Most animals have three major pairs of salivary glands that differ in the type of secretion they produce: parotid glands produce a serous, watery secretion. submaxillary (mandibular) glands produce a mixed serous and mucous secretion; sublingual glands secrete a predominantly mucous saliva.

1. ACCESSORY GLANDS OF
THE DIGESTIVE SYSTEM

2. The SALIVARY GLANDS
 SALIVA
The fluid mixture produced by the serous and
mucous salivary glands.
Mixed glandular secretion which constantly
bathes the teeth & oral mucosa.

3. The SALIVARY GLANDS
 Major salivary glands
Paired and drain into the oral cavity
Parotid, mandibular, sublingual, zygomatic
 Minor salivary glands*
Contained within the submucosa of the oral mucosa
Palatine, buccal, pharyngeal, labial, lingual

4. Salivary glands. (The right half of the mandible was removed.)
Mandibular gland
Parotid gland

5. Parotid gland and accessory gland. (With permission from Elsevier)
Submandibular and sublingual glands. Note the many small ducts from the
sublingual gland. (With permission from Elsevier)

7. The SALIVARY GLANDS
 Type of secretion
 Serous – parotid glands*
 Mucous – sublingual glands**
 Seromucous – submaxillary (mandibular) glands

8. Structure of salivary glands
 Salivary gland tissue
1. Secretory end piece (acini*)
2. Duct system (intercalated, striated, excretory
ducts)

11. REGULATION OF
SALIVARY SECRETION

13. Autonomic nervous system
 Secretion of saliva is a nerve-mediated reflex.
 The volume and type of saliva secreted is controlled by the
autonomic nervous system.
 The glands receive both parasympatheic and sympathetic
nerve supplies.

16. Salivary Reflex Regulation

17. Unconditioned reflex
 This reflex is present at birth.
 This is due to the stimulation of receptors in the mouth
by chemical substances and even mechanical
stimulation brought about by food in mouth*
 The most important stimulus is the presence of food in
the mouth.

19. Conditioned reflex
 This reflex is acquired during the life.
 The stimulation originates not from the mouth but from
special senses (sight, smell, hearing).
 In man, previous experiences associated with the supply of
food like the sight, smell can give rise to secretion of saliva.
In animals, this can be experimentally produced for sight,
sound or smell of food.

20. Salivary Flow Rate

21. Salivary flow rate
 Saliva secretion is continuous process.
 Spontanenous secretion – saliva secreted without stimulation*
 Unstimulated secretion – saliva secreted at rest, with involvement
of nervous activity; range of 0.3–0.5 mL/min.
 Stimulated secretion – saliva secreted during intensive
mechanical-chemical stimulation (during the eating); amount of saliva
increases and its amount is up to 2.0 mL/min**
 During 8-hours, sleep secretion is the smallest, up to 0,05 mL/min
 Adult (human) generates about 1-2 L per 24 h.

22. Different rates of salivary flow

23. TABLE 12. SALIVA PRODUCTION AND RATE OF DIETARY INTAKE
Saliva production Intake rate
Feed g/g food ml/min g food/min
Dairy cubes .68 243 357
Fresh grass .94 266 283
Silage 1.13 280 248
Dried grass 3.25 270 83
Hay 3.63 254 70
Bailey, C.B. 1959. Proc. Nutr. Soc. 18:1

24. TABLE 13. SALIVA PRODUCTION IN CATTLE AS INFLUENCED
BY TYPE OF FEED AND PHYSICAL FORMa
Feed Kg saliva/kg ingested feed
Hay
Concentrates
5.02
Meal 1.30
Cubed 1.01
Flaked corn 1.37
Beet Fodder 5.00
Grass .93
Balch, C.C. 1958. Brit. J. Nutr. 12:330.

25. TABLE 14. FEED INTAKE AND SALIVARY PRODUCTION IN STEERSa
Feed intake, % of body weight
.8 1.4 2.0 2.6
Feed intake, kg 2.5 4.5 6.3 8.3
Saliva, l/d 33.5 45.2 52.0 54.1
Saliva, l/kg of feed 13.4 10.0 8.3 6.5
Buffering capacity 47.1 46.6 51.1 52.2
aPutnam, P.A., R. Lehmann, and R.E. Davis. 1966. J. Anim. Sci.
25:817. bMilliliters of .1N HCl required to decrease the pH of 50 ml
of saliva from 7 to 5.

