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PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler DIGESTION NTC Dr. Ann Wechsler Room 1135 McIntyre 398-4341 [email protected] Lecture 01 – March 22th 2006 GIT role in homeostasis The main reason that systems fct the way they do is to maintain homeostasis. It insures balance. We will examine the way the GIT maintains a constant supply of nutrients. Provide nutrients - We obtain food from the external environment (food = large particles) digestive system converted into absorbable molecules - in order to be absorbed, food must be converted into smaller molecules (monosaccharide, amino acids, lipids,…) they enter the internal environment (circulation) further processed and they are ultimately distributed to cells, which use to provide the energy (ATP) and raw materials for growth & repair and permits function & regulation of this latter which are all derived from food So, this is the way the GIT contributes to homeostasis. * We will examine what becomes of food within the digestive system; how is the food converted into absorbable molecules and how do they enter the internal environment? GI structure External environment earthworm internal environment GI tract: it is a tube, which extends from the anterior to the posterior end, and it communicates w/ the exterior environment at both ends. The lumen (central cavity of the GIT) is a continuation of the external environment. Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 1 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler There are 2 lines of development. 1- Growth (enormous elongation and expansion of the tube) 2- Differentiation (diff. regions of the tube become specialized to perform diff. digestive functions) * The tubular nature as well as the communication w/ the external environment is always maintained throughout these 2 steps. These two elements are conserved thru growth and differentiation a) Tubular nature b) Communication w/ external environment Growth 1 – Elongation of the GI tube In mammals (including humans), the tube is much longer than the trunk and even the height of the individual The GI tract has a length of 4.5m in an adult human (1.5 m) In the cadaver, the length of the adult GI tract is 9m. It is only half that length in a living adult b/c the muscular elements in the wall of the GIT are always in partial contraction, making the tube shorter. Growth 2 – of the internal surface area of the tube Lumen – Central cavity * (continuation of the external environment) The internal lining has many folds and outpushings, inpushings. The result: internal surface area is 600 X greater than the external surface area of the tube, reaching up to 200-250 m2 (about the size of tennis court). It allows for efficient absorption. * Surface area is a major component in allowing efficient diffusion. Different iation Instead of having a simple tube, there are specialized cells that differentiate so that they can perform different fcts of the digestive system efficiently. We therefore have a sequence of organs (mouth, esophagus, stomach, small intestine, large intestine, colon, rectum, and anus). Notice the maintenance of the connection w/ exterior and of the tubular nature. There are 3 major accessory structures associated w/ GIT (from a functional and Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 2 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler developmental point of view): Salivary glands – release the product into the oral cavity Liver – largest organ in the body; performs large # of functions, including release of digestive secretions. Pancreas – endocrine and exocrine gland Anatomy of the GI tract wall – Vander’s p.581 If we took a cross-section b/w mid-esophagus and terminal portion of the large intestine, you find same typical 4 layers that make up the wall. There are quantitative and qualitative changes that occur. If you start from the outside of the tube towards the lumen serosa - Thin but tough layer of connective tissue. It surrounds the tube. In the abdominal tract, it is continuous w/ the membrane lining the abdominal cavity. muscularis externa layer of muscle (consists of 2 layers of muscle) 1- longitudinal fibers - outer layer of longitudal fibres = parallel to the tube) (when it contracts, the GIT shortens), 2- Circular fibers (innermost, wound around the tube, at right angles to the tube, making the lumen narrow) inner layer circular fibres (when these fibers contracts, lumen narrows) - musculature in oral cavity, pharynx, upper 1/3 esophagus and external anal sphincter are made up of striated (or skeletal muscles) The rest of the GIT musculature is smooth muscle. There is no skeletal type muscle in this area. Smooth muscle has specific properties (to be discussed later on) submucosa – loose connective tissue, housing neuronal network (nerve ganglia and fibers), lymphatics, blood vessels mucosa - innermost, which consists of 3 layers muscularis mucosae – muscular; outermost layer of the mucosa; wherever the musculature of the GIT is smooth in the muscularis externa, you will also find smooth muscularis mucosae - lamina propria (loose connective tissue) – Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 3 PHGY 210 – Digestion Lecture01 - Wednesday, March 