Salivary Glands and Secretion

Question 1

General Principles of GI secretions

Answer 1

Basic Mechanism

• contact of food with epithelium stimulates secretion
• types of stimuli that activate this system are
  1) tactile stimuli
  2) chemical irritation
  3) distension of gut wall

Autonomic Stimulation

• parasympathetic stimulation increases the alimentary tract glandular secretion rate
• sympathetic stimulation has dual effect
  a) alone it usually increases secretion
  b) if parasympathetic is already causing secretion, its stimulation usually reduces the secretion.

By hormones
- several hormones regulate the volume and character of secretions. they are usually Polypeptide or its derivatives

Basic Mechanism of secretion by glandular cells
a) nutrient materials needed for Secretion must diffuse or be actively transported by blood into the base of the glandular cells

b) Energy (ATP) produced by mitochondria, along with appropriate substrates is then used to synthesize secretory substances; entirely in the ER and Golgi complexes. Ribosomes helping for protein synthesis
c) secretory Substance from tubules of the ER ——20 minutes—> vescicles of golgi complex
d) in golgi body materials are modified, concentrated, and discharged into cytoplasm as secretory vescicles
e) vescicles remain stored until the nervous / hormonal control signals cause the cells to extrude the vesicular contents

Question 2

Types of Salivary glands

Answer 2

there are major and minor salivary glands

1) principal or major salivary glands are
a) parotid
b) sub mandibular
c) sub lingual

2) minor salivary glands are buccal, labial and glossopalatine

saliva contains two major types of protein secretions

a) serous secretion - contains ptyalin
b) mucus secretion - mucin for lubrication and protection

daily secretion: 800-1500 ml (average 1000 ml)

pH - 6 - 7.4

Parotid glands

• largest salivary glands
• entirely serous glands
• parotid Secretion is rich in water and electrolytes and lacks mucin
• located near the angle of neck
• 25% of the total salivary Secretion

Sub lingual glands

• below the tongue or in the floor of the mouth
• serous and mucous glands
• 8-20 ducts open on the summit of sublingual fold

Sub mandibular glands

• aka sub maxillary
• below the inner ramus of mandible
• serous and mucous

Question 3

Composition of Saliva

Answer 3

• contains large quantities of K+ and Hco3-
• Na+ and cl- are several times less in saliva than the plasma

                     composition
                              |
       a) water                     b) solutes
                                                   |
     Organic                                Inorganic • ptyalin (alpha amylase)               • Na • mucin                                            • Cl • proline rich proteins                   • K • igA.                                                • HCO3- • lysozymes                                    • Ca • lipase                                     • Phosphate • peroxidase                          • Thiocyanate • kallikrein  • histatins  • lactoferrin • statherin

Question 4

Functions of Saliva

Answer 4

1. Defense
   a) Antibacterial (Thiocyanate, Lysozyme and proteolytic enzymes)
   b) Antifungal
   c) Antiviral
   d) Immunological
2. Digestive functions
   a) Digestive enzymes - ptyalin and lipase
   b) Formation of bolus
   c) Taste
3. Protective Function
   a) Protective coating for soft tissues
   b) Protective coating for hard tissues - teeth
4. Lubrication function
   a) Keeps the oral cavity moist
   b) Facilitates speech
   c) Helps in mastication and swallowing
5. Buffering function

Question 5

Mechanism of Salivary secretion

Answer 5

Salivary Secretion occurs in two stages: secretion in the acini and secretion in the salivary duct

Secretion in the Acinus of Gland
- The acini secrete a primary secretion that contains ptyalin and/or mucin in a solution of ions with concentrations not greatly different from those of typical extracellular fluid.

• as the primary secretion flows through the ducts, two major active processes take place that markedly modify the ionic composition of the fluid in the saliva

1) first, Na+ is actively reabsorbed from all the salivary ducts and k+ is actively Secreted in exchange for na+.
so na+ conc in the saliva becomes greatly reduced whereas the k+ conc becomes increased.
- however there is excess na+ reabsorption as compared to k+ secretion which creates electrical negativity of -70 mv in the salivary ducts
- this negativity causes cl- to be reabsorbed passively.
- therefore the chloride ion conc un the salivary fluid falls to a very low level

second, hco3- is secreted by the ductal epithelium into the lumen of the duct

• the net result of all these transport processes is that under resting conditions the conc of na+ and cl- in the saliva are only about 15 mEq/L each,, about 1/7 to 1/10th their conc in the plasma
• conversely k+ = 30 mEq/L is 7 times as great as in plasma
• and HCO3- = 50-70 mEq/L is about 2-3 times that of plasma

during maximal salivation, the salivary ionic conc change considerably because the formation rate of primary secretion by the acini can increase as much as 20 fold.
- this acinar secretion then flows through the ducts so rapidly that the ductal reconditioning of the secretion is considerably reduced.
- therefore, when copious amounts of saliva are being Secreted, the sodium chloride conc is about 1/2 or 2/3 that of plasma
and potassium conc rises to only about 4 times that of plasma

Question 6

Regulation of Salivary Secretion

Answer 6

controlled exclusively by neural mechanisms. both sympathetic and parasympathetic stimuli influence salivary Secretion.

salivary glands are controlled mainly by the parasympathetic nervous signals all the way from superior and inferior salivary nuclei in brainstem

• located at the juncture of 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 sour taste elicit copious secretion of saliva — often 8-20 times the basal rate
• also certain tactile stimuli like presence of smooth objects in the mouth causes marked salivation, whereas rough objects cause less salivation and occassionally even inhibit salivation

higher centers (CNS) tactile and
| taste stimuli
|_____________________________|
|
superior and inferior
salivatory nuclei
|
salivary glands
|
secretion or inhibition

• nervous signals arriving in the salivatory nuclei from higher centers of the central nervous system can also stimulate or inhibit salivation
• for instance, when a person smells or eats favourite foods, salivation is greater than when food that is disliked is smelled or eaten
• appetite area which partially regulates this is located close to the parasympathetic centers of anterior hypothalamus
• Salivary production ↑ - vasodilation by parasympathetic nerves
• kallikrein secreted by Salivary cell which in turn acts as an enzyme to split one of the blood proteins, an alpha 2- globulin to form bradykinin, a strong vasodilator.
• in turn Salivary secretion increases.