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Gastrointestinal Physiology

Mrs C Mahachi

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Objectives of GIT course

General Instructional Objective • An understanding of basic gastrointestinal

physiology and it’s application– Motility– Digestion– Secretion – Absorption and general utilization of nutrients– Elimination/defecation

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Functions of Gastrointestinal organsSummary of motility, secretion, digestion, and absorption in

different regions of the digestive system

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Functions of GIT

• Motility: movement through the GI tract

• Digestion: breakdown of food

• Secretion and absorption: across the epithelial

layer either into the GI tract (secretion) or into

the blood (absorption)

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Other functions of GIT• Immune Response

– Produces acid in stomach which gives an inhospitable environment for microorganisms

– There are microorganisms that kill pathogenic microorganisms (Probiotics)

– GALT (gut-associated lymphoid tissue). Peyer’s patches are a component of GALT found in the lining of the small intestines.

– Peyer’s patches (which are secondary lymphoid tissue)and other gut-associated lymphoid tissue contain macrophages, dendritic cells, B-Lymphocytes, and T-Lymphocytes.

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GALT

• About 70% of the body's immune system is found in the digestive tract– Tonsils (Waldeyer's ring)– Adenoids (Pharyngeal tonsils)– Peyer's patches– Lymphoid aggregates in the appendix and large intestine– Small lymphoid aggregates in the oesophagus

• Lymphoid tissue accumulates with age in the stomach

– Diffusely distributed lymphoid cells and plasma cells in the lamina propria of the gut

Back of pharynx

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GALT

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How each GIT section is divided

• Intestinal tract functionally divided into segments by sphincters

• Helps restrict the flow of intestinal contents to optimize digestion and absorption

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Sphincter Locations

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• Upper Esophageal Sphincter (UES): – Made up entirely of skeletal muscle. – under tonic stimulation between swallows. Upon swallowing, the UES relaxes, allowing the food to move from the mouth and pharynx into the

esophagus.• B. Lower Esophageal Sphincter (LES):

– made entirely of smooth muscle – separates the esophagus from the proximal stomach. – The function of the LES is critical to preventing stomach acid from refluxing up into the esophagus.

• C. Pyloric Sphincter: – the distal stomach from the small intestine (duodenum). – regulates gastric emptying. – Dysfunction of the pyloric sphincter causes dumping of a high acid load into the small intestine

• may lead to duodenal ulceration and problems with digestion.

• D. Ileocecal Sphincter (Valve): – regulates the flow of food material from the small intestine into the large intestine.– Activity in this sphincter limits the movement of bacteria from the cecum to the ileum. – An incompetent sphincter is associated with intestinal bacterial overgrowth and malabsorption.

• E. Internal Anal Sphincter (IAS): – composed of smooth muscle – is important for initiating the defecation reflex. – Distension of the walls of the rectum cause the IAS to relax to facilitate defecation.

• F. External Anal Sphincter (EAS): – is composed of striated muscle – therefore is under somatic (voluntary) control. – the initiation of a defecation reflex by stretch of the IAS can be overcome through contraction of the EAS, a learned behavior (toilet training).

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Layers of the GIT wall

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Mucosa• Concerned with

– secretion of digestive juices – secretion of certain hormones– absorption of the various nutrients.

• Layers– Epithelial layer – made up of columnar cells

• Contains endocrine gland cells and exocrine gland cells and cells specialized for absorption of digested nutrients

– Lamina propria- connective tissue layer• Houses GALT

– Muscularis mucosa – smooth muscle layer• Circular and longitudinal muscle

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Mucosa

• It contains blood capillaries, lymph vessels. • Generally highly folded to increase surface

area for absorption– Ridges and valleys

• Degree of folding varies with region of tract– Greatest in the small intestine

• Pattern of folding can be modified by contraction of the muscularis mucosa

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Submucosa

• This is a dense/thick connective tissue layer• Provides distensibility and elasticity • contains

– larger blood – lymph vessels – network of neurons called submucous or

Meissner’s plexus.

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Muscularis externa

• Major smooth muscle coat of the GIT• An outer longitudinal layer and inner circular layer of

smooth muscle. – In between myenteric or Aurbach’s plexus.

• Contraction of circular muscles decreases diameter of the lumen– layer prevents food from traveling backward.

• Contraction of longitudinal muscles shortens tube– peristalsis

• Contraction of both muscles provides propulsive and mixing motions.

