Unit Twelve: Gastrointestinal Physiology

1. Unit Twelve: Gastrointestinal Physiology Chapter 62: General Principles of GI Function—Motility, Nervous Control, and Blood Circulation Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Alimentary Tract • Provides the Body with Water, Nutrients, Electrolytes, • and Vitamins By: • Movement of food through the alimentary tract • Secretion of digestive juices and digestion of the food • Absorption of water, various electrolytes, vitamins, • and digestive products • Circulation of blood through the GI organs to carry • away the absorbed substances • Control of all these functions by local, nervous, and • hormonal systems

3. Alimentary Tract Fig. 62.1

4. General Principles of GI Motility • Physiologic Anatomy of the GI Wall- Layers from • the outer to inner • Serosa • Longitudinal smooth muscle layer • Circular smooth muscle layer • Submucosa • Mucosa

5. General Principles of GI Motility • Physiologic Anatomy of the GI Wall Fig. 62.2 Typical cross section of the gut

6. General Principles of GI Motility • GI Smooth Muscle Functions As a Syncytium • Individual smooth muscle fibers are 200-500 um • in length, 2-10 um in diameter, and arranged in • bundles containing as many as 1000 fibers • Fibers are electrically connected through large • numbers of gap junctions allowing rapid movement • of electrical signals for contraction • Muscle bundles fuse with each other at many points • so in reality each layer is a branching latticework • of smooth muscle bundles

7. General Principles of GI Motility • GI Smooth Muscle Functions As a Syncytium • When an AP is elicited anywhere within the • muscle mass, it generally travels in all directions • Electrical Activity of GI Smooth Muscle • Slow waves-most GI contractions occur rhythmically, • and this is determined mainly by the frequency of • slow-waves of smooth muscle

8. General Principles of GI Motility Fig. 62.3 Membrane potentials in intestinal smooth muscle.

9. General Principles of GI Motility • Slow Waves • Not APs, but slow undulating changes in the • resting membrane potential • Appear to be caused by interactions between • smooth muscle cells and the interstitial cells of • Cajal (act as electrical pacemakers for smooth • muscle cells • Do not cause muscle contraction by themselves • but excite the appearance of intermittent spike • potentials, which then excite the muscle

10. General Principles of GI Motility • Spike Potentials • True action potentials • Occur automatically when the resting • membrane potential of the GI smooth muscle • becomes more positive than -40 mV. • Last 10-40X as long in GI smooth muscle as in • large nerve fibers

11. General Principles of GI Motility • Spike Potentials • Channels responsible are calcium-sodium channels • Channels are much slower to open and close than • those of nerves

12. General Principles of GI Motility • Changes in Voltage of the Resting Membrane • Potential • Under normal conditions the resting potential is • -56 mV • Factors that depolarize • Stretching of the muscle • Stimulation by AcH (parasympathetic) • Stimulation by specific GI hormones

13. General Principles of GI Motility • Changes in Voltage of the Resting Membrane • Potential • Factors that hyperpolarize • Effect of epinephrine or norepinephrine • Stimulation of sympathetic nerves that • secrete mainly norepinephrine

14. General Principles of GI Motility • Calcium Ions and Muscle Contraction • Calcium ion, acting through a calmodulin mechanism • activate the myosin fibers, causing interaction with • the actin fibers to initiate contraction • Slow waves do not cause calcium ions to enter the • smooth muscle fiber (only sodium)-so no contraction • Spike potentials allow significant calcium to enter • and cause most of the contraction

15. General Principles of GI Motility • Tonic Contraction of Some GI Smooth Muscle • Tonic contraction is continuous and not • associated with the basic electrical rhythm of • the slow waves • Sometimes caused by continuous repetitive • spike potentials • Can be caused by hormones • Continuous entry of calcium in ways not associated • with changes in membrane potential

16. Neural Control of GI Function-Enteric Nervous System • Enteric Nervous System • Lies entirely within the wall of the gut • Composed of 100 million neurons • Composed of mainly two plexuses • Myenteric plexus-outer plexus between the • longitudinal and circular muscle layers • 2. Submucosalpleuus-lies in the submucosa

17. Neural Control of GI Function-Enteric Nervous System • Enteric Nervous System Fig. 62.4

18. Neural Control of GI Function-Enteric Nervous System • Sensory nerve endings that originate in the GI wall or • epithelium send afferent fibers to both plexuses as well • as • Prevertebral ganglia of the sympathetic system • To the spinal cord • In the vagus nerves all the way to the brain stem

