Saliva is made by salivary glands that are found in the underlying tissues of our mouths.
The basic unit of salivary glands are clusters of cells called acini. These cells secrete a fluid that contains water, electrolytes, mucus and enzymes all of which flow out into a series of collecting ducts. The main salivary glands are the:
* Parotid glands * Submandibular glands * Sublingual glands
Each gland is found in symmetrical pairs in the head. Each gland has a tube shaped duct that carries the saliva produced into the mouth. There are also smaller saliva-producing glands that are dotted throughout the mouth and contribute to the overall amount of saliva produced.
How the glands make Saliva?
The type of nerve system that controls saliva production is the autonomic nervous system, which controls both the volume and type of saliva secreted. The secretion of saliva by each gland is controlled by two different types of nerves; sympathetic and parasympathetic nerves.
The parasympathetic nerve supply is most active during the day, whilst eating and creates more watery, or serous saliva; predominantly produced by the parotid gland, and partly by the submandibular gland.
The parasympathetic system turns up the flow of saliva by releasing a chemical, acetylcholine, which stimulates the glands to make more saliva. If these glands get diseased, damaged, or affected by drugs, they may not make enough saliva, leading to dry mouth.
The sympathetic nerve supply produces predominantly thicker mucous saliva mainly by the sublingual and partly the submandibular glands. This may occur when in certain situations, fear, stress or anger are aroused. This is also the case during hard physical exercise.
The Parotid gland
The Parotid glands are the largest of the glands and lie between the back of the jaw and each ear and secrete about 25% of the total saliva amount at rest. Each gland is surrounded by a hard capsule called the parotid capsule. The parotid mainly produces watery, or serous saliva. It’s duct opens in the mouth just opposite the crown of the 2nd upper molar tooth. The gland’s productions are predominantly controlled by a nerve called the glossopharyngeal nerve which originates in the superior salivatory nucleus of the medulla in the brainstem.
The Submandibular Gland
The submandibular glands are of intermediate size and lie just inside the lower-back parts of the mandible (jaw) in the floor of the mouth. A part of the gland curls inwards around the mylohyoid muscle. The submandibular produces most of the saliva at rest (about 60%) and its ducts are 5cm long, emptying underneath the tongue at the floor of the mouth. The type of saliva produced is both serous and mucous saliva: the amount of each is altered depending on which nerves (parasympathetic or sympathetic) are in control.
The submandibular gland is innervated predominantly by the facial nerve (CNVII). The nerve fibres begin in the superior salivatory nucleus in the pons of the brainstem.
The Sublingual Glands
The sublinguals are the smallest glands and are located just under the floor of the mouth, above the mylohyoid muscle. You can feel it as a ridge under your tongue. These secrete around 5% of total saliva at rest and produce mainly mucous saliva secreting glands, although some serous saliva is also produced. It’s major and minor ducts also empty at the floor of the mouth in a row along with the submandibular duct. The nerve controlling sublingual production follows the same pathway as the nerve controlling the submandibular gland (CNVII).
Where does Saliva go?
Saliva is primarily involved at the beginning of the swallowing process.
When food is placed before the nose or eyes, the sight and smell of food stimulates the autonomic nervous system which in turn sends messages to the glands instructing them to produce saliva. The saliva created is secreted into the mouth. It is mixed up with the food and swallowed.
Why have Saliva?
Saliva has many uses within the human body. It primarily:
- Begins digestion. The enzyme Amylase in saliva begins the process of the breaking down of carbohydrates of food in the mouth.
- Lubricates the tongue and lips for smooth and clear articulation of speech.
- Protects the lining of the mouth from damage caused by abrasive foods and objects.
- Assists with acidity levels in the digestive tract. Bicarbonate ions regulates the pH levels in the mouth and eosphagus.
- Acts as a solvent so that substances in the mouth can be tasted.
- Maintains a clean and hygienic mouth and carries anti-bacterial agents (immunoglobulins). They destroy micro-organisms and remove toxic substances.
Influences on saliva production
The general pattern of saliva production is that it is greater during the day than at night and when upright rather than lying down. However, from person to person, the production of saliva can vary. Many external and internal elements can influence saliva production:
- Mood (eg. anxiety, depression).
- Gender. Evidence has shown that males produce greater amounts and rates of flow of saliva than females.
- Age. Although there is conflicting evidence in regards to this, several studies have found that the prevalence of oral dryness increases with aging and that the resting flow rate of saliva decreases with age.
- The amount of water you drink. Reducing bodywater content may lead to less saliva flow at rest.
- Chewing. It is generally agreed that chewing creates more whole saliva flow, particularly of the parotid secretions and thus serous saliva. Nerve endings or receptors (periodontal mechanoreceptors) ascertain the force and frequency of chewing and feedback the information so that the amount of saliva secreted from the parotid is adapted accordingly.
- Taste. Research has found that saliva contains specific proteins that are growth factors that make taste buds develop and mature. Without these growth factors, taste buds degenerate. Decreased salivary flow results in a clinically significant oral imbalance that may manifest as altered taste sensation.
- Smoking. Saliva production may be increased.
- Sight of Food. It is commonly thought that saliva is produced upon the sight of food.