1. The mouth and esophagus play important roles in digestion, including mastication, lubrication, and protection. Saliva produced by salivary glands is key to these functions through its lubricating, antimicrobial, and buffering properties. 2. Mastication involves chewing food into smaller pieces using the teeth and tongue, mixing it with saliva to form a bolus. Swallowing is initiated once the food bolus reaches a certain size and lubrication. 3. Saliva secretion is regulated by the autonomic nervous system, with parasympathetic stimulation increasing saliva production and sympathetic stimulation decreasing it. Saliva aids digestion through amylase and protects the oral cavity.

1. GIT PHYSIOLOGY

2. Structural and Functional Organization of the GIT

6. MOUTH AND AESOPHOGUS
oThe functions of the mouth and oesophagus:
oMastication
oTaste
oSwallowing
oLubrication
oDigestion
oSpeech
oSignalling of thirst
oProtection of the body from harmful ingested substances
oThe performance of all of these functions depends on the
presence of saliva

7. Function of the mouth and oesophagus
Lubrication:
o Saliva aid chewing, swallowing and speech bcs of the lubricant properties conferred on
it by its mucin content
o Compromised in xerostomia
o Saliva useful for mastication bcs it coats the food and lubricates it, making it more
easily moved about in the mouth
o In swallowing it is more easily moved back into the pharynx where it stimulates the
pressure receptors that initiate the process, enabling it to pass smoothly into the
oesophagus
o Lubricant properties of saliva also assist the passage of the food bolus down the
oesophagus and prevent abrasion of the walls by hard material in the swallowed food
Digestion:
o Lack of salivary -amylase does not result in malabsorption of starch if pancreatic  -
amylase secretion is adequate, as the amylases from the two sources have similar
catalytic actions in starch digestion, and adequate amounts of -amylase are secreted
by the pancreas

8. Solution:
• Saliva is important for taste as it depends on substances dissolved in saliva
• It is also important for the solution of substances that are absorbed by mouth
Moistness:
• Lack of saliva signals thirst
• Thirst is therefore a constant sensation in xerostomia
Protection:
• Saliva is also important for oral and dental health: it washes the mouth, buffers
acids in the food, and contains antimicrobial substances
• Infections of the mouth are rare following dental surgery even though it is difficult
to maintain aseptic conditions in the mouth
• However, infections of the mouth and associated structures are common in
xerostomia without the protective properties of saliva
• In addition the constant rinsing of the oesophagus by saliva and its buffering and
antimicrobial properties, help to protect the oesophagus from damage by acids, and
to prevent infections

9. The Mouth (Oral Cavity)
• aka the oral cavity
• The oral cavity (mouth) is closed by the apposition of the lips. The
lips and the cheeks are composed mainly of skeletal muscle
embedded in elastic fibro-connective tissue
• Figure below shows the anatomical features of the oral cavity and
the structures within it, including the tongue and the teeth
• It also shows associated structures, such as the olfactory mucosa,
which are important for the functioning of the digestive system
• Mouth is a hollow cavity that allows food and air to enter the body
• other organs in the mouth – the teeth, the tongue, & ducts of the
salivary glands

10. • Oral physiology includes the physiological process of mastication
and deglutition, role of saliva in maintaining oral health and
facilitating digestion & actions of minerals on the oral cavity
• Mouth and oral organs aid ingestion and digestion of food
• Digestion starts in the mouth - mechanical process of chewing
breaks the food down into digestible pieces
• Saliva secretions mix with food and aid the process of breaking
it down into a form the body can absorb and use
• Saliva vital in lubricating food, digestion, and protecting the oral
environment, thus regulation of saliva production ensures
efficient digestion

11. Functions of the mouth
Chewing, swallowing and speech:
• Chewing depends on activation of periodontal receptors that results in impulses in sensory
nerve fibres in the inferior dental nerve being transmitted to the chewing centre
• The tongue, which is innervated by the lingual nerve, moves the food around in the mouth to
aid chewing
• The swallowing reflex is initiated when the tongue moves the food bolus to the back of the
mouth where it activates pressure receptors in the pharynx
• Articulation of many sounds depends on fine control of the movements of the muscles of the
tongue
Taste:
• Some nerve fibres in the lingual nerve carry sensory information from the taste buds on the
tongue (via the chorda tympani)