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Serosa or Adventitia

• An outer fibrous coating• A thin layer of connective tissue that in some

regions is also wrapped in a thin membrane of cells – connective tissue covering is called an adventitia

• Where there is an additional covering of a membrane of cells, the covering is collectively called a serosa. .– Secretes a serous fluid

• Lubricates and prevents friction between digestive organs and surrounding viscera

Structure of the GIT Wall

• Villi – – fingerlike projections extending from luminal surface

of small intestine: each villi surface is covered with layer of epithelial cells whose surface membranes form small projections called microvilli or the brush-border.

– This combination of folded mucosa, villi, microvilli increases small intestine surface area 600-fold.

• Total surface area of human small intestine is 300 m2 = area of tennis court

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Intestinal Villi

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Smooth muscle layout of the GIT

• Individual smooth muscle fibers in the gastrointestinal tract are – 200 to 500 micrometers in length – 2 to 10 micrometers in diameter, – arranged in bundles of as many as 1000 parallel fibers.

• Each bundle of smooth muscle fibers is partly separated from the next by loose connective tissue

• The muscle bundles fuse with one another at many points, – so that in reality each muscle layer represents a branching

latticework of smooth muscle bundles. • each muscle layer functions as a syncytium (when an action potential is

elicited anywhere within the muscle mass it generally travels in all directions);

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Smooth muscle layout of the GIT

• The distance that a muscle bundle travels depends on the excitability of the muscle; – sometimes it stops after only a few millimeters – other times it travels many centimeters or even the entire

length (longitudinal) and breadth (circular) of the intestinal tract.

• A few connections exist between the longitudinal and circular muscle layers, so that excitation of one of these layers often excites the other as well– E.g Gap Junctions that allow low resistance movement of

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SMOOTH MUSCLE OF G.I.

TWO SMOOTH MUSCLE CLASSIFICATIONS• Unitary type

- Contract spontaneously in the absence of neural or hormonal influence but in response to stretch (such as in stomach and intestine)- Cells are electrically coupled via gap junctions

• Multiunit type - Do not contract in response to stretch or

without neural input (such as gall bladder)

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SMOOTH MUSCLE OF G.I.-Contractions

• Phasic contractions

- periodic contractions followed by relaxation; such as in gastric antrum, small intestine and esophagus

• Tonic contractions

- maintained contraction without relaxation; such as in orad region of the stomach, lower esoghageal, ileocecal and internal anal sphincter

- not associated with slow waves

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SMOOTH MUSCLE OF G.I.

• Tonic contractions (continued):- Caused by:

• Continuous repetitive spike potential• Hormonal effects• Continuous entry of Calcium

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The Musculature of the Digestive Tract

• Three main muscle layers:– Longitudinal muscle layer– Circular muscle layer– Oblique muscle layer (stomach only)

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• Longitudinal Muscle:– Contraction shortens the segment of the

intestine and expands the lumen – Innervated by ENS, mainly by excitatory

motor neuron – Calcium influx from out side is important

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• Circular muscle:– Thicker and more powerful than longitudinal – Contraction reduces the diameter of the lumen and

increases its length – Innervated by ENS, both excitatory and inhibitory

motor neurons– More gap junctions than in longitudinal muscle– Intracellular release of Calcium is more important

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Coupling Trigger Contractions in GI Muscles

• Depolarization opens the voltage-gated Ca channels (electromechanical coupling)

• Ligands open the ligand-gated Ca channels (pharmacomechanical coupling)

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Slow Waves & Action potentials are Forms of Electrical Activity in GI Muscles

• Slow waves- slow, undulating changes in the resting membrane potential - Unknown cause– intensity usually varies between 5

and 15 millivolts,

- Responsible for triggering AP in G.I.- Interstitial cells of Cajal, ICCs (pacemaker)

• Lie at boundary between the longitudinal and circular smooth muscle layers

• ICCs are muscle-like cells– display autonomous activity

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SLOW WAVES

• Occur at different frequency – 4/min in stomach; – 12/min in duodenum; – 8/min in distal ileum; – 9/min in cecum– 16/min in sigmoid;

- May or may not accompanied by AP

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• The slow waves do not cause calcium ions to enter the smooth muscle fiber (only sodium ions)

• During the spike potentials, generated at the peaks of the slow waves, – action potentials occur– significant quantities of calcium ions do enter the

fibers and cause most of the contraction.

