parts of the gastrointestinal tract. ✓ Describe the four basic processes of digestion (secretion, digestion, motility, and absorption). ✓ Explain the neural, endocrine, and paracrine mechanisms of the control of GIT function. 80