12. Salivary secretion:
• When food is eaten or at the approach of food over half of
the increased flow comes from the parotid glands
• The lingual nerve innervates the submandibular and
sublingual salivary glands but not the parotids (Fig)
• The later are innervated by fibres in the glossopharyngeal
nerve

13. Innervation of mouth structures is via the four branches of
the mandibular division of the trigeminal nerve
Mandibular division of the trigeminal nerve - lingual nerve
innervates the anterior two-thirds of the tongue and the
sublingual and submandibular salivary glands. The inferior
dental nerve innervates the tooth pulp, periodontal ligaments
and gums

14. 1. The anterior division, which
innervates the lateral
pterygoid, temporal, and
masseter muscles which are
involved in mastication
2. The auriculotemporal nerve,
which innervates structures of
the ear
3. The inferior dental nerve,
which innervates the lower lip
and the tooth pulp, periodontal
ligaments and gums
4. The lingual nerve, which
innervates the anterior two-
thirds of the tongue, the floor
of the mouth, and the gum on
the lingual side of the lower
teeth
• The lingual nerve is joined by
the chorda tympani that run
through the lateral pterygoid
muscle
• The chorda tympani carries
sensory taste fibres from the
lingual nerve to the facial nerve,
and secretomotor
(parasympathetic) fibres from
the facial nerve to the lingual
nerve
• These fibres innervate the
submandibular and sublingual
salivary glands

15. Role of Saliva
• Saliva is a dilute aqueous solution that contains both inorganic and
organic constituents
• It is secreted by three pairs of major salivary glands i.e. parotid,
submandibular and sublingual
• In addition, there are numerous minor salivary glands that secrete
saliva and are widely distributed in the oral mucosa
• pH of saliva ranges from 6.2 – 7.6.
• It bcms more alkaline during rapid secretions considering the high
bicarbonate content
• Saliva plays an essential role during mastication, swallowing and speech
• Daily secretion of saliva constitutes to about 1500 mL/day
• Salivary biomarkers have also been introduced lately that aid in early
detection of systemic disorders like dental caries, oral cancer and
periodontitis thus validating saliva as a potential diagnostic tool

16. Composition of Saliva
• Digestive enzymes:
• ptyalin or salivary –amylase, lysozymes (bactericidal in action),
kallikrein (proteolytic enzyme) & lipase
• Mucin (glycoprotein)
• Immunoglobulins – IgA, IgG, IgM
• Ions - sodium, potassium and chloride
• Organic constituents - urea, uric acid and creatinine
• pH of saliva is <8 during active secretion

18. Saliva Functions
• Protection - flush pathogens (bacteria), antimicrobial proteins e.g., lysozyme, lactoferrin,
peroxidase, alpha-defensins, beta-defensins; IgA, IgG and IgM - prevent adhesion of microbes
to oral tissues
• Protects mucosae of oral cavity from adhering to each other through mucinous secretions by
providing a lubricating barrier, protecting the mucosa from toxins, trauma, and noxious stimuli;
Viruses e.g., rabies, mumps and poliomyelitis viruses are excreted through saliva
• Buffering – increase pH by aid of bicarbonate, phosphate, other ions; thus saliva maintain a
relatively neutral pH in the oral cavity
• Maintenance of tooth integrity - forms a film (salivary pellicle) - a binding spot for calcium that
aid protect the surface of teeth, but can be binding site for bacteria and increasing plaque
formation; interaction of teeth with saliva increases surface hardness, decreases permeability
and increases resistance to caries thus maintaining tooth integrity
• Tissue repair - Growth factors and other biologic active proteins in saliva and regenerate tissue
and promote wound healing
• Digestion - begins in the mouth; salivary amylase breaks down starches/glycogen into maltose and
dextrin; & reduces sugars available thus inhibit their growth; salivary lipase - fat digestion
• Assistance with taste - Saliva assists with the sensation of taste by solubilizing food so that
the taste receptors can interact with the molecules that cause receptor activation
• Regulation of temperature - saliva is also a significant feature in animal