28. COMPOSITION OF SALIVA

29. Saliva Composition
 99% of saliva is water
 Rest of the volume are inorganic and organic components
 pH of fresh saliva is about 6.6 (humans); ruminants (8.0)
Unstimulated Stimulated
Water 99.55 % 99.53%1
Solids 0.45% 0.47%1

31. Organic components
 Proteins – immunoglobulins, albumins, glycoprotein,
enzymes
 Non-proteins nitrate compounds – urea, urea acid,
amino acids, creatinine
 Lipids– unsaturated fatty acids, cholesterol, lecithin,
phospholipids
 Hormones – steroids

32. Organic components
 Glycoproteins (mucins)*
 In non-stimulated saliva glycoproteins are about 20–30%
of total amount of proteins
 With increased amount of mucins in saliva volume,
density and viscosity increases as well
 Glycoproteins in saliva allow forming and swallowing of
food and protection of soft tissues against mechanical
irritation.

33. Organic components
 Agglutinin – glycoproteins with high molecular weight to
be able to agglutinate a certain Streptococcus spp.
 Phosphoproteins (acidic glycoprotein)
 Have large amount of negative charge and asymmetric
structure.
 Prevents increase of calcium phosphates crystals in
saturated saliva*

34. Organic components
 Histatin
 Molecule contains positive charge in amino acid side
chain and is bonding with negatively charged
phospholipids from cell membrane of bacteria or fungus.
 Statherins
 Acidic proteins which participate in remineralization of
enamel and protect it from physical factors.

35. Organic components
 Cystatins*
 Inhibit precipitation of calcium phosphates from saliva.
 Play important role in prevention of periodontal diseases.
 Lactoferrin
 Glycoprotein able to combine Fe3+ ions. Some bacteria of
oral cavity need those ions to grow, so their attachment to
lactoferrin limits their development.

36. Organic components
 α-amylase
 Participates in preliminary food digestion
 Responsible for creating glycoprotein complex of layer on the
surface of the tooth just after teethes brushing.

37. Organic components
 Urea
 Has significant role in buffer system of saliva – from its
decomposition ammonia is generated which combines
with excess of H+ ions.
 Uric acid
 Exhibit antioxidative effect (has about 70-80% of
oxidative properties of saliva).

38. Non-nitrate organic compounds
 Carbohydrates
 Lipids
 Hormones

39. Inorganic components
 Content in saliva is not constant.
 Origin mainly from blood (except HCO3-)
 Occur in ionic form.

40. Inorganic components
 Na+
 Low concentration in resting saliva (5.76 mmol/L)
 Increased concentration in stimulated saliva (20.67
mmol/L)
 Participate in transport of active compounds through cell
membrane
 K+
 Transports active compounds through cell membrane.

41. Inorganic components
 Ca2+
 Constant amount in stimulated saliva
 Has the same form as in apatite
 Participate in enamel growth and remineralization of primary
damages
 Activator of some saliva’senzymes
 Mg2+
 Participate in tooth structure creation
 Activator of some enzymes

42. Inorganic components
 Chloride
 Osmoregulator; α-amylase activator
 Fluoride
 Influences structure and remineralization processes of enamel
 Iodide
 Bicarbonate
 The strongest buffer system (carbonic acid/bicarbonate system)
 Quantity increases in stimulated saliva

43. Inorganic components
PO4
3-, HPO4
2-, H2PO4
 Mineral parts of saliva
 Present in the same for enamel
 Participate in enamel growth and remineralization of
primary damages

44. FUNCTIONS OF SALIVA

46. Fluid/lubrication & binding
 Coats hard and soft tissue which helps to protect against
mechanical, thermal and chemical irritation and tooth wear*
 Assists smooth air flow, speech and swallowing.
 Mucus is extremely effective in binding masticated food into
a slippery bolus that slides easily through the esophagus
without inflicting damage to the mucosa.

47. Ion reservoir
 Solution supersaturated with respect to tooth mineral
facilitates remineralization of the enamel (calcium).
 Statherin and acidic proline-rich proteins in saliva
inhibit spontaneous precipitation of calcium phosphate
salts.

48. Buffer
 Helps to neutralize plaque pH after eating, thus reducing
time for demineralization
 Buffer system maintains acid-base equilibrium through
neutralization of organic acids*
 Buffers maintain resting saliva pH in the range of 5.7
and 6.2.

