Git Physiology Chapter No 62 Guyton By Dr. Roomi 335ye

Physiology of GIT Chapter No=62 By Dr. Mudassar Ali Roomi (MBBS, M.Phil.) Assistant Professor Physiology

GIT •

Function: The GI tract provides a continuous

supply of water, nutrients and electrolytes to the body.

•

To achieve this target we need: 1. 2. 3. 4. 5.

Movement (Motility) Secretion of glands Absorption of nutrients Circulation of blood (splanchnic circulation) Control of all these functions (nervous and hormonal)

Two Types of Digetsive Systems • Incomplete digestive system – One-way, saclike digestive cavity • Complete digestive system – A tube with an opening at each end

Major Components of GIT • • • •

Mouth (oral cavity) Pharynx (throat) Esophagus (food pipe) Gut – Stomach – Small intestine • Duodenum • jejunum • ileum

– Large intestine • • • • •

Cecum appendix colon Rectum Anus

Accessory Organs • Salivary glands (sublingual, submandibular and parotid glands)

– Secrete saliva. • Pancreas: – Secretes digestive enzymes. • Liver: – Secretes bile. • Gallbladder: – Stores and concentrates the bile.

Typical cross section of intestine •

A typical cross section shows: 1. Mucosa. 2. Submucosa. 3. Muscularis externa: – –

Circular muscle layer. Longitudinal muscle layer.

4. Serosa.

Mucosa: First Layer (innermost layer) • Mucosa= epithelium and lamina propria • Small amount of smooth muscle (muscularis mucosae) is also present • Lumen: ageway. • Epithelium may have mucus secreting glands (also secrete digestive enzymes) • Secretes and absorbs.

Second Layer: Submucosa • Contains loose Connective Tissue. • Contains Glands, blood vessels, lymphatics and nerves. • Carries away absorbed materials.

Third Layer: Muscularis externa • Provides the tube movements (peristalsis) • Has two coats of smooth muscle tissue: – Inner Circular fibers (increase and decrease tube diameter) – Outer Longitudinal fibers (lengthen and shorten the tube)

Fourth Layer: Serosa (serous layer) • Outer covering of the tube. • Visceral peritoneum. • Secretes serous fluid to keep the outside of the organs lubricated and moist, thus performs a protective function.

Neural Control of the GI Function •

Neural Controls:

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–

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Extrinsic (3)

• • •

Parasympathetic. Sympathetic. Somatic.

Intrinsic (2)

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Myenteric plexus. Submucosal plexus.

SANS and PANS modulate the enteric nervous system as opposed to directly controlling the smooth muscle of the bowel.

Extrinsic control Autonomic Neural Pathways: Parasympathetics • Upper/Cranial via Vagus nerve: – Innervates the upper segments of GI tract to Splenic flexure along with the pancreas. • Lower/Sacral via Pelvic Splanchnic nerves (nervi erigentes) – S2-S4 to the descending colon, rectum and anus. – Post Ganglionic Neurons: • Located mainly in the Myenteric and Submucosal plexuses.

– Neurotransmitter: • Acetylcholine. – Function: – Stimulates GI secretion, motor activity. – Relaxes sphincters

Extrinsic control Autonomic Neural Pathways: Sympathetics • Fibers originate in the spinal cord between segments T-5 and L-2. • Preganglionic fibers enter the Sympathetic chains, then on to Coeliac and Mesenteric ganglia (PREVERTEBRAL ganglia)

– Post Ganglionic Fibers: • They originate in these ganglia and on to all parts of the GIT through sympathetic nerves.

– Neurotransmitter: • Norepinephrine. • Some amounts of Epinephrine.

– Function: • Inhibition of GI secretion, motor activity. • Contraction of GI sphincters and blood vessels.

Extrinsic control Neural pathways: Somatic fibers

• Along Pudendal Nerve:

– S2-S4. – External anal sphincter and the pelvic floor.

Intrinsic/ Enteric Nervous System: – Submucosal (Meissner’s) plexus. – Myenteric (Auerbach’s) plexus.

• Regulate segment-tosegment movement of the gastrointestinal tract. • May be considered a 3rd part of the ANS.

Enteric Nervous System Myenteric plexus (Auerbach’s plexus): – Located between the longitudinal and circular layers of muscle in the wall of the GIT. – Controls tonic and rhythmic contractions. – Exerts control primarily over digestive tract motility. – Principal effects: • Increased tone. • Increased intensity of contractions. • Increased rate of contractions. • Increased velocity of conduction.

