The Purposes of Spit
The Purposes of Saliva and Where it Comes From
We secrete between 1 and 2 liters of saliva every day. Saliva helps us chew, taste, swallow, and speak. It is created by 3 main sets of glands that are found in the mouth:
- the sublingual glands, which are under the tongue
- the submandibular glands, which are found under the jawbone (which is also known as the mandible)
- the parotid glands, which are near the hinge of the jaw
In addition to these glands, there are 800-1000 minor salivary glands through the tissue of the mouth, lips, cheeks, palate, and tongue. These minor glands are surrounded by connective tissue, but not encased by them. They are 1-2 mm in diameter and may share a duct with another gland.
Location of Major Salivary Glands
Location of Medulla Oblongata in the Brain
What Makes Us Salivate
Smelling, seeing, or thinking about food send messages to the medulla oblongata, a portion of the brain, to prepare the body to digest incoming food. These messages are strengthened once food is put into one’s mouth. When the medulla oblongata receives these signals that warn of food’s imminent arrival, it sends a message to the salivary glands that it’s time to begin secreting saliva.
Overview of Salivary and Digestive Glands
How Saliva Protects Us
The high water and mucus contents of saliva help coat the food so it’s easier to swallow. Mucus is produced by the goblet cells of the sublingual glands and the submandibular glands. It is largely composed of glycoproteins, molecules that contain sugars and amino acids or peptides chemically bonded together, called mucins. Mucus’ primary jobs in the digestive system are to coat the lining of the GI tract and to lubricate the contents of the digestive system.
Saliva also protects our mouths from invaders. It contains salivary lysozyme which is an enzyme that helps fight bacteria, fungi and viruses in the mouth. Saliva also contains salivary immunoglobulins, which can incapacitate the bacteria and viruses they encounter. In addition, saliva helps rid our mouth of particles and washes our tongue and teeth.
Location of the Von Ebner's glands
How Saliva Helps Us Taste Our Food
In addition to coating the food, saliva also helps begin to break the food down as we chew. Chemical digestion is the breaking down of the bonds and linkages that connect the molecules in our food to each other. It is accomplished by enzymes, proteins that aid in breaking other proteins and substances down into more elemental building blocks.
Chemical digestion begins with the secretion of salivary amylase in the mouth. Salivary amylase is found in the serous fluid of saliva, which is produced by the parotid glands and the submandibular glands. Salivary amylase is an important enzyme that breaks starch into maltose, a simpler form of carbohydrate and sugar.
Lingual lipase, which helps break down fats, is secreted by specialized Von Ebner's glands which only produce serous fluid. These glands are found just in front of the terminal sulcus, a shallow V-shaped grove on the surface of the tongue that separates the posterior third of the tongue from the anterior two-thirds. The Von Ebner's glands lie among the circumvallate and foliate papillae of the tongue, as depicted in the figure above.
If the initial chemical digestion that is accomplished by the salivary amylase and lingual lipase were to happen after the food left the mouth our sense of taste would be considerably diminished. This is because our taste buds would be unable to recognize the various chemicals they interpret as tastes.
How Spit Changes as we Eat a Meal
Our saliva contains different ratios of mucus, enzymes and watery secretion, depending on the phase of digestion we’re in. Salivary secretion is controlled largely by our autonomic nervous system, which has 2 branches: the sympathetic and parasympathetic nervous systems.
When the first signals announce the arrival of food, the sympathetic nervous system, which is responsible for our flight or fight responses, activates the salivary glands. At this stage, the saliva has more mucus and enzymes, in order to properly digest and coat the food to prepare for swallowing.
Once we have finished chewing and swallowing our food, the parasympathetic nervous system, the part of our nervous system that is responsible for our “rest and digest” reflexes, tells our salivary glands to secrete less mucus and enzymes and waterier secretions.
Overview of the Autonomic Nervous System
Salivary Glands: Their Structures and the Cells they are Made of
Salivary glands are made of little clumps of cells, called lobules, which receive blood vessels and nerves needed to integrate digestion throughout the body. The lobules are held together as a gland by a capsule of connective tissue.
There are 3 main types of cells in the salivary glands:
- Serous cells-which are shaped like a pyramid
- Mucous cells-which are shaped like cubes
- Myoepithelial cells
The serous cells are shaped like pyramids. These pyramidal shaped cells than clump together in spheres. These spheres secrete the amylase used in chemical digestion of starches that was discussed earlier.
The second type of cell in the glands, the mucous cells, are shaped like cubes and join together to make little tubes called tubules. Those tubules empty into ducts that are known as intercalated ducts. (Ducts are tubes that lead from an exocrine gland or organ and deliver the gland’s product to wherever it’s going to be used.) Those intercalated ducts gather into larger ducts which have little stripes, or striations on them. These striated ducts gather into ducts that travel between the lobes of the gland, and are known as interlobar or excretory ducts. Finally, the main ducts secrete the saliva into the mouth.
As the saliva travels along the duct to get to the mouth, Na+ is reabsorbed by the cells and K+ is excreted into the duct. Typically, water would follow the Na+ out of the duct, but the cell membranes of the duct cells don’t permit water to pass. This leads to waterier and more dilute saliva.
The third kind of cell, the myoepithelial cells, surround the opening of the duct, where the saliva is secreted into the mouth. They can squeeze the gland so that the saliva comes out faster.