Secretion of Saliva
The principal glands of salivation are:
(1)parotid,
(2)submandibular,
(3)sublingual glands;
(4)in addition, there are many very small buccal glands.
Daily secretion of saliva normally ranges between 800 and 1500 milliliters, as shown by the average value of 1000 milliliters in Table 64–1.
Saliva contains two major types of protein secretion:
(1)a serous secretion that contains ptyalin (alpha-amylase), which is an enzyme for digesting starches,
(2)mucus secretion that contains mucinfor lubricating and for surface protective purposes.
- The parotid glands secrete almost entirely the serous type of secretion,
- while the submandibular and sublingual glands secrete both serous secretion and mucus.
- The buccal glands secrete only mucus.
Saliva has a pH between 6.0 and 7.0, a favorable range for the digestive action of ptyalin.
The functions of saliva include:
- initial digestion of starches and lipids by salivary enzymes;
- dilution and buffering of ingested foods, which may otherwise be harmful;
- and lubrication of ingested food with mucus to aid its movement through the esophagus.
Structure of the Salivary Glands
The three major salivary glands are the parotid glands, the submandibular glands, and the sublingual glands. Each gland is a paired structure that produces saliva and delivers it to the mouth through a duct. The parotid glands are composed of serous cells and secrete an aqueous fluid composed of water, ions, and enzymes. The submaxillary and sublingual glands are mixed glands and have both serous and mucous cells. The serous cells secrete an aqueous fluid, and the mucous cells secrete mucin glycoproteins for lubrication.
The acinar cells produce an initial saliva composed of water, ions, enzymes, and mucus. This initial saliva passes through a short segment, called an intercalated duct, and then through a striated duct, which is lined with ductal cells. The ductal cells modify the initial saliva to produce the final saliva by altering the concentrations of various electrolytes.
Myoepithelial cells are present in the acini and intercalated ducts. When stimulated by neural input, the myoepithelial cells contract to eject saliva into the mouth.
Salivary acinar cells and ductal cells have both parasympathetic and sympathetic innervation. While many organs have such dual innervation, the unusual feature of the salivary glands is that saliva production is stimulated by both parasympathetic and sympathetic nervous systems (although parasympathetic control is dominant).
The salivary glands have an unusually high blood flow that increases when saliva production is stimulated. When corrected for organ size, maximal blood flow to the salivary glands is more than 10 times the blood flow to exercising skeletal muscle!
Formation of Saliva
Saliva is an aqueous solution whose volume is very high considering the small size of the glands. Saliva is composed of water, electrolytes, α-amylase, lingual lipase, kallikrein, and mucus.
When compared with plasma:
- saliva is hypotonic (i.e., has a lower osmolarity),
- has higher K+ and bicarbonate (HCO3-) concentrations,
- has lower Na+ and chloride (Cl-) concentrations.
Saliva, therefore, is not a simple ultrafiltrate of plasma, but it is formed in a two-step process that involves several transport mechanisms.
- The first step is the formation of an isotonic plasma-like solution by the acinar cells.
- The second step is modification of this plasma-like solution by the ductal cells.
- The acinar cells secrete the initial saliva, which is isotonic and has approximately the same electrolyte composition as plasma. Thus, in initial saliva, osmolarity, Na+, K+, Cl-, and HCO3- concentrations are similar to those in plasma.
- The ductal cells modify the initial saliva. The transport mechanisms involved in this modification are complex, but they can be simplified by considering events in the luminal and basolateral membranes separately and then by determining the net result of all the transport mechanisms.
The luminal membrane of the ductal cells contains three transporters:
Na+-H+ exchange, Cl--HCO3- exchange, and H+-K+ exchange.
The basolateral membrane contains theNa+-K+ ATPase and Cl- channels.
The combined action of these transporters working together is absorption of Na+ and Cl- and secretion of K+ and HCO3-. Net absorption of Na+ and Cl- causes the Na+ and Cl- concentrations of saliva to become lower than their concentrations in plasma, and net secretion of K+ and HCO3- causes the K+ and HCO3- concentrations of saliva to become higher than those in plasma. There is net absorption of solute because more NaCl is absorbed than KHCO3 is secreted. And as ductal cells are water-impermeable, water is not absorbed along with the solute, making the final saliva hypotonic.
The acinar cells also secrete organic constituents such as α-amylase, lingual lipase, mucin glycoproteins, IgA, and kallikrein.
α-Amylase begins the initial digestion of carbohydrates, and lingual lipase begins the initial digestion of lipids. The mucus component serves as a lubricant.
Kallikrein is an enzyme that cleaves high molecular weight kininogen into bradykinin, a potent vasodilator. During periods of high salivary gland activity, kallikrein is secreted and produces bradykinin.
Effect of Flow Rate on Composition of Saliva
Under resting conditions, the concentrations of sodium and chloride ions in the saliva are only about 15 mEq/L each, about one seventh to one tenth their concentrations in plasma. Conversely, the concentration of potassium ions is about 30 mEq/L, seven times as great as in plasma; and the concentration of bicarbonate ions is 50 to 70 mEq/L, about two to three times that of plasma.