Myenteric plexus (Auerbach’s plexus): • It is not totally excitatory in nature. – Some of the neurons of the myenteric nervous system secrete VIP. This is an inhibitory peptide. – These inhibitory signals are useful for controlling the intestinal sphincters e.g. pyloric sphinter

• Partially controlled by autonomic nervous system.

Enteric Nervous System Submucosal plexus (Meissner’s plexus) – Buried in the Submucosa. – Senses the environment within the lumen. – Regulates GI blood flow. – Controls epithelial cell function (local intestinal secretion and absorption). – May be sparse or missing in some parts of GI tract.

• Partially controlled by autonomic nervous system.

Enteric Nervous System •

Types of Neurons in the Enteric System: 1. Sensory neurons: – Chemoreceptors sensitive to acid, glucose and amino acids have been demonstrated which, in essence, allow "tasting" of luminal contents. Sensory receptors in muscle respond to stretch and tension.

2. Motor neurons: • Control GI motility and secretion, and possibly absorption. 3. Interneurons: • Largely responsible for integrating information from sensory neurons and providing it to motor neurons.

Types of Enteric Neurotransmitters in enteric neurons – Acetylcholine: • Excitatory – – – –

Stimulates smooth muscle contraction. Increases intestinal secretions. Releases enteric hormones. Dilates the blood vessels.

– Norepinephrine: • Derived from extrinsic sympathetic neurons. • Inhibits the GI activity. • Causes vasoconstriction

– Others (some are excitatory some are inhibitory) • Adenosine tri phosphate (ATP), Serotonin, Dopamine, Cholecystokinin, VIP, Somatostatin, Substance P, Leu-enkephalin, Met-encephalin and Bombesin.

Electrical activity in the Smooth Muscle of GIT • Smooth muscle in the GIT is excitable. • Slow, intrinsic electrical activity takes place along the membranes of this smooth muscle. • The GI smooth muscle acts as a functional syncytium. – Due to presence of Gap junctions.

• Therefore, whenever an action potential is generated within the muscle mass, it travels in all directions.

Electrical activity in GIT • The RMP is -50 to -55 mV. This can be changed to different levels to control the motor activity of the gut. • There are two types of electrical waves: – Slow waves– Spike potentials.

Slow Waves/ Basic Electrical Rhythm of the gut. • These are spontaneous, rhythmic fluctuations in the RMP between -55 to 40 mV. • These are not true action potentials, but are localized/graded potential • They do not cause muscle contraction in the GIT. • Frequency of slow waves in different parts of gut: – 3/ min in the body of the stomach. – 12/ min in the duodenum. – 8-9/ min in the terminal ileum.

Cause of Slow Waves – Pacemaker cells/Interstitial cells of Cajal: – These pacemaker cells undergo cyclic changes in the membrane potential due to unique Na ion channels which periodically open and produce inward, pacemaker currants which in turn generate slow waves.

Slow Waves • Function:

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– They lead the RMP to threshold value so that the spike potential can take place. Stimulated by (which causes hypopolarization): – Stretch. – Acetylcholine. – Parasympathetic nervous system (vagus) – Parasympathomimetic drugs e.g. Pilocarpine

• Slow wave activity is greatly reduced by Vagotomy, Norepinephrine and sympathetics (which causes hyperpolarization)

Spike Potentials • These are true action potentials. • They occur whenever the RMP of the smooth muscle becomes more positive than -40 mV (threshold) • Higher the slow wave potential, greater is the frequency of the spike potentials • Each spike lasts for up to 10- 20 m sec which is 10- 40 times longer than the action potential in the nerves fibers.

• Ionic basis of action potentials/spike potential: – In GIT, the ion channels involved are slow Calcium- Sodium channels. – The slowness s for the long duration of the action potential in the GI smooth muscle.

Three types of Gastrointestinal Reflexes 1. Reflexes that are integrated entirely within the enteric nervous system. These are the reflexes which control: – Secretion. – Peristalsis. – Contractions etc.

2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract. – Gastrocolic reflex. – Enterogastric reflex. – Colonoileal reflex.

3. Reflexes from the gut to the spinal cord or brain stem and then back to the GI tract. – Reflexes from stomach and duodenum to the brain stem and back via the vagus nerve. – Pain reflexes. – Defecation reflexes.

Hormonal Control of the GI Motility • Following hormones affect the motility of the gut in addition to their digestive functions: 1. 2. 3. 4. 5.

Gastrin. Cholecystokinin (CCK) Secretin. Gastric Inhibitory Peptide (GIP) Motilin.