49. Carbonic acid/bicarbonate system
 Major buffer in stimulated saliva.
CO2+ H2O H2CO3 HCO3-+ H+
Hydrogen and bicarbonate ions form carbonic acid, which forms
carbon dioxide and water. Carbon dioxide is exhaled and thus the
acid is removed.

50. Stephan Curve illustrating the changes in plaque pH over time following a sucrose rinse

51. Buffer
 Bicarbonate secretion is of tremendous importance to
ruminants because it provides a critical buffer that
neutralizes the massive quantities of acid produced in
the forestomachs.

52. Oral hygiene
 The oral cavity is almost constantly flushed with saliva,
which floats away food debris and keeps the mouth
relatively clean.
 Saliva also contains lysozyme, an enzyme that lyses
many bacteria and prevents overgrowth of oral microbial
populations.

53. Antimicrobial actions
 Specific (e.g. sIgA) and non-specific (e.g. Lysozyme,
Lactoferrin and Myeloperoxidase) anti-microbial
mechanisms help to control the oral microflora.
 α-defensins, β-defensins

54. Agglutination
 Agglutinins in saliva aggregate bacteria, resulting in
accelerated clearance of bacterial cells.
 Examples are mucins and parotid saliva
glycoproteins.

55. Protective functions
 Pellicle formation
 Thin (0.5 μm) protective diffusion barrier formed on
enamel from salivary and other proteins.
 Wound healing
 Growth-stimulating factors (epidermal growth hormone,
statherines, histatines).
 Trophic effects*

56. Endocrine functions
 Circulating non-protein-bound fractions of hormones,
such as of melatonin, cortisol, and sex steroids,passively
move into the saliva.
 Salivary substances may appear in the blood as indicated
by amylase and epidermal growth factor.

57. Excretory functions
 Excretion of:
 Harmful products of bacteria metabolism,
 Bacteria
 Food residue from oral cavity and teeth surface
 This is a very inefficient excretory pathway as reabsorption
may occur further down the intestinal tract.

58. Water balance
 Under conditions of dehydration, salivary flow is
reduced, dryness of the mouth and information from
osmoreceptors are translated into decreased urine
production and increased drinking (integrated by the
hypothalamus.

59. Saliva: functions (ruminants)
 Source of nutrients for ruminal microorganisms
 Mucoprotein and urea serve as N source – electrolytes,
particularly sodium are growth factors for ruminal
bacteria .
 Influence nutrient removal rate from the rumen*
 Antifoaming agent**
Bartley, E.E. 1976. p. 61-81. In: Buffers in Ruminant Physiology and Metabolism. M.S.
Weinburg and A.L. Sheffner (Ed.), Church and Dwight, Inc., N.Y.

60. Other functions
 Thermoregulation
 Grooming
 Mark territory or attract mates

61. MECHANISMS OF
SALIVARY SECRETION

62. Salivary secretion
 A unidirectional movement of fluid, electrolytes
and macromolecules into saliva in response to
appropriate stimulation.
 Stimulation – mechanisms that integrate the response
to salivary stimuli and communication between the
nervous system & the secretory machinery.
 Fluid, electrolytes, macromolecules*
 Unidirectional

64. Salivary secretion
 Fluid & electrolyte secretion
 Regulated through the parasympathetic efferent pathway
by releasing ACh.
 Macromolecule secretion
 Regulated by noradrenalin (NorAd or norepinephrine,
US) release from sympathetic nerves*
The first step in stimulus-secretion coupling is release of
neurotransmitter (1st messenger).

65. Second step
 The binding of neurotransmitter to receptor and activation
of a G-protein coupled intracellular enzyme*
 G-protein coupled receptor (GPCR)
 ACh – muscarinic M3 ACh receptors
 NorAdrenaline – β adrenergic receptors

67. Target enzymes
 Phospholipase C
 Target enzyme in fluid secretion.
 Adenylate cyclase
 Target enzyme in protein secretion.
 Activation of the target enzyme (activated G-protein)
will generate molecules of second messengers.

68. Third step (macromolecule)
 Macromolecule (protein) secretion is activated by binding of
NorAdrenaline to β adrenergic receptors.
 The G-protein is called Gs and the target enzyme is
adenylate cyclase which converts ATP into cAMP.
 The next and all subsequent steps in macromolecule
secretion are regulated by cAMP.

