In this lesson, we will discus the process involved in the formation of saliva. Here we will explain the mechanism of secretion of saliva. In the end of the lesson, you will also have access to mechanism of salivary secretion ppt (we are working on this)
The first secretion food encounters after ingestion is the saliva. Saliva is a hypo-osmotic fluid that contains ions, mucus, enzymes such as lysozyme and phospholipase, and other proteins such as immunoglobulins dissolved in water.
There Are Three Major Salivary Glands
Associated with the oral cavity are several complex glands that provide saliva for digestion of food as well as maintenance of oral hygiene. These glands called salivary glands are compound acinous glands that lie outside the wall of the gastrointestinal tract.
Salivary glands secrete saliva into the mouth, the principal glands being the parotid, submandibular, and sublingual glands. In addition to the three major salivary glands, we have pockets of small buccal glands that secrete small amounts of saliva into the mouth. Together, the salivary glands secrete about 800 to 1500 mL of saliva every day. Salivary glands are exocrine glands
Mechanism And Control Of Salivary Secretion
Salivary Secretion Is A Two-step Epithelial Transport Operation
It is necessary to describe the internal structure of the salivary glands before one can understand fully the mechanism of salivary secretion.
Salivary glands are exocrine glands with a secretory epithelium that forms many acina (plural of acinus).
The two steps in salivary secretion determine the ionic/electrolyte composition of saliva
According to the diagram above a hollow center or lumen forms in exocrine glands, secretions of the gland move to the surface of the epithelium through the lumen commonly called duct. The secretory epithelium of salivary glands (also called acinus) opens to the oral cavity through series of ducts.
Structurally, a salivary gland therefore constitutes two units: the acinus and the salivary duct.
A typical salivary gland contains millions of acini that feed into a system of ducts that coalesce to form ducts that finally empty into the mouth (oral cavity). The acini contain epithelial cells that serve secretory functions whereas the salivary ducts deliver secreted saliva to the oral cavity.
The first epithelial transport operation involves the acini whereas the second involves the salivary ducts.
Primary And Secondary Saliva Secretion
The acini secrete the primary secretion. Primary secretion contains mucus and/or ptyalin, in a solution of electrolytes in concentrations similar to that of plasma. As the primary secretion flows through the salivary ducts on its way to the oral cavity, two significant active epithelial transport processes occur that markedly change the ionic composition of the formed saliva.
Ductal Reconditioning Changes The Ionic Composition Of The Primary Secretion
First, ductal cells actively reabsorb sodium ions (Na+) and at the same time actively secreting potassium ions (K+). Consequently, the sodium ion concentration of saliva becomes significantly lower than that of plasma. Concomitantly, the potassium ion concentration of saliva becomes higher than that of plasma. However, reabsorbed sodium outweighs secreted potassium creating an electrical gradient that drives the passive reabsorption of chloride ions.
In the second phase of saliva secretion, ductal cells secrete bicarbonate ions into the lumen of the duct. The net effect of these epithelial transport processes is that during resting flow rate, the concentrations of sodium and chloride ions in saliva falls to 15mEq/L each which is about one ninth of their concentrations in plasma. On the other hand, the concentration of potassium ions rises to about 30mEq/L, which is about seven times as great in plasma.
At maximal flow rates, the rate of formation of primary secretion by the acini may increase as much as 20-fold. Moreover, at such flow rates, acinar secretion flows through the ducts so rapidly that the ductal reconditioning of the primary secretion is minimal. Therefore, at maximal flow rates (i.e. when copious amounts of saliva are being secreted) sodium chloride concentration may only rise to one half or two thirds that of plasma whereas the potassium concentration rises to only four times that of plasma.
Ionic Composition Of Saliva
The ionic composition of fluid secreted by the acinar cells (the primary secretion) is similar to that of plasma (high in sodium and low in potassium). However, as the fluid passes through the duct on its way to the oral cavity (mouth), epithelial cells along the duct reabsorb sodium ions (Na+) and secrete potassium (K+). This epithelial transport along the ducts, called ductal reconditioning, makes the ionic composition of the ductal fluid significantly different from that of plasma. Therefore, the final secreted saliva is high in potassium and low in sodium.
Saliva contains significantly large amounts of potassium and bicarbonate ions. However, saliva contains sodium and chloride ions in concentrations that are several times less when compared to those of plasma.
The Regulation Of Saliva Secretion Is Purely Neural, With No Hormonal Component
Tactile and taste stimuli from the oral tissues regulate and control salivation. Fibers of the autonomic nervous system carry taste and tactile stimuli, which arise from the tongue, mouth and pharynx, to the superior and inferior salivatory nuclei situated in the brainstem. Parasympathetic fibers leave the salivatory nuclei and innervate the salivary glands where they stimulate saliva secretion. Of all the tastes, sour taste induces the greatest amount of saliva secretion by the salivary glands. Both superior and inferior salivatory nuclei lie where the medulla and the pons meet.
Moreover, higher brain centers can inhibit or stimulate salivation by sending nervous signals to the salivatory nuclei in brainstem. Among the higher centers of the brain that regulate salivary secretion are the appetite center of the brain, taste and smell areas of the cerebral cortex, and certain center in the amygdala.
Interestingly, both parasympathetic and sympathetic stimulation increases the volume of saliva secreted. However, parasympathetic stimulation of salivary glands increases the volume of secretion far more than sympathetic stimulation does. Sympathetic nerves to the salivary glands originate from the superior cervical ganglia and runs along the surfaces of blood vessels to the salivary glands. The regulation of saliva secretion is purely neural. Parasympathetic stimulation increases volume of saliva secreted primarily by increasing blood flow to the salivary glands secondary to vasodilation of their blood vessels.
Saliva Protects The Oral Tissues
At basal flow rate, salivary glands secrete about 30mL of saliva every hour. Several pathogenic bacteria colonize the mouth and can easily invade the body to destroy oral tissues and cause dental carriers. Secreted saliva helps to maintain oral hygiene in different ways.
One of the several proteolytic enzymes saliva contains is lysozyme; lysozyme attacks and destroys bacteria. Moreover, saliva contains significant amounts of secretory antibodies called immunoglobulin A, which can destroy some microbes in the mouth.
The role of saliva in maintaining optimum oral health is obvious when we consider persons that lack salivary glands and are unable to secrete saliva. A medical condition called xerostomia is the absence of saliva and mostly results from the congenital absence of salivary glands. Individuals with xerostomia often develop raw patches (ulcers) in their mouth and they see the dentist more often because of frequent dental carriers.