Smooth Muscle Excitation/Contraction Coupling

• Slow Waves cause “Spike Potentials” during Ach stimulation

• Spike Potentials lead to increased [Ca++]

• [Ca], hormones (through PIP2

pathway)– activate MLCK– phosphorylates MLC

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Slow Waves & Action potentials are Forms of Electrical Activity in GI Muscles Factors that depolarize the membrane:

– Stretching of the muscle – Ach– Parasympathetic stimulation – Hormonal stimulation

Factors that hyperpolarize the membrane:– Norepinephrine – Sympathetic stimulation

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CONTROL OF DIGESTIVE FUNCTIONS BY NERVOUS SYSTEM

• Autonomic nervous system (ANS) is divided into

- Parasympathetic

- SympatheticExtrinsic nervous system

CONTROL OF DIGESTIVE FUNCTIONSBY NERVOUS SYSTEM

• Parasympathetic Nerves:– Located in brain stem & sacral region– Projection to the G.I. are preganglionic efferents– Vagus & pelvic nerves– Vagus nerves synapse with neurons of ENS in esophagus,

stomach, small intestine, colon, gall bladder & pancreas– Pelvic nerves synapse with ENS in large intestine– Neurotransmitter is Ach

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• Sympathetic nerves:– Located in thoracic & lumbar regions– Neurotransmitter is NE– NE increases sphincter tension– Inactivate the motility

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• Enteric Nervous System (minibrain) Has as many neurons as spinal cord• Located close to the effector systems such as:

- Musculature- Glands- Blood vessels (from esophagus to the anus)

• Consists of ganglia & fibers projecting to the effector systems

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Enteric nervous system (Mini brain)

• Enteric nervous system contains– adrenergic and cholinergic neurons– nonadrenergic & noncholinergic neurons that

release neurotransmitters e.g NO, CO, serotonin, GABA, ATP & several neuropeptides

Enteric Nervous System (minibrain)

• Composed of two plexuses:1- myenteric plexus: excitatory or inhibitory

(outer plexus)– increases intensity of rhythm of contraction– increases tone– increases rhythm rate– increases velocity of conduction of excitatory wave

2- Submucous plexus (inner plexus)

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Myenteric and Meissner Plexus• Many axons leave the myenteric (Auberch’s)

plexus and synapse with neurons in submucosal (meissner) plexus and vice-versa.

• Neural activity in one plexus influences the activity of/in the other

• Stimulation at one point in plexuse lead to impulses conducted both up and down tract.(stimulation of small intestine can affect smooth muscle and gland activity of stomach)

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Myenteric and Meissner Plexus

• Myenteric (Auberch’s) plexus influences mostly smooth muscle

• Submucosal plexus (Meissner) influences mostle secretory activity

• Many effectors (muscle cells, exocrine glands) are supplied by neurons that are part of the ENS & this allows neural reflexes that are completely within the tract independent of the CNS

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Gastrointestinal reflexes• The anatomical arrangement of the ENS and its connection to the ANS

supports 3 types of reflexes1. Reflexes that are integrated entirely within the enteric nervous system

– Reflexes control secretion, peristalsis, mixing contractions, local inhibitory effects

2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GIT

– Transmit signals over long distances in the GIT. “Law of the GUT” eg what happens in the stomach affecting what happens in the colon (gastrocolonic reflex)

3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract eg

– Reflexes from the stomach and the duodenum to the brain stem and back to the stomach by way of the vagus nerve (control gastric motor and secretory activity

– Pain reflexes that cause general inhibition of the entire GIT– Defecation reflexes that travel from the colon and rectum to the spinal cord and

back again to produce the powerful colonic, rectal and abdominal contractions required for defecation

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Long and short neural reflexes

• Most reflexes are initiated by luminal stimuli• Distension• Osmolarity• Acidity• Digestion products

• Other stimuli• Hunger• Sight• Smell• Emotional state

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• Long and short reflex pathways can be activated by stimuli in the GIT.– Long reflexes utilize neurons that link the central

nervous system to the GIT– Short reflexes mediated by the enteric NS to

effector cells

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CNS & Enteric Nervous System (ENS)

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Types of Neurotransmitters Secreted by Enteric Neurons

• Acetylcholine • Norepinephrine . • adenosine triphosphate, • serotonin, • dopamine, • cholecystokinin, • substance P, • vasoactive intestinal polypeptide, • somatostatin, • leu-enkephalin, • met- enkephalin, • bombesin.

