1. Gastrointestinal Physiology
Mrs C Mahachi
1

2. 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
2

3. Functions of Gastrointestinal organs
Summary of motility, secretion, digestion, and absorption in
different regions of the digestive system
3

4. 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)
4

5. 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.
5

6. 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
6
Back of pharynx

7. GALT
7

8. 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
8

9. 9
Sphincter Locations

10. • 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).
10

11. Layers of the GIT wall
11

12. 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
12

13. 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
13

14. 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.
14

15. 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.
15

16. 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
16

17. 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
17

18. Intestinal Villi
18

19. 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);
19

20. 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 ions
20

21. 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)
21

22. 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
22

23. SMOOTH MUSCLE OF G.I.
• Tonic contractions (continued):
- Caused by:
• Continuous repetitive spike potential
• Hormonal effects
• Continuous entry of Calcium
23

24. The Musculature of the Digestive
Tract
• Three main muscle layers:
– Longitudinal muscle layer
– Circular muscle layer
– Oblique muscle layer (stomach only)
24

25. The Musculature of the Digestive
Tract
• 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
25

26. The Musculature of the Digestive
Tract
• 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
26

27. 27

28. Coupling Trigger Contractions in GI
Muscles
• Depolarization opens the voltage-gated
Ca channels (electromechanical coupling)
• Ligands open the ligand-gated Ca
channels (pharmacomechanical coupling)
28

29. 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
29

30. 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
30

31. • 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.
31

32. 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
32

33. 33

34. 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
34

35. CONTROL OF DIGESTIVE
FUNCTIONS BY NERVOUS SYSTEM
• Autonomic nervous system (ANS) is
divided into
- Parasympathetic
- Sympathetic
35
Extrinsic nervous system

36. CONTROL OF DIGESTIVE FUNCTIONS
BY 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
36

37. CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
• Sympathetic nerves:
– Located in thoracic & lumbar regions
– Neurotransmitter is NE
– NE increases sphincter tension
– Inactivate the motility
37

38. 38

39. CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
• 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
39

40. 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
40

41. 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)
41

42. 42

43. 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)
43

44. 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
44

45. Gastrointestinal reflexes
• The anatomical arrangement of the ENS and its connection to the ANS
supports 3 types of reflexes
1. 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
45

46. Long and short neural reflexes
• Most reflexes are initiated by luminal stimuli
• Distension
• Osmolarity
• Acidity
• Digestion products
• Other stimuli
• Hunger
• Sight
• Smell
• Emotional state
46

47. • 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
47

48. CNS & Enteric Nervous System (ENS)
48

49. 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.
49

50. 50

51. Autonomic Control of the
Gastrointestinal 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
51

52. 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
52

53. • 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
53

54. Excitatory Motor Neurons Evoke Muscle
Contraction & Intestinal Secretion
• Neurotransmitters of motor neurons:
1. Substance P
2. Ach
• Neurotransmitters of secretomotor neurons (releasing
of water, electrolytes and mucus from crypts of
Lieberkuhn):
1. Ach
2. VIP
3. Histamine (neurogenic secretory diarrhea)
54

55. Inhibitory Motor Neurons Suppress
Muscle Contraction
• Neurotransmitters:
1. ATP
2. NO
3. VIP
N.B. Longitudinal muscles do not have inhibitory
motor innervation
55

56. ENDOCRINE AND PARACRINE ACTIVITY
OF THE GIT
57

57. 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
58

58. 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
59

59. 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.
60

60. Peptide families
Gastrin
CCK
gastrin
Secretin
secretin glucagon
PHI, GIP, VIP,
PACAP, GLP17-36
Pancreatic polypeptide
pancreatic
polypeptide
neuropeptide Y
peptide YY
Other
GRP
motilin
galanin
neurotensin
somatostatin
62

61. 63

62. 64

63. 65

64. 66

65. 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
67

66. Hormone production sites
68

67. 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
69

68. Factors affecting Ghrelin function
70

69. 71

70. 72

71. 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.
74

72. • 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
78

73. Effects of GLP-1
79

74. 80

75. 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
81

76. Incretin effect of GIP and GLP-1
82
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

77. 83

78. 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
84

79. • 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
85

80. Splanchnic Circulation
• Vasoconstrictors- Ang II, endothelin, NE (a2-
agonists), PGF2a, Vasopressin
• Vasodilators- Ach, Adenosine, Bradykinin,
CGRP, histamine, NO, VIP, b2-agonists
dilator
G-PR
Vascular Smooth Muscle Cell
Gs
AC
cAMP Decreased
free Ca++
Vasorelaxation
cGMP
86

81. 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
98

82. 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
99

83. Control of food intake
• Two control centres in the hypothalamus
– Feeding centre (encourages eating behaviour)
– Satiety centre (discourages eating behaviour)
100

84. Polypeptides affect food intake
101
Increase intake
(orexigenic)
Decrease intake
(antiorexigenic)
AgRP,b 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

85. Ghrelin effect on the brain
102

86. Glucagon related hormones effect on
appetite
104

87. Nutrient sensing mechanism by L cells
and effect on nutrient absorptions
105

88. Phases of Gastrointestinal Control
• There are three phases
– Cephalic
– Gastric
– Intestinal
106

89. 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.
107

90. Gastric phase
• Initiated by
– Distension
– Acidity
– Amino acids
– Peptides
• Mediated by
– short and long neural reflexes
– Gastrin
108

91. Intestinal Phase
• Initiated by
– Distension
– Acidity
– Osmolarity
– Various digestive products
• Mediated by
– Short and long reflexes
– Secretin, CCK, and GIP
109

92. Figure 21-9
Phases of Gastrointestinal Control
110

93. Mouth (salivary glands), pharynx
and oesophagus
111

94. 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
112

95. Main Salivary Glands
• 1. Parotid gland
• 2. Submandibular
gland
• 3. Sublingual gland
113

96. • The salivary glands are compound acinous
glands
114

97. 115

98. 116
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

99. 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
117

100. 118
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

101. 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.
119

102. 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.
120

103. • 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.
121

104. 122

105. 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
123

106. 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.
124

107. 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
125

108. Figure 21-24
Swallowing Reflex
126