19. Regulation of Saliva Production
• 1.5 litres of saliva is produced daily, essential for lubricating
food, digestion, and protecting the oral environment
• Production and composition of saliva is under neural
control - via the parasympathetic and sympathetic nervous
systems
• Secretory activity is mediated by cholinergic agents in
parasympathetic system and by adrenergic agents in
sympathetic system
• The secretory motor nerve endings are seen in relation to
secretory cells, cells of striated and intercalated ducts,
myoepithelial cells, smooth muscles of arterioles, etc.

20. Production of Saliva
• Saliva is produced by the salivary glands of body -
the parotid, submandibular and sublingual glands
• Within the glands, the acinar cells are responsible for the
volume of saliva secreted, and the duct cells are responsible
for the composition of saliva

22. Molecular Mechanism of Saliva Secretion
• Mechanism of salivary gland secretion involves primarily cholinergic signaling by
the parasympathetic nerves and signaling by neuropeptides like substance P, but
also adrenergic signaling in sympathetic nerves
• Parasympathetic stimulation: activates acetylcholine receptors to activate protein
kinase C (PKC), releasing diacylglycerol (DAG) and inositol triphosphate (IP3)
which stimulate increased intracellular calcium levels
• The rise in calcium mediates the increased volume of saliva and amylase output
• Substance P activates neurokinin-1 (NK-1) receptor to stimulate PKC to increase
the formation of IP3 and DAG, which then increase amylase output and volume
flow
• Sympathetic stimulation: increase alpha receptor stimulation by norepinephrine
(NE) which causes smooth muscle contraction and increases volume flow and
amylase output
• NE also act on beta receptors and activate the cAMP cascade, increasing
protein kinase A (PKA) activity, amylase output, & transient saliva volume flow

25. • There are two main stages to secretion of saliva:
1) Once stimulated, acinar cells secrete primary saliva which is
isotonic and contains amylase, mucus, & extracellular fluid
– this isotonic form of saliva is made by secreting sodium chloride
2) the primary saliva gets modified as it passes down the ductal tree
– the sodium gets actively reabsorbed, potassium is actively secreted, chloride
is passively absorbed, and bicarbonate secreted
– the ductal epithelium has poor water permeability
– the final saliva product will be hypotonic

26. Autonomic Control (sympathetic and parasympathetic nerve fibres)
• Saliva is produced and secreted by the salivary glands of the body
• These glands are under the control of the autonomic nervous system,
comprised of sympathetic and parasympathetic nerve fibres
Sympathetic Innervation
• Sympathetic control of salivary production is via the superior cervical
ganglion
• Sympathetic stimulation results in the release of noradrenaline, which
acts upon alpha- and beta-adrenergic receptors
• This results in the following effects:
• Decreased production of saliva by acinar cells
• Increased protein secretion
• Decreased blood flow to the glands
• There is variable sympathetic innervation between the salivary glands
• On the whole, this system is far less important than the
parasympathetic innervation in terms of regulating production of saliva

28. • Sympathetic stimulation can also increase salivation by slight amount,
much less than parasympathetic stimulation
• Sympathetic nerves originate from superior cervical ganglia and travel
along the surfaces of the blood vessel walls to the salivary glands
• A secondary factor also affecting salivary secretion is the blood
supply to the glands bcs secretion always requires adequate nutrients
from blood
• Parasympathetic nerve signals that induce copious salivation also
moderately dilate the blood vessels
• Salivation itself also directly dilates the blood vessels, thus increasing
salivatory gland nutrition as needed by the secreting cells
• Part of this additional vasodilator effect is caused by kallikrein
secreted by the activated salivary cells, which in turn acts as an
enzyme to split one of the blood proteins, an alpha2-globulin, to form
bradykinin, a strong vasodilator

