Salivation

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Control of Salivation

Mastication is one of the most important regulators of salivation and it is nearly as powerful a stimulus as taste. The smell of food can also trigger salivation but, in humans at least, anticipation of feeding has only a small effect. It is possible that the mouthwatering sensation that occurs when you think of something tasty is due to a very small amount of previously formed saliva that is squeezed out the glands by the action of the myoepithelial cells.

The stimuli for salivation act through the salivary nuclei in the pons and the medulla which in turn stimulate via cranial nerve VII (facial) to activate the submandibular and sublingual glands and cranial nerve IX (glossopharyngeal)to activate the parotid. See also "Neural control of Salivation"

Fear is one of the very few ways of inhibiting salivary secretion. Think about how dry your mouth becomes just before an exam

Functions of Saliva

Everyone knows about the digestive function of saliva: salivary amylase; lingual lipase, yadda, yadda, yadda. This is the least important function of saliva and the one that you would be least likely to miss, where it to go away. Perhaps you might be slightly less able to digest chips, but the pancreas is king of digestive enzymes (including amylase) and in comparison the contribution of saliva is trivial.

MUCH more important is the role of saliva in protecting the teeth. The relationship is very simple. No saliva, no teeth. The image of rampant caries was taken from an individual who had lost salivary function following radiotherapy for head and neck cancer. It is to be hoped that a dentist would be included in the care team for such an individual because they might at least be able to at least slow down the process.

The next or possible equally most important function of saliva is lubrication. The thin film of saliva that coats the oral surfaces enables speech as well as chewing and swallowing. Think again about a frightening experience, could you talk with no saliva?

The same thin film of saliva also protects the mucosa from infection. The pretty gross image of a Candida infection was also taken from someone with a dry mouth.

See also "Functions of Saliva"

Saliva protects teeth by diluting any sugar or acid in the mouth and then by removing it. Every time you swallow, sugar and acid moves to the stomach, where it can do no (or at least less) harm. Equally important, saliva (at least stimulated saliva) contains bicarbonate which buffers acid, especially important is the buffering of acid produced by plaque bacterial.

The Stephan curve shows the effectiveness of saliva in reversing the change in plaque pH following exposure to sugar. Notice that, with "normal" salivary flow, the teeth are exposed to a pH less than 5.5 for about 15 minutes. This is called the Critical pH because at pH 5.5 or less saliva is no longer supersaturated with calcium and phosphate and so these minerals dissolve out of the teeth. This plaque-acid mediated demineralisation is better known as caries.

If you stimulate more salivary flow then the Stephan curve will be shorter and the teeth will not be vulnerable to demineralisation for so long. If you have a low stimulated salivary flow rate, then you are in trouble because your teeth will be exposed to acid for longer. If you have NO stimulated saliva (this happens) then your teeth are pretty much doomed.

Even though saliva is 99% water, it is a much better lubricant than plain water because it contains salivary glycoproteins which have a relatively small protein core and massive side chains that stick out, a bit like the tines on a hairbrush. In fact, I think of salivary glycoproteins to be a bit like little hairbrushes (without handles, obviously) that stick together when they touch and naturally form into a thin film.

The layer of saliva also prevents bacteria and fungi from adhering to the mucosal surfaces. Saliva also contains many antibacterial proteins including IgA. See the book "Saliva and Oral Health" of which there are many copies in the library for much more detail. Both the 3rd and 4th editions are available and both are fairly current. The 2nd edition is OK in parts but the 1st edition is pretty obsolete (I didn't write the section on the control of salivary secretion for this edition.).

The lubricating properties of saliva allow speech and swallowing

Deglutition

Deglutition (swallowing): a process with three phases. Click on the movie still to start it.