• It is the first part of gastrointestinal tract.
• Mouth is formed by:
  • Cheeks
  • Lips
  • Palate
• It encloses:
  • Teeth
  • Tongue
  • Salivary glands

Openings of Mouth

• Anteriorly → to the exterior through lips.
• Posteriorly → through fauces into pharynx.

Juice present in the mouth is saliva, which is secreted by salivary glands.

Functions of Mouth

1. Mastication

• Teeth cut and grind the food.
• Lips and cheeks hold the food in the mouth with the help of tongue.
• Muscles of the mouth along with jaw movements help in chewing the food properly and mixing the food with saliva.
• Saliva lubricates and softens the food to facilitate chewing and swallowing.

2. Taste

• Taste buds present on tongue and other structures of mouth help to appreciate and differentiate the taste of food.
• Saliva helps in appreciation of taste by dissolving the foodstuffs.

3. Speech

• Mouth coordinates with larynx, pharynx, lips and tongue during speech.
• Saliva helps in speech by moistening and lubricating soft parts of mouth and lips.

4. Appearance

• Shape of mouth along with jaws, lips and teeth together contributes to the appearance of face.

5. Expression

• Facial expressions like smiling and laughing are mostly centered on mouth along with movements of lips and cheeks.

6. Breathing

• Occasionally, when nose breathing is inadequate, as in case of running or nasal block, mouth is used for breathing.

In humans, saliva is secreted by three pairs of major salivary glands and some minor salivary glands in the oral and pharyngeal mucous membrane.

Major Salivary Glands

Minor Salivary Glands

1. Lingual mucous glands
   • Situated in posterior 1/3 of tongue, behind circumvallate papillae and at the tip and margins of tongue.
2. Lingual serous glands
   • Located near circumvallate papillae and filiform papillae.
3. Buccal glands
   • Present between mucous membrane and buccinator muscle.
   • Four to five buccal glands are larger and situated outside buccinator around terminal part of parotid duct.
   • These glands are called molar glands.
4. Labial glands
   • Situated beneath the mucous membrane around orifice of mouth.
5. Palatal glands
   • Found beneath mucous membrane of soft palate.

Salivary glands are classified into three types based on nature of secretion:

• Salivary glands are made up of acini or alveoli.
• Each acinus is formed by a small group of cells around a central cavity.
• Central cavity of each acinus is continuous with lumen of the duct.
• Fine duct draining each acinus is called intercalated duct.
• Many intercalated ducts join together to form intralobular duct.
• Few intralobular ducts join to form interlobular ducts.
• Interlobular ducts join to form the main duct of the gland.
• Gland with this type of structure and duct system is called racemose type (racemose = bunch of grapes).

Properties of Saliva

Composition of Saliva

Mixed saliva contains:

Organic Substances

Enzymes

1. Amylase
2. Maltase
3. Lingual lipase
4. Lysozyme
5. Phosphatase
6. Carbonic anhydrase
7. Kallikrein

Other Organic Substances

1. Mucin
2. Albumin
3. Proline-rich proteins
4. Lactoferrin
5. IgA
6. Blood group antigens
7. Free amino acids
8. Non-protein nitrogenous substances:
   • Urea
   • Uric acid
   • Creatinine
   • Xanthine
   • Hypoxanthine

Inorganic Substances

1. Sodium
2. Calcium
3. Potassium
4. Bicarbonate
5. Bromide
6. Chloride
7. Fluoride
8. Phosphate

Gases

1. Oxygen
2. Carbon dioxide
3. Nitrogen

Saliva is an essential digestive juice. Since saliva has many functions, its absence leads to many inconveniences.

1. Preparation of Food for Swallowing

• When food is taken into the mouth, it is moistened and dissolved by saliva.
• Mucous membrane of mouth is also moistened by saliva.
• This facilitates chewing.
• By movement of tongue, moistened and masticated food is rolled into a bolus.
• Mucus of saliva lubricates the bolus and facilitates swallowing.

2. Appreciation of Taste

• Taste is a chemical sensation.
• Saliva, by its solvent action, dissolves the solid food substances.
• Thus, dissolved substances can stimulate the taste buds.
• Stimulated taste buds recognize the taste.

3. Digestive Function

Saliva has three digestive enzymes:

1. Salivary amylase
2. Maltase
3. Lingual lipase

Salivary Amylase

• Salivary amylase is an amylolytic enzyme (carbohydrate-digesting enzyme).
• It acts on cooked or boiled starch.
• It converts starch into:
  • Dextrin
  • Maltose
• Though starch digestion starts in mouth, major part of it occurs in stomach because food stays only for a short time in mouth.
• Optimum pH necessary for activation of salivary amylase is 6.
• Salivary amylase cannot act on cellulose.

Maltase

• Maltase is present only in traces in human saliva.
• It converts maltose into glucose.