29. Parasympathetic Innervation
• Parasympathetic outflow is coordinated via centres in
the medulla, and innervation occurs via the facial and
glossopharyngeal nerves
• Afferent information from the mouth, tongue, nose and
conditioned reflexes are integrated within the brain - and in the
presence of food, parasympathetic stimulation occurs
• Parasympathetic outflow results in the release
of acetylcholine (ACh) onto M3 muscarinic receptors
• This results in the following effects:
• Acinar cells increase secretion of saliva
• Duct cells increase HCO3
– secretion
• Co-transmitters result in increased blood flow to the salivary glands
• Contraction of myoepithelium to increase the rate of expulsion of saliva
• Overall, increased parasympathetic stimulation results in an
increased flow of saliva that is more watery in composition

32. • Figure 64–3 shows parasympathetic nervous pathways for regulating salivation, demonstrating
that the salivary glands are controlled mainly by parasympathetic nervous signals all the way from
the superior and inferior salivatory nuclei in the brain stem
• Salivatory nuclei are located at the juncture of the medulla and pons and are excited by both
taste and tactile stimuli from the tongue and other areas of the mouth and pharynx
• Many taste stimuli, esp the sour taste (caused by acids), elicit copious secretion of saliva - often
8 to 20 times the basal rate of secretion
• Also, certain tactile stimuli, such as the presence of smooth objects in the mouth (e.g., a pebble),
cause marked salivation, whereas rough objects cause less salivation and occasionally even inhibit
salivation
• Salivation also be stimulated or inhibited by nervous signals arriving in the salivatory nuclei from
higher centers of the central nervous system
• E.g, when a person smells or eats favorite foods, salivation is greater than when disliked food is
smelled or eaten
• Appetite area of the brain, partially regulates these effects, is located in proximity to the
parasympathetic centers of anterior hypothalamus, and it functions to in response to signals from
the taste and smell areas of the cerebral cortex or amygdala
• Salivation also occurs in response to reflexes originating in the stomach and upper small
intestines esp when irritating foods are swallowed or when a person is nauseated bcs of some GI
abnormality
• Saliva, when swallowed, helps remove the irritating factor in the GIT by diluting or neutralizing
the irritant substances

34. Mastication
• The first and foremost step in the process of digestion is
mastication or chewing
• Mastication = act of chewing food whereby the ingested
food is crushed into small pieces, mixed with saliva and
formed into a bolus
• Chewing is important for most fruits and raw vegetables bcs
they have indigestible cellulose membrane around their
nutrient portions which must be broken down b4 food can be
acted upon by enzymes
• Process of mastication ensures healthy growth and
development of oral tissues

36. • The initiation of swallowing process depends on separate
thresholds for food particle size and particle lubrication
• It has also been suggested that swallowing is initiated when it is
sensed that a batch of food particles is bound together under
viscous forces for the purpose of forming a bolus
• The oral cavity is of prime importance in the masticatory
framework
• Another important factor in mastication is the bite force
• Coordination btn. the chewing muscles (the temporalis, masseter,
medial pterygoid and lateral pterygoid muscles – attach to
mandible and movement at TMJ) is also an important
• The movement of the jaws and neuromuscular control of chewing
plays an important role in the fragmentation of the food

40. Forces of mastication
• Males exert more masticatory force than females
• The ~masticatory force exerted on anterior teeth is 10–15
kg and on molars is 50 kg
• Maximum biting force up to 150 kg has been recorded
• Food characteristics influence the number of chewing cycles
needed to prepare the food for swallowing
• Dry and hard products require more chewing cycles
• Evidently, more time is needed to break the food down and
to add enough saliva to form a cohesive bolus that is
suitable for swallowing

41. Neuromuscular control of chewing
• Jaw movements and neuromuscular control of chewing play
an important role in the comminution (reduce to small
fragments) of food
• More muscle activity is generated if the closing movement is
counteracted by food resistance