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Autonomic Control of theGastrointestinal Tract

• Parasympathetic Innervation. – The parasympathetic supply to the gut is divided into 2

1. Cranial parasympathetic- nerve fibers are almost entirely in the vagus nerves provide extensive innervation to the esophagus, stomach, and pancreas and somewhat less to the intestines down through the first half of the large intestine – Excludes mouth and pharyngeal regions

2. sacral divisions- originate in the second, third, and fourth sacral segments of the spinal cord.– pass through the pelvic nerves to the distal half of the large intestine and all

the way to the anus – sigmoidal, rectal, and anal regions are considerably better supplied with

parasympathetic fibers than are the other intestinal areas. – ibers function especially to execute the defecation reflexes

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Sympathetic Innervation.

• Fibers to the gastrointestinal tract originate in the spinal cord between segments T-5 and L-2.

• Most of the preganglionic fibers that innervate the gut enter the sympathetic chains that lie lateral to the spinal column,

• many of these fibers then pass on through the chains to outlying ganglia such as to the celiac ganglion and various mesenteric ganglia.

• Most of the postganglionic sympathetic neuron bodies are in these ganglia,

• innervate all of the gastrointestinal tract • the sympathetic nervous system inhibits activity of the

gastrointestinal tract

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• It exerts its effects in two ways: 1. to a slight extent by direct effect of secreted

norepinephrine to inhibit intestinal tract smooth muscle (except the mucosal muscle, which it excites)

2. to a major extent by an inhibitory effect of norepinephrine on the neurons of the entire enteric nervous system.

• Strong stimulation of the sympathetic system can inhibit motor movements of the gut

Excitatory Motor Neurons Evoke Muscle Contraction & Intestinal Secretion

• Neurotransmitters of motor neurons: 1. Substance P2. Ach

• Neurotransmitters of secretomotor neurons (releasing of water, electrolytes and mucus from crypts of Lieberkuhn):

1. Ach 2. VIP3. Histamine (neurogenic secretory diarrhea)

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Inhibitory Motor Neurons Suppress Muscle Contraction

• Neurotransmitters:

1. ATP

2. NO

3. VIP

N.B. Longitudinal muscles do not have inhibitory motor innervation

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ENDOCRINE AND PARACRINE ACTIVITY OF THE GIT

Gastrointestinal Peptides • Hormones

- endocrine cells - via portal circulation and liver - e.g., gastrin, CCK, secretin and GIP

• Paracrines- endocrine cells- through diffusion at the same tissue- e.g., somatostatin (mucosa), to inhibit gastric H secretion

• Neurocrines

- neuronal cells in GI tract - e.g., VIP, GRP and Enkephalins

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Peptides as endocrine, neurocrine or paracrine substances

ENDOCRINE NEUROCRINE PARACRINE Somatostatin Somatostatin

Somatostatin

Cholecystokinin CCK Peptide YY

Gastrin GRP

Secretin Opioids

Insulin Substance P

Glucagon VIP

Enteroglucagon Neuropetide Y (NPY)

Pancreatic polypeptide Neurotensin

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Hormonal regulation

• GIT is an endocrine gland• Hormones secreted by enteroendocrine cells

– More than 15 types of cells excist– Many secret 1 hormone, others more than 1– Identified by letters– Serotonin secreting cells called enetrochromaffin– Histamine secreting cells called enterochromaffin-like cells

• Enteroendocrine cells found scattered throughout stomach, small intestine and colon.

• Enteroendocrine cells on the luminal surface are stimulated to secrete their respective hormones when they come into contact with various substances in the chyme from the opposite side of the cell into the blood.

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Peptide families

GastrinCCKgastrin

Secretinsecretin glucagon

PHI, GIP, VIP, PACAP,

GLP17-36

Pancreatic polypeptidepancreatic polypeptideneuropeptide Y peptide YY

OtherGRPmotilin galanin neurotensin somatostatin

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Hormones ctd

• Each hormone participates in a feedback control system that regulates some aspect of the GI luminal environment

• Each hormone affects more than one type of target cell

• In many cases a single effector cell contains– receptors for more than one hormone,– Receptors for neurotransmitters and paracrine

agents

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Hormone production sites

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Hormones cotd

• A variety of inputs affect the cell’s responses i.e. Synergism of inputs can potentiate responses – e.g. Secretin stimulates pancreatic bicarbonate secretion,

whereas CCK has a weak stimulus of bicarbonate secretion

– Therefore a stronger stimulus than one stimulus

• Therefore consequence of potentiation is that small changes in the plasma concentration on GI hormone can have large effects on the action of other GI hormones