Lingual Lipase

• Lingual lipase is a lipolytic enzyme (lipid digesting enzyme).
• It digests milk fats which are pre-emulsified fats.
• It hydrolyzes triglycerides into:
  • Fatty acids
  • Diacylglycerol

4. Cleansing and Protective Functions

1. Due to constant secretion of saliva, mouth and teeth are rinsed and kept free of food debris, shed epithelial cells and foreign particles.
   • In this way, saliva prevents bacterial growth by removing materials which may serve as culture media for bacterial growth.
2. Enzyme lysozyme of saliva kills some bacteria such as:
   • Staphylococcus
   • Streptococcus
   • Brucella
3. Proline-rich proteins present in saliva have antimicrobial property and neutralize the toxic substances, e.g. tannins.
4. Tannins are present in many food substances including fruits.
5. Lactoferrin of saliva also has antimicrobial property.
6. Proline-rich proteins and lactoferrin protect the teeth by stimulating enamel formation.
7. Immunoglobulin A (IgA) in saliva also has:
   • Antibacterial action
   • Antiviral actions
8. Mucin present in saliva protects the mouth by lubricating the mucous membrane.

5. Role in Speech

• By moistening and lubricating soft parts of mouth and lips, saliva helps in speech.
• If the mouth becomes dry, articulation and pronunciation become difficult.

6. Excretory Function

• Many organic and inorganic substances are excreted in saliva.
• It excretes substances such as:
  • Mercury
  • Potassium iodide
  • Lead
  • Thiocyanate
• Saliva also excretes some viruses such as those causing:
  • Rabies
  • Mumps
• In some pathological conditions, saliva excretes certain substances which are not found in saliva under normal conditions, such as glucose in diabetes mellitus.
• In certain conditions, some normal constituents of saliva are excreted in large quantities.

Examples

1. Excess urea is excreted in saliva during nephritis.
2. Excess calcium is excreted during hyperparathyroidism.

7. Regulation of Body Temperature

• In dogs and cattle, excess dripping of saliva during panting helps in loss of heat and regulation of body temperature.
• However, in human beings, dogs do not play any role in this function.

8. Regulation of Water Balance

• When body water content decreases, salivary secretion also decreases.
• This causes dryness of the mouth and induces thirst.
• When water is taken, it quenches thirst and restores the body water content.

Digestive Enzymes of Saliva

Salivary secretion is regulated only by nervous mechanism.

Autonomic nervous system is involved in regulation of salivary function.

Nerve Supply to Salivary Glands

Salivary glands are supplied by:

• Parasympathetic division
• Sympathetic division

of autonomic nervous system.

Parasympathetic Fibers

Parasympathetic Fibers to Submandibular and Sublingual Glands

• Parasympathetic preganglionic fibers to submandibular and sublingual glands arise from superior salivatory nucleus situated in pons.
• After taking origin from this nucleus, preganglionic fibers run through:
  • Nervus intermedius of Wrisberg
  • Geniculate ganglion
  • Motor fibers of facial nerve
  • Chorda tympani branch of facial nerve
  • Lingual branch of trigeminal nerve
• Finally, they reach the submaxillary ganglion.
• Postganglionic fibers arise from this ganglion and supply the submaxillary and sublingual glands.

Parasympathetic Fibers to Parotid Gland

• Parasympathetic preganglionic fibers to parotid gland arise from inferior salivatory nucleus situated in upper part of medulla oblongata.
• From here, fibers pass through:
  • Tympanic branch of glossopharyngeal nerve
  • Tympanic plexus
  • Lesser petrosal nerve
• Finally, they end in otic ganglion.
• Postganglionic fibers arise from otic ganglion and reach the parotid gland by passing through auriculotemporal branch in mandibular division of trigeminal nerve.

Functions of Parasympathetic Fibers

• When parasympathetic fibers to salivary glands are stimulated:
  • A large quantity of watery saliva is secreted.
  • It contains less quantity of organic constituents.
  • Parasympathetic fibers activate acinar cells and cause vasodilation in salivary glands.
  • Neurotransmitter involved is acetylcholine.

Sympathetic Fibers

• Sympathetic preganglionic fibers to salivary glands arise from lateral horns of first and second thoracic segments of spinal cord.
• Preganglionic fibers leave the cord through anterior nerve roots and end in superior cervical ganglion of sympathetic chain.
• Postganglionic fibers from this ganglion are distributed to salivary glands along nerve plexus around arteries supplying the glands.

Functions of Sympathetic Fibers

• Stimulation of sympathetic fibers causes secretion of saliva which is thick and rich in mucus.
• This is because these fibers activate the acinar cells and cause vasoconstriction in salivary glands by secreting noradrenaline.

Salivary secretion is regulated by nervous mechanism through reflex action.

Salivary reflexes are of two types:

1. Unconditioned reflex
2. Conditioned reflex

1. Unconditioned Reflex

• Unconditioned reflex is an inborn reflex that is present since birth.
• It does not need any previous experience.
• This reflex induces salivary secretion when any substance is placed in the mouth.
• It is due to stimulation of nerve endings in mucous membrane of oral cavity.