45. Deglutition
• Deglutition or swallowing refers to a series of co ordinated
muscular contractions that move the ingested food and
pooled saliva from the oral cavity into the stomach, passing
through the esophagus
• Swallowing is a reflex response controlled by the vagus
nerve with its centre located in the medulla oblongata
• It can be divided into 3 stages:
1) Oral Stage (voluntary)
2) Pharyngeal Stage (involuntary)
3) Esophageal Stage (involuntary)

47. Oral stage
• Chewed food is softened by saliva and rolled into a bolus
• Bolus is pressed against the hard palate as a result of contraction of
the front part of the tongue
• Swallowing commences by closure of the mouth and contraction of
mylohyoid muscle
Pharyngeal stage
• Elevation of soft palate takes place thus preventing food from entering
into the nasal cavity
• Larynx rises with elevation of hyoid bone which leads to approximation
of the vocal cords
• Cricopharyngeal muscle relaxes and bolus enters the upper esophagus
• Vocal cords then eventually open to allow resumption of rhythmic
breathing

48. Esophageal stage
• Bolus moves down esophagus to reach the stomach
• The peristaltic movements (alternative contraction and
relaxation of muscle fibers of GIT) aid in the movement of
food in esophagus

50. Taste
Taste buds
• There are several thousand taste buds on the human tongue
• Each circumvallate papilla contains several hundred taste buds
• Taste buds contain gustatory (taste) receptor cells
• Located in the oral (stratified squamous) epithelium, in association with papillae, but can be located anywhere in the oral cavity
e.g., palate and the epiglottis
• They lie within the epithelium
• They have a pale, barrel-shaped appearance with a depression, the taste pore, in the surface (aperture that provides
communication with the exterior)
• Contains three types of cell: supporting (sustentacular) cells, neuro-epithelial taste cells, and basal cells
• Supporting cells lie at the periphery, and are arranged like the staves of a barrel
• Neuroepithelial cells, 10–14 in each taste bud, lie more centrally
• Two types of neuroepithelial sensory cell can be distinguished under the electron microscope; one type contains clear vesicles
within its cytoplasm and the other contains dense core vesicles
• Presence of different vesicles is consistent with the presence of different transmitter substances
• are stored in the vesicles prior to being released from the cell
• Both sensory cells and the support cells have long apical microvilli, or taste hairs, which project into the taste pore
• Taste hairs lie in amorphous polysaccharide material that is secreted by the supporting cells
• Basal cell is located peripherally near the basal lamina (stem cells for other cell types)
• There are club-shaped endings of sensory nerves lying between the cells
• Chemical (taste) stimuli are received by the neuroepithelial cells and transmitted via the release of neurotransmittersNTs
from the cells to the nerve endings
• Secretions of the serous glands of the papillae wash away food material and permit new taste stimuli to be received by the
receptors.

51. Taste sensation
• The solubilization of food constituents by saliva enables the sense of taste
to be experienced
• Thus, taste depends on the detection of chemicals that are dissolved in the
saliva and for this reason, taste is compromised in xero-stomia
• There are four submodalities: salt, sour (acid), sweet and bitter
• Dissolved substances with these properties stimulate the receptors (the
taste buds) on the tongue
• Acid is the most potent stimulus; in humans, sucking a lemon can lead to the
maximum rate of secretion, which can be 7–8mL of saliva per minute
• A taste bud responds to several or all of these submodalities, but each taste
bud is most sensitive to one particular taste
• However, all taste buds respond to all four stimuli, given high enough
concentrations of the appropriate chemicals
• The taste buds that respond primarily to each submodality are situated as
follows:

52. • Sour: the posterior sides of the tongue
• Salt: the anterior sides
• Sweet: the front of the tongue
• Bitter: the rear of the tongue.
• but, no obvious structural differences in taste buds in the different
regions present
• The differences in sensitivity are partly due to precise projections of
the afferent nerves to the central nervous system and partly to the
patterns of impulses from a population of chemoreceptors
• The nerves from the taste buds in the anterior of the tongue pass in
the chorda tympani (a branch of the facial nerve) and those from the
taste buds in the posterior third travel in the glossopharyngeal nerve.
These nerves project to the tractus solitarius (Fig.)
• The sensory nerves from the taste buds in the palate and epiglottis
ascend in the vagus nerve