• GI hormones also have trophic effects

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Factors affecting Ghrelin function

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Glucagon-like peptides • Are released from enteroendocrine cells in response to nutrient

ingestion • Two types of GLPs

– glucagon-like peptide-1 (GLP-1)– glucagon-like peptide-2 (GLP-2)

• GLP-1 and GLP-2 exhibit a diverse array of metabolic, proliferative and cytoprotective actions with important clinical implications for the treatment of diabetes and gastrointestinal disease, respectively

• L cells that produce GLP-1 and GLP-2 – the vagus nerve, the neurotransmitter gastrin-releasing peptide and the

hormone glucose-dependent insulinotropic peptide all contribute to the rapid release of GLP-1 and GLP-2 from distal L-cells in response to nutritional stimuli.

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• The GLP-1 receptor (GLP-1R) has a widespread distribution and is expressed in a number of tissues, including – the pancreas, – intestine, – stomach, – central nervous system (CNS), – heart, – pituitary, – Lung – kidney

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Effects of GLP-1

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Glucagon-like peptide-2

• GLP-2 is an intestinal trophic peptide that stimulates cell proliferation and inhibits apoptosis in the intestinal crypt compartment.

• The GLP-2 receptor (GLP-2R) is expressed in a highly tissue-specific manner, – the gastrointestinal tract – brain

• GLP-2 also regulates intestinal glucose transport, food intake and gastric acid secretion and emptying, and improves intestinal barrier function

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Incretin effect of GIP and GLP-1

Incretins are a group of metabolic hormones that stimulate a decrease in blood glucose levels. Incretins do so by causing an increase in the amount of insulin released from pancreatic beta cells of the islets of Langerhans after eating, before blood glucose levels become elevated

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GIT blood flow

• Blood vessels of the GIT are part of the Splanchnic circulation– Includes blood flow through the gut itself through

the spleen, pancreas and liver– Blood flows from gut to the spleen and pancreas

then flows immediately into the liver by way of the portal vein

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• In liver, blood passes through millions of minute liver sinusoids and finally leaves the liver via the hepatic veins that empty into the vena cava of the general circulation

• Parasympathetic stimulation of the stomach and lower colon increases local blood flow that causes an increase in secretion. (Secondary effect)

• Symapthetic stimulation causes vasoconstriction reducing blood flow

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Splanchnic Circulation

• Vasoconstrictors- Ang II, endothelin, NE ( 2a -agonists), PGF2a, Vasopressin

• Vasodilators- Ach, Adenosine, Bradykinin, CGRP, histamine, NO, VIP, b2-agonists

dilatorG-PR

Vascular Smooth Muscle Cell

Gs

AC

cAMP Decreasedfree Ca++

VasorelaxationcGMP

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Interactive motile events (reflexes)

• Law of the intestine- the intrinsic contractile wave in response to bolus of material

– Distension in one segment affecting activity in another segment

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Interactive motile events (reflexes)

1. Intestino-intestinal reflex– Overdistension of one segment causes relaxation in the

rest of the intestine

2. Ileogastric reflex– Ileal distension leads to decrease gastric motility

3. Gastroileal reflex– Increased gastric activity causes increased ileal motility

& movement through ileocecal sphincter

4. Gastrocolic and duodenocolic reflex– Increased gastric and duodenal distension increases colon

motility

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Control of food intake

• Two control centres in the hypothalamus– Feeding centre (encourages eating behaviour)– Satiety centre (discourages eating behaviour)

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Polypeptides affect food intake

Increase intake (orexigenic)

Decrease intake (antiorexigenic)

AgRP, endorphin, MCH (found in hypothalamus), Galanin,neuropeptide , ghrelin, orexin A & B

Bombesin, leptin, CRH, CCK, oxytocin, glucagon, somatostatin, neurotensin, αMSH,GRP, CGRP GLP-1 &2, Oxytocin, Peptide YY,

Read Chapter on hypothalamus in Ganong

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Ghrelin effect on the brain

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Glucagon related hormones effect on appetite

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Nutrient sensing mechanism by L cells and effect on nutrient absorptions

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Phases of Gastrointestinal Control

• There are three phases– Cephalic– Gastric– Intestinal

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Cephalic Phase• Initiated when receptors in the head are stimulated by

– Sight – Smell– Taste– Chewing– Emotional state

• Efferent pathways are mediated by parasympathetic fibers in the vagus

• Fibers activate neurons in the GI nerve plexuses, in turn affect secretory and contractile activity.