22, 2006 Ann Wechsler Connective tissue. Contains immune cells – these cells play important role against invasion of microorganisms from external environment. epithelial layer (secretory – exocrine and endocrine & absorptive cells) – critical role in secretion (enzymes, into lumen) & it also contains cells where absorption takes place. * There may quantitative and qualitative differences as you go down the GIT, but all these layers are typically recognized. GIT functions ROLE OF GIT: To convey food along the GIT, allowing it to be disrupted into small molecules/particles, so that they may be absorbed into circulation. * Absorption is the raison d’être of the GIT in all its complexities 3 principal activities of the GI tract: 1. Motility (Muscular activity in the wall = contractile activity): - Propulsion (allows food to move along the tract) - Physical breakdown (disruption of large masses into smaller particles) 2. Secretion (Glandular activity of the epithelial cells + liver and pancreatic cells): Chemical breakdown – production of fluids containing enzymes, produced by glands that permit chemical breakdown of food into small molecules. The term digestion refers to the chemical breakdown of the food into progressively smaller molecules 3. Absorption – Transfer of molecules from the lumen of the tract into the circulation, where it will be distributed to the cells of the body. Digestive / Absorbtive efficiency (Vander’s, pp.588-590) Almost everything we eat is digested and ultimately absorbed: * This efficiency results from the fact that the 3 activities of the GI tract is highly integrated by NEURAL and HORMONAL activity. Enteric innervation (ENS) The GI tract has its own innervation (ENS), such that it can fct independently from the CNS or ANS. This innervation allows for the integration and regulation of all the aforementioned activities. GIT is capable of initiating activity Activities of muscular and secretory elements (within a particular organ): initiates, programs, regulates, coordinates activities of muscular and secretory elements within particular organs. GI tract wall innervation Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 4 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler The ENS manifests itself as a huge number of neurons and interconnected fibers found in ganglia, which are organized into 2 plexuses. * Plexus: integrated collection of ganglia 1- Submucosal plexus – in the submucosa 2- Myenteric plexus - b/w the circular and longitudinal muscle Structurally they are different; but for our purposes, the plexuses are considered as 1: ~ enteric plexus/innervation - a functional unit that comprises the submucosal and myenteric plexus. This is b/c there are too many connections b/w the two to distinguish them one from the other, from a functional stand. Within the enteric innervation, we find all the elements of the reflex arc. Sensory neurons (purple): can bring info from receptors (located in either the mucosa → chemoreceptors, osmoreceptors or mechanoreceptors [stretch receptors] of the muscle). Effector neurons (red): can activate secretory cells in the mucosa, or muscular cells in either layer of the smooth muscle. Large # of interneurons: integration of activity in a given region of the GIT. If you stimulate at one point: 1- Activation a particular sensory fiber 2- This sensory fiber activates an effector fiber - Cause activation of a muscle cell. * B/c of the presence of interneurons, there is also activation of an effector neuron that may cause contraction in the longitudinal fibers (for example). ** It can activate enteric effector neurons which may cause secretion at another point. So, you can cause secretion and contraction at a slight distance. Whereas many of the effector neurons are excitatory (cause contraction or secretion by mucosal cells), many of the enteric nerve fibers are inhibitory (orange). * The predominant innervation of the circular layer of muscle is inhibitory. Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 5 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler Summary: - ENS = 2 diff. plexuses (anatomically distinct, behave as a functional unit) Include all the elements required for reflex arcs - Sensory neurons, effector neurons and interneurons. - ENS consists of ganglion cells and their processes. They synapse w/ smooth muscle cells, endocrine and exocrine cells and other ganglion cells. - Some enteric neurons are excitatory (release mostly Ach – acting on muscarinic receptors on both smooth muscle and secretory cells) - Some are inhibitory (release NANC ~ non adrenergic, non cholinergic transmitters) * Also present are enteric sensory fibers w/ cell bodies in plexuses Enteric neurons and transmitters ACh (Muscarinic receptors) blocked by atropine - the enteric innervation involves excitatory enteric neurons that release ACh, acting on the muscarinic receptors on the muscle (contract) or on secretory cell. NANC Non-adrenergic non-cholinergic - The muscle cell may also be innervated by an inhibitory enteric neurons. It releases NANC (non-adrenergic, non-cholinergic neurotransmitter). Nitric Oxide (NO) is a very common inhibitory neurotransmitter; there is also peptide. There might even be some Purine like ATP that can also act like a neurotransmitter for these neurons in the gut. Short /intramural reflexes The enteric innervation has all the elements for the reflex arc: These are known as short/intramural reflexes. Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 6 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler Stimulus Activation of chemoreceptors, osmoreceptors & mechanoreceptors Sensory fibers activate nerve plexus Efferent enteric neurons smooth muscle or gland Response * Although the gut can fct when separated from the CNS and ANS, there is normally an intact ANS and CNS Autonomic innervation of GIT – Vander’s pp.199-202 The gut wall receives innervation from CNS via the ANS a) Parasympathetic (preganglionic): Synapse ONLY w/ enteric neurons (not with muscle or anything else) by releasing ACh (acts on nicotinic receptors) b) Sympathetic (postglanglionic) – There is a synapse within a ganglia OUTSIDE the gut wall. The postganglionic cell synapse also w/ the enteric neurons only. Autonomic innervation of the gut wall (diagram not to memorize) Sympathetic Emerges form the spinal cord. There are synapses and the ganglia outside the wall of the GI tract = postganglionic fibers innervate it Parasympathetic Vagus X (main parasympathetic nerve that reaches the wall as preganglionic fibers) – esophagus, stomach, small intestine & colon Pelvic nerves – takes over the parasympathetic function at colon & rectum. Elements present in the ENS: sensory, effector and interneurons. *There are also many sensory fibers sending CONSTANTLY info from the gut to the CNS (medulla or the spinal cord). Parasympathetic innervation = Preganglionic fibers that synapse w/ enteric neurons on plexuses and release ACh acting on nicotinic receptors . Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 7 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler Sympathetic innervation = Postganglionic fibers (presence of a synapse in a ganglia outside the gut wall) that release NA (noradrenaline) in the vicinity of enteric neurons. * Basis for long reflexes – Stimulation of a receptor causes a lot of integration. There are effector fibers that can change the activity of the enteric neurons, either via sympathetic or parasympathetic innervation. Enteric Neurons & Autonomic Innervation - of muscle or glandular cells Parasympathetic (EXCITATORY): 1 – Preganglionic neurons releasing ACh acting on nicotinic receptor of enteric neurons 2 – The enteric neurons release ACh acting on muscarinic receptors present on effector cells *Parasympathetic can also act on inhibitory interneurons. In this case, it activates these neurons to release NANC transmitters to the receptor cells. Sympathetic (INHIBITORY): Postganglionic cells releasing NA Effects: a) on excitatory enteric neurons (decreases the excitation of enteric neurons and, thus, diminish the contractile activity of the muscle) b) on inhibitory enteric neuron (diminish the inhibition of effector cells by enteric inhibitory cells. Thus, the effector cells will be more active = EXCITATION) * When you stimulate via ANS, you can stimulate long lengths of the GIT Long reflexes: Receptors on gut wall send info to the CNS that react to these stimulus by sending efferent Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 8 PHGY 210 – Digestion Lecture01 Wednesday, March 22, 2006 Ann Wechsler info to the GI plexuses via the ANS (Sympathetic/Parasympathetic) * Many emotional states, as well as sensory (visual, sound, taste) also modulate or modify the activity of the enteric neurons via ANS. ANS role on the activity of ENS Allows for integrated activity over longer distances along gut Permits long and extrinsic reflexes. In general – Parasympathetic: Excitatory (may also excite inhibitory neurons) - Sympathetic: Inhibitory (may also inhibit inhibitory neurons) Hormonal regulation of gut activity – Vander’s pp.589-590 Gut is regarded as the largest and most diversified endocrine system in the body. There are a variety of substances that regulate it; DES = diffuse endocrine system (scattered in mucosa) There are 3 types of glandular cells: Autocrine regulates itself Paracrine regulates nearby cell Endocrine a glandular cell releases its product into the circulation (capillary) modulates activity of cell at a distance Gut regulatory peptides No steroid hormones released in the gut 1. released from mucosa into portal blood liver systemic circulation 2. have multiple targets (released by gut and affects nearly any muscular or glandular cell): excitatory & inhibitory 3. interact w/ one another and w/ neurotransmitters a) synergistically (enhance each other’s activity) b) antagonistically (decrease activity) * A number of peptide agents are released from endocrine cells in the mucosa of the stomach and mechanical stimulation, coincident w/ intake of food Released into the portal circulation, the gut peptides pass thru the liver to the heart, and back to the digestive system to regulate its mvts and secretion Summary of GI tract regulation There are 3 diff. component participating in the regulation of the GI tract activity: 1. Short enteric (intramural) reflexes (key regulation) 2. Long extrinsic (ANS) reflexes Regulate the activity 3. Hormonal factors of the short reflex Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh page 9