53. • Dissolved chemicals in the saliva diffuse into the taste pore
from the fluid layer on the tongue
• Appropriate chemicals, such as NaCl, are detected by
receptor molecules on the taste hairs, and this results in a
depolarization of the cell membrane (receptor potential) of
the taste bud cell which causes the release of an excitatory
transmitter that evokes a generator potential in the sensory
nerve endings

55. Clinical significance
• Saliva testing - detect hormone levels e.g. cortisol; screen
for HIV, detect drugs and identify viruses or bacteria
infections
• Sialometry - diagnose salivary gland hypofunction. Drug-
stimulated and unstimulated salivary flow rates are
measured
• Salivary gland imaging with CT or MRI head and neck region
for neoplasms of the salivary glands then salivary gland
biopsy

56. • Xerostomia: Reduction in the quantity of saliva produced
• Xerostomia is the subjective complaint of dry mouth due to lack of saliva caused by
multitude of factors - diseases, treatments, and medications can alter the secretion of
saliva
• Vary from the sensation of dry mouth (xerostomia) despite adequate salivary flow,
decreased flow of saliva, or abnormal constituents of saliva
• Causes:
• Psychological (anxiety and depression)
• Dehydration (diarrhoea, vomiting)
• Auto-immune diseases (Sjogren syndrome, SLE, and rheumatoid arthritis)
• Hyposecretory conditions (e.g., biliary cirrhosis, atrophic gastritis)
• Irradiation of head and neck and surgical trauma
• Drugs for urinary urge incontinence, antihistamines, and antidepressant - disrupt
parasympathetic signaling pathways
• Interruptions in normal salivation affect normal protective and digestive functions of saliva
• Xerostomia cause:
• difficulty in speech and eating
• predispose to halitosis (bad breath)
• increase in dental caries, , bacterial, and fungal infections
• reduce protection of the oral cavity mucosa and teeth, buffering, antimicrobial activity,
altered taste, digestion, and tissue repair

57. • Sialorrhea (drooling) - production of excessive saliva/excessive flow of saliva from the
mouth
• Caused by uncontrolled secretion of saliva, inability to retain saliva within the mouth or
swallowing impairment
• Aka ptyalism or hypersalivation
• Normal in 15 months and 3 years of age (neurodegenerative conditions)
• Pathologic after age 4 years esp (cerebral palsy)
• Causes:
• excess production and stimulation of salivary glands; decreased clearance of saliva
from the oral cavity (lack of muscle strength and coordination with disruption of
neuromuscular activity); inability to retain saliva within the mouth or swallowing
impairment
• side effect of drugs e.g., atypical antipsychotic drugs (clozapine – schizophrenia
Tx), direct acting muscarinic agents (pilocarpine), and acetylcholinesterase
inhibitors (donepezil)
• Excess salivation - risk for aspiration and choking
• Hypersalivation Rx: physical therapy (by strengthening oral cavity musculature),
removal of offending drugs, or adding an anti-muscarinic drug (counteract the
hypersalivation and decrease salivation)

58. Dental and oral health
• Dental health depends on saliva for many reasons including its
continuous rinsing of the oral and buccal cavities to wash away
particles in which microorganisms grow, its ability to buffer acids
(saliva is more alkaline at high rates of flow), its specific and non-
specific immune functions due to the presence of immunoglobulins, the
antimicrobial constituents sialoperoxidase, thiocyanate and lysozyme,
and the presence of calcium phosphate, which prevents
demineralization of the teeth
• As a consequence of the decreased production of saliva various oral
diseases are associated with xerostomia
• These are dental caries, gum disease, mucosal ulceration and atrophy,
infections of the mouth (e.g. candida), and ascending infection of the
salivary glands
• In addition, in the absence of saliva, retention of dentures is more
difficult. Patients who have a dry mouth because they are being
treated with tricyclic antidepressants, ganglion-blocking drugs, or
parasympathomimetic drugs (for hypertension), may experience
difficulty in oral function if they wear dentures