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Gastric phase

• Initiated by– Distension– Acidity– Amino acids– Peptides

• Mediated by – short and long neural reflexes– Gastrin

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Intestinal Phase

• Initiated by– Distension– Acidity– Osmolarity– Various digestive products

• Mediated by– Short and long reflexes– Secretin, CCK, and GIP

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Phases of Gastrointestinal Control

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Mouth (salivary glands), pharynx and oesophagus

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Mouth• Where digestion starts.• Chewing (breaking up large pieces of food to smaller

particles that can be swallowed)· Saliva secreted by three pairs of salivary glands

loacted in head, (parotid, submandibular and sublingual) drains into the mouth through short ducts.

· There are 600 other minor glands in the oral cavity and Von Ebner's Glands found in circumvallate papillae of the tongue

· Saliva contains mucus, moistens and lubricates the food particles before swallowing.

· hypothalamic centers increase or decrease salivation

Main Salivary Glands

• 1. Parotid gland• 2. Submandibular

gland• 3. Sublingual gland

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http://en.wikipedia.org/wiki/File:Illu_quiz_hn_02.jpg

• The salivary glands are compound acinous glands

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Unique Features of Saliva• Hypotonic in relation to plasma• Potassium level 2x to 30x higher than plasma

• Parasympathetic - low organic material concentration• resulting in copious quantities of watery

secretion • Both sympathetic & parasympathetic NS

stimulate increased secretion• Sympathetic – high organic material concentration

• Rate of secretion not hormonally controlled

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Mouth contd

· Human saliva is composed · mostly of water (99.5%),

· Electrolytes (low in Na+ and Cl-, high in K+ and HCO3-)

, · mucins, · antibacterial compounds, (IgA, lactoferrin,

lactoperoxidase, lysozymes) · Amylase and various other enzymes

· 800 to 1500 ml secreted per day· pH of 6.5-7.5

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Functions of Saliva• Digestion - amylase & lingual lipase• Lubrication - chewing, swallowing & speech

Mucins, and glycoproteins

• Protection– Against hot fluids - increased production– Against gastric acid if vomit– Against bacteria - lysozyme & lactoferrin &

immunoglobulins– Keeps mouth and teeth clean by dissolving and

washing food particles from between the teeth

• Taste – dissolves food particles and carries food particles to taste buds

Regulation of Salivary secretion

A) Simple or unconditioned: The presence of food in the mouth results in reflex secretion of saliva.

• Stimulus: presence of food in the mouth.• Receptors: taste buds.• Afferent: nerves from taste buds carry impulses

to salivary centre.• Centre: salivary centre in medulla oblongata

(in brain stem).• Efferent: autonomic nerves supplying salivary

glands.

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B) Conditioned

• An acquired reflex and needs training• Eg Bell to indicate meal time.

• The centre is in the cerebral cortex. • The sight, smell, thought of food in the

absence of food in the mouth increase salivary secretion.

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• Salivary secretions are regulated by nervous mechanisms only

• Parasympathetic stimulation, produces flow of watery saliva that is rich in enzymes.

• Sympathetic stimulation produces a much smaller volume of thick saliva that is rich in mucus.

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Chewing/Mastication• Controlled by the somatic nerves to the skeletal

muscles of the mouth and jaw (voluntary)• Also controlled by skeletal muscles and includes

reflex rhythmic chewing motions, activated by the pressure of food against the gums, hard palate at the roof of the mouth, and tongue– Activation of mechanoreceptors leads to reflex

inhibition of the muscles holding the jaw closed – The resulting relaxation of the jaw reduces the pressure

on the various mechanoreceptors, leading to a new cycle of contraction and relaxation

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Chewing cont

• Chewing prolongs pleasure of food.

• Does not alter rate at which food is digested and

absorbed.

• Chewing reduces risk of choking from large food particles that may lodge over the trachea.

• Symptoms of choking similar to a heart attack.

• Heimlich maneuver can be used to dislodge

obstructing particles from the airways.

Swallowing• Swallowing is divided into three stages

– Voluntary stage- initiates swallowing process– Pharyngeal stage- involuntary stage,

constitutes the passage of food through the pharynx into the oesophagus

– Oesophagal stage- involuntary phase that promotes passage of food

from the pharynx to the

stomach

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126Figure 21-24

Swallowing Reflex

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