Examples of Unconditioned Reflex

1. When food is taken.
2. When any unpleasant or unpalatable substance enters the mouth.
3. When the oral cavity is handled with instruments by dentists.

2. Conditioned Reflex

• Conditioned reflex is acquired by experience and needs previous experience.
• Presence of food in the mouth is not necessary to elicit this reflex.
• Stimulus for this reflex is:
  • Sight
  • Smell
  • Hearing or thought of food

Substances Which Increase Salivary Secretion

1. Sympathomimetic drugs like:
   • Adrenaline
   • Ephedrine
2. Parasympathomimetic drugs like:
   • Acetylcholine
   • Pilocarpine
   • Muscarine
   • Physostigmine
3. Histamine

Substances Which Decrease Salivary Secretion

1. Sympathetic depressant like:
   • Ergotamine
   • Dibenzamine
2. Parasympathetic depressants like:
   • Atropine
   • Scopolamine
3. Anesthetics such as:
   • Chloroform
   • Ether

Hyposalivation

It is of two types:

1. Temporary hyposalivation
2. Permanent hyposalivation

Common consequence of hyposalivation → Dental caries.

Temporary Hyposalivation

Temporary hyposalivation occurs in:

1. Emotional conditions such as fear.
2. Fever.
3. Dehydration.

Permanent Hyposalivation

Permanent hyposalivation occurs in:

1. Sialolithiasis
   • Obstruction of salivary duct by formation of salivary calculus (stone).
2. Congenital absence or hypoplasia of salivary glands.
3. Bell’s palsy
   • Paralysis of facial nerve.

Hypersalivation

Hypersalivation occurs in the following conditions:

1. Decay of tooth or neoplasm
   • Abnormal new growth or tumor in tooth or tongue.
   • Due to continuous irritation of nerve endings in the mouth.
2. Disease of esophagus, stomach and intestine.
3. Neurological disorders such as:
   • Mental retardation
   • Cerebral stroke
   • Parkinsonism
4. Some psychological and psychiatric conditions.
5. Nausea and vomiting.

Other Disorders

1. Xerostomia

Causes of Xerostomia

1. Dehydration or renal failure.
2. Sjögren syndrome.
3. Radiotherapy.
4. Trauma to salivary gland or their ducts.
5. Side effect of drugs like:
   • Antihistamines
   • Antidepressants
   • Antiparkinsonian drugs
6. Shock.
7. After smoking marijuana (psychoactive compound from the plant cannabis).

Xerostomia causes difficulties in:

• Mastication
• Swallowing

It also causes halitosis (bad breath).

2. Drooling

Uncontrolled flow of saliva outside the mouth is called drooling.

Drooling occurs because of excess production of saliva in association with inability to retain saliva within the mouth.

Conditions When Drooling Occurs

1. During teeth eruption in children.
2. Upper respiratory tract infection or nasal allergies in children.
3. Difficulty in swallowing.
4. Tonsillitis.
5. Peritonsillar abscess.

3. Chorda Tympani Syndrome

Chorda tympani syndrome is the condition characterized by sweating while eating.

• During trauma or surgical procedure, some of the parasympathetic nerve fibers to salivary glands may be severed.
• During regeneration, some of the nerve fibers, which run along with chorda tympani branch of facial nerve, may deviate and join nerve fibers supplying sweat glands.
• When food is placed in mouth, salivary secretion is associated with sweat secretion.

4. Paralytic Secretion of Saliva

• When parasympathetic nerve to salivary gland is cut in experimental animals:
  • Salivary secretion increases for first 3 weeks.
  • Later it diminishes.
  • Secretion stops at about 6th week.
• Increased secretion of saliva after cutting parasympathetic nerve fibers is called paralytic secretion.
• It is because of hyperactivity of sympathetic nerve fibers to salivary glands when parasympathetic fibers are cut.

5. Augmented Secretion of Saliva

• If nerves supplying salivary glands are stimulated twice:
• Amount of saliva secreted by second stimulus is more than amount secreted by first stimulus.
• This is because first stimulus increases the excitability of acinar cells.
• Therefore, when second stimulus is applied, salivary secretion is augmented.

6. Mumps

• Mumps is the acute viral infection affecting parotid glands.
• The virus causing this disease is paramyxovirus.
• It is common in children who are not immunized.
• It occurs in adults also.

Features of Mumps

• Puffiness of face due to swelling of parotid glands
• Fever
• Sore throat
• Weakness

Mumps affects:

• Meninges
• Gonads
• Pancreas also

7. Sjögren’s Syndrome

It is an autoimmune disorder during which the immune cells destroy exocrine glands such as:

• Salivary glands
• Lacrimal glands

Common Symptoms

• Dryness of mouth due to lack of saliva
• Xerostomia
• Persistent cough
• Dryness of eyes

Severe Conditions

In severe conditions, organs like:

• Kidneys
• Lungs
• Liver
• Pancreas
• Thyroid
• Blood vessels
• Brain

are affected.