During maximal salivation,the salivary ionic concentrations change considerably because the rate of formation 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 quantities of saliva are being secreted, the sodium chloride concentration drops to one half or two thirds that of plasma, and the potassium concentration rises to only four times that of plasma.
Regulation of Salivary Secretion
There are two unusual features in the regulation of salivary secretion. (1) Salivary secretion is exclusively under neural control by the autonomic nervous system, whereas the other gastrointestinal secretions are under both neural and hormonal control. (2) Salivary secretion is increased by both parasympathetic and sympathetic stimulation, although parasympathetic stimulation is dominant. (Usually, the parasympathetic and sympathetic nervous systems have opposite actions.)
There is parasympathetic and sympathetic innervation of acinar and ductal cells. Stimulation of salivary cells results in increased saliva production, increased HCO3- and enzyme secretions, and contraction of myoepithelial cells.
Parasympathetic innervation.
The parasympathetic input to the salivary glands is carried on the facial and glossopharyngeal nerves. Postganglionic neurons release ACh, which interacts with muscarinic receptors on the acinar and ductal cells.
At the cellular level, activation of muscarinic receptors leads to production of inositol 1,4,5-triphosphate (IP3) and increased intracellular calcium (Ca2+) concentration, which produce the physiologic action of increased saliva secretion. Several factors modulate the parasympathetic input to the salivary glands. Parasympathetic activity to the salivary glands is increased by food, smell, and nausea and by conditioned reflexes (e.g., as demonstrated by Pavlov's salivating dogs). Parasympathetic activity is decreased by fear, sleep, and dehydration.
Thesalivary glands are controlled mainly byparasympatheticnervous signals all the way from thesuperior andinferior salivatory nuclei in the brain stem.
The salivatory nuclei are located approximately at the junction 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.The Many tastestimuli, especially the sour taste (caused by acids),elicit copious secretion of saliva—often 8 to 20 timesthe 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, whereasrough objects cause less salivation and occasionallyeven inhibit salivation.
Salivation can also be stimulated or inhibited bynervous signals arriving in the salivatory nuclei fromhigher centers of the central nervous system. Forinstance, when a person smells or eats favorite foods,salivation is greater than when disliked food is smelledor eaten.Theappetite areaof the brain, which partiallyregulates these effects, is located in proximity to theparasympathetic centers of the anterior hypothalamus,and it functions to a great extent in response to signalsfrom the taste and smell areas of the cerebral cortexor amygdala.
Salivation also occurs in response to reflexes originatingin the stomach and upper small intestines; particularlywhen irritating foods are swallowed or whena person is nauseated because of some gastrointestinalabnormality. The saliva, when swallowed, helps toremove the irritating factor in the gastrointestinal tractby diluting or neutralizing the irritant substances.
Sympathetic innervation.
The sympathetic input to the salivary glands originates in thoracic segments T1 to T3 with preganglionic nerves that synapse in the superior cervical ganglion. The postganglionic sympathetic neurons release norepinephrine, which interacts with β-adrenergic receptors on the acinar and ductal cells. Activation of β-adrenergic receptors leads to stimulation of adenylyl cyclase and production of cyclic adenosine monophosphate (cAMP). The physiologic action of cAMP, like that of the parasympathetic IP3/Ca2+ mechanism, is to increase saliva secretion. Sympathetic stimulation also activates α-adrenergic receptors on acinar cells, although the activation of β-adrenergic receptors is considered more important.
Role of Blood Supply
A secondary factor that also affects salivary secretionis the blood supply to the glands because secretionalways requires adequate nutrients from the blood.
The parasympathetic nerve signals that induce copioussalivation also moderately dilate the blood vessels.
Inaddition, salivation itself directly dilates the bloodvessels, thus providing increased salivatory glandnutrition as needed by the secreting cells.
Part of thisadditional vasodilator effect is caused by bradykinin.
Function of Saliva for Oral Hygiene.
Under basal awake conditions, about 0.5 milliliter of saliva, almost entirely of the mucous type, is secreted each minute;
but during sleep, secretion becomes very little.
This secretion plays an exceedingly important role for maintaining healthy oral tissues. The mouth is loaded with pathogenic bacteria that can easily destroy tissues and cause dental caries. Saliva helps prevent the deteriorative processes in several ways.
First, the flow of saliva itself helps wash away pathogenic bacteria as well as food particles that provide their metabolic support.
Second, saliva contains several factors that destroy bacteria.
(a)Thiocyanate ionsand
(b)Several proteolytic enzymes(most important, lysozyme)which
- attack the bacteria,
- aid the thiocyanate ions in entering the bacteria where these ions in turn become bactericidal, and
- digest food particles, thus helping further to remove the bacterial metabolic support.
Third, saliva often contains significant amounts of antibodies that can destroy oral bacteria, including some that cause dental caries.
In the absence of salivation, oral tissues often become ulcerated and otherwise infected, and caries of the teeth can become out of control.