1. GASTRIN: Secretion: By G cells in gastric pits of the mucosa. Stimulus: Stomach distention , the products of proteins and Gastrin releasing peptide, which is released by the nerves of the gastric mucosa as a result of Vagal stimulation. Actions: – – – – –

1. Increases HCl production in stomach. 2. Increases gastric motility. 3. Stimulates growth of gastric mucosa. 4. Contracts lower esophageal sphincter. 5. Relaxes pyloric sphincter.

2. CHOLECYSTOKININ (CCK): Secretion: By I cells in the mucosa of the duodenum and jejunum. Stimulus: Chyme rich in digestive products of triglycerides and fatty acids and monoglycerides and amino acids. Actions: – Contracts the gallbladder. – Opens the Sphincter of Oddi. – Increases secretion of pancreatic juice rich in digestive enzymes. – Inhibits gastric secretion and motility. – May reduce hunger.

3. SECRETIN Secretion: By S cells in the mucosa of the duodenum. Stimulus: Acid chyme (H+), fatty acids in the small intestine causes secretion of Secretin. Actions: – Stimulates the watery secretion of pancreatic juice and bile that is rich in bicarbonate ions. MCQ – Inhibit production of HCl in stomach. – Promotes growth and maintenance of the pancreas. – Enhances the effects of Cholecystokinin (CCK)

4. Gastric Inhibitory Peptide (GIP)/Glucose dependent insulinotropic peptide Secretion: By mucosa of the upper small intestine. Stimulus: Chyme rich in fatty acids and amino acids, glucose Actions: – Inhibits gastric secretion and motility. – Stimulates release of Insulin by beta cells.

5. Motilin Secretion: By the cells of the upper duodenum. Stimulus: It is released during fasting. Actions: • Increases the gastrointestinal motility. • Causes formation of Interdigestive myoelectric complexes/Migratory motor complexes (MMC) in a fasting person. • House keeping function

Functional Types of Movements in the GIT 1. Propulsive movements/peristalsis 2. Mixing movements. • Both are brought about by the Enteric nervous system, but are influenced by the Extrinsic nervous system, especially the Parasympathetic nervous system.

Propulsive movements/peristalsis • It is an inherent property of many syncitial smooth muscle tubes. • Occurs in the gut, bile ducts, glandular ducts and ureters. • Stimulus: – Distension of the gut, physical or chemical irritation of the surface epithelium and stimulation by the Parasympathetic nervous system.

• Controlled by: – Actual peristalsis is brought about by the Myenteric nervous system. – But it is influenced by the Parasympathetic nervous system.

Propulsive movements/peristalsis • Mechanism of myenteric reflex or the peristaltic reflex: – Distension of the gutStimulation of the enteric nervous system. – The gut wall contracts 2-3 cm behind the point of stimulusformation of a contractile ring. – Initiation of a wave of contraction- travels in the anal direction. – Relaxation of the gut wall distal to the point of stimulation- receptive relaxation.

• Function of the Myenteric plexus in peristalsis: – Weak or no peristalsis in areas of the gut where the Myenteric plexus is absent ( e.g. congenital megacolon/Hirschprung’s disease) – If the Cholinergic nerve endings of the Myenteric plexus are blocked by Atropine- No peristalsis. – Therefore, effectual peristalsis requires an intact functional Myenteric plexus.

Properties of peristalsis: 1. Directional movement: – – – –

Theoretically, it can occur in both the orad and anal directions. But the wave of peristalsis dies out if it travels in the orad direction. Exact reason is not known. Probably because of the fact that the Myenteric plexus is itself polarized.

2. Receptive Relaxation: – Part of the gut immediately after the peristaltic ring relaxes to allow the easy movement of food. – Is a function of the Myenteric plexus. – This relaxation takes place only the anal direction, ahead of the contractile ring. – Therefore, peristalsis moves towards the anus.

3. The anal ward moving complex in association with the Myenteric plexus is called the Myenteric/ peristaltic reflex. – The peristaltic reflex Plus the anal direction of the movement is called the Law of the Gut.

Mixing Movements • Differ in most parts of the gut. – Segmentations- in small intestine – Haustrations- in large intestine

• In many cases, most of the mixing is provided by peristalsis itself, especially when it pushes the food against a closed sphincter. • Local , intermittent, constrictive contractions occur at some places in the gut. They are present after every few cms. • They last for 5 to 10 secs. • Then new contractions appear at new places. • This pattern is called chopping and shearing of food.