69. The third step in macromolecule stimulus-secretion coupling is production of cAMP.

70. Protein kinase A (pKA)
 Referred to be the sole mediator of the actions of cAMP.
 At rest, it is a tetramer composed of 2 catalytic subunits &
2 regulatory subunits.
 When cAMP binds to pKA, the catalytic subunits separate
from the regulatory SU and become active.
 pKA phosphorylates

71. cAMP and pKA

72. Third step (fluid & electrolyte)
 Fluid and electrolyte secretion is activated by binding of ACh
to muscarinic M3 receptors.
 The G-protein is called Gq and the target enzyme is
phospholipase C (PLC).
 PLC splits phosphatidyl 4,5, bisphosphate (PIP2) into
diacylglycerol (DAG) and Inositol 1,4,5 trisphosphate
(IP3).

75. Salivary secretion
 Fluid and electrolyte secretion is activated by binding
of ACh to muscarinic M3 receptors. The target enzyme
is phospholipase C (PLC) which splits phosphatidyl 4,5,
bisphosphate (PIP2) into diacylglycerol (DAG) and Inositol
1,4,5 trisphosphate (IP3)
 Macromolecule (protein) secretion is activated by binding
of NorAdrenaline to β adrenergic receptors. The G-protein is
called Gs and the target enzyme is adenylate cyclase which
converts ATP into cAMP.

76. Macromolecules
 Macromolecules cannot cross the plasma membrane.
 Proteins are secreted when the containing vesicle
fuses with the plasma membrane in the process of
exocytosis.
 The secretory process may be divided into four
stages: synthesis, segregation & packaging,
storage and release*

79. Exocytosis
 SNAREs*
 Present in salivary gland acinar cells.
 Secretory vesicles have SNAREs (v-SNARES) which
recognise plasma membrane SNARES (t-SNARES).
 The two form tight complexes that link the two
membranes and mediate the three steps in regulated
exocytosis.

80. A cAMP-dependent ‘brake’ point

81. Transcellular transport protein
 Not all secreted proteins originate in salivary gland
cells. Saliva also contains plasma proteins, for
example the immunoglobulin, IgA.

82. Transcellular protein transport

83. Fluid & electrolyte secretion
 Binding of ACh to muscarinic ACh receptors
increases intracellular IP3 levels.
 IP3 causes Ca2+ release from intracellular stores and
the increase in intracellular [Ca2+] triggers fluid
secretion.
 The remaining question is how? How does an
increase in [Ca2+] trigger fluid secretion?

84. Fluid & electrolyte secretion
 Fluid secretion is driven by electrolyte secretion, so
the trick is to secrete the electrolytes.
 Acinar cells concentrate Cl- within themselves which
they release across the apical membrane in response
to an increase in [Ca2+].
 The secretion of Cl- leads to Na+ secretion and
together NaCl secretion drags water across the cells
by osmosis.

89. Bicarbonate secretion
 One of the most important functions of saliva is to buffer
plaque acid.
 Bicarbonate (HCO3-) is the acid buffer secreted in saliva.
 In short, HCO3- can pass through Cl- channels and so
will be secreted when ACh triggers the signal
transduction cascade that results in increased [Ca2+]i.
 The question here is, "How does HCO3
- get concentrated
inside the cells?"

91. Bicarbonate secretion
 At low salivary flow rates, HCO3- is reabsorbed by the striated
duct cells and so very little reaches the mouth. At high flow
rates, the striated ducts can't keep up and so HCO3- reaches
the mouth at high concentration (<25mM)).
 HCO3- is valuable to the body and not something to throw
into the gut for no reason.
Unstimulated Stimulated
Bicarbonate (mmol/L) 5.47 16.03

92. Calcium and phosphate transport

93. Water channels
 There are two possible routes for water to take across
the cell, either through the tight junctions between the
cells (paracellular) or across both the apical and
basolateral membranes (transcellular).
 The intrinsic water permeability of the plasma membrane
is very low*
 Water channels in salivary acinar cells are members of
the aquaporin (AQP) family.

94. Water channels
 Aquaporins come in two types, one of which transports
only water and another which is also permeable to
glycerol.
 Neither type conducts ions.
 AQP5 has been localised to the apical membrane of
salivary gland acinar cells.

96. Unidirectional
 One Way Only
 Saliva is always secreted into the lumen of the gland
(and then onwards down the ducts).
 Crudely speaking, material is taken up at one side of the
cell and dumped out the other which means that one end
of a salivary gland acinar cells is specialized for influx
and the other for efflux.

97. Histological polarity