19. Neural Control of GI Function-Enteric Nervous System • Differences Between the Myenteric and • Submucosal Plexuses • Stimulation of myenteric plexus causes • Increased tonic contraction of the gut wall • Increased intensity of rhythmical contractions • Slightly increased rate of the rhythm of • contraction • Increased velocity of conduction of excitatory • waves along the gut wall

20. Neural Control of GI Function-Enteric Nervous System • Differences Between the Myenteric and • Submucosal Plexuses • Some neurons of the myenteric are inhibitory • Submucosal plexus • Mainly concerned with controlling function within • the inner wall of each minute segment of the • intestine

21. Neural Control of GI Function-Enteric Nervous System • Types of Neurotransmitters Secreted by Enteric • Neurons • Acetylcholine-most often excitatory • Norepinephrine and epinephrine-most often • inhibitory • ATP • Serotonin • Dopamine • CCK • Substance P • Somatostatin • Enkephalins

22. Neural Control of GI Function-Enteric Nervous System • Autonomic Control of the GI Tract • Parasympathetic stimulation increases activity of • the Enteric Nervous System • Sympathetic stimulation usually inhibits GI tract • activity • By the direct effect of norepinephrine of smooth • muscle • By the inhibitory effects of norepinephrine on • the neurons of the Enteric Nervous System

23. Neural Control of GI Function-Enteric Nervous System • Afferent Sensory Nerve Fibers From the Gut- • cell bodies may be in the Enteric Nervous System or • in the dorsal root ganglia of the spinal cord; • stimulated by • Irritation of the gut mucosa • Excessive distension of the gut • Presence of specific chemicals in the gut

24. Neural Control of GI Function-Enteric Nervous System • Gastrointestinal Reflexes • Reflexes that are integrated entirely within the gut • wall enteric nervous system • Reflexes from the gut to the prevertebral sympathetic • ganglia and then back to the GI tract • Reflexes from the gut to the spinal cord or brain stem • and back to the GI tract

25. Neural Control of GI Function-Enteric Nervous System • Hormonal Control of GI Motility (Table 62.1)

26. Functional Types of Movements in the GI Tract • Propulsive Movements-Peristalsis Fig. 62.5 Peristalsis

27. Functional Types of Movements in the GI Tract • Propulsive Movements-Peristalsis • Usual stimulus is distension of the gut • Other stimuli can include chemical or physical • irritation of the gut or strong parasympathetic • stimulation • Function of the myenteric plexus-effectual • peristalsis requires a functional myenteric plexus

28. Functional Types of Movements in the GI Tract • Propulsive Movements-Peristalsis • Directional movement of peristaltic waves is • toward the anus • Peristaltic Reflex and the “Law of the Gut”- • alternating contraction and relaxation as peristalsis • occurs; the peristaltic reflex plus the direction of • movement is called the law of the gut

29. Functional Types of Movements in the GI Tract • Mixing Movements • Differ in different parts of the alimentary tract • Other than typical peristalsis, there is local • intermittent constrictive contractions • also, if peristalsis is blocked by a sphincter then • only churning occurs

30. Gastrointestinal Blood Flow- Splanchnic Circulation • Mixing Movements • Differ in different parts of the alimentary tract • Other than typical peristalsis, there is local • intermittent constrictive contractions • Also, if peristalsis is blocked by a sphincter then • only churning occurs

31. Gastrointestinal Blood Flow- Splanchnic Circulation Fig. 62.6 Splanchnic circulation

32. Gastrointestinal Blood Flow- Splanchnic Circulation • Anatomy of the GI Blood Supply Fig. 62.7 Arterial blood supply to the intestines through the mesenteric web

33. Gastrointestinal Blood Flow- Splanchnic Circulation • Effect of Gut Activity and Metabolic Factors on GI • Blood Flow • Blood flow in each area of the GI tract and layers of • the gut wall is directly related to the level of local • activity • Causes of increased blood flow during GI activity • Vasodilators released from the mucosa of the • intestinal tract during digestion (CCK, gastrin, • secretin, vasoactive intestinal peptide)

34. Gastrointestinal Blood Flow- Splanchnic Circulation • Release of kallidin and bradykinin • Decreased oxygen cocentration in the gut wall; • decrease in oxygen can lead to a fourfold increase • in adenosne (vasodilator)

35. Gastrointestinal Blood Flow- Splanchnic Circulation • Countercurrent Blood Flow in the Villi Fig. 62.8

36. Gastrointestinal Blood Flow- Splanchnic Circulation • Nervous Control of GI Blood Flow • Parasympathetic nerves increase local blood flow • and increases glandular secretion • Sympathetic causes intense vasoconstriction of • the arterioles and decreases blood flow • Sympathetic vasoconstriction allows skeletal • muscle and the heart to get extra flow when • needed