Saliva produced in the mouth and is responsible for two major functions – the taste and digestion of food, and as a defense against microbes in the mouth. The main components of saliva are mucus and serous fluids with enzymes, electrolytes and antibodies. The composition of saliva does not remain the same all the time. It changes during the course of the day, and also in response to food and other stimuli. Similarly the quantity of saliva fluctuates, often reaching its highest levels when hungry and during a meal and the lowest levels are when sleeping. Sometimes the salivation increases to a point where it is considered excessive, and this can be a sign of certain diseases.
It is important to understand Saliva is mainly produced by three pairs of salivary glands in the mouth. These salivary glands are classified as:
- Parotid glands: Parotid glands are the largest salivary glands in the mouth. They are shaped in the form of inverted pyramids, and are located between the angle of the jaws and the beginning of the ears.
- Submandibular glands: As the name implies, submandibular glands are located below the mandibles (the lower jaw bone).
- Sublingual glands: Sublingual glands lie below the tongue on both sides of the mouth. They are the smallest salivary glands and are oval-shaped. The sublingual glands on both sides of the floor of the mouth unite with each other around the root of the tongue.
In addition to the above mentioned salivary glands, several small accessory glands in the oral cavity also contribute to the production of saliva.
Saliva in Quantity
On an average, we secrete about 1 liter of saliva every day. The actual amount may vary from 800 mL to 2 liters, depending on a number of factors. Saliva production is a continuous process, with at least 0.5 mL of saliva being produced every minute. It occurs 24 hours a day. However, the rate of production varies greatly with the time of the day (more in daytime and less during sleep) and other regulatory factors.
Some of the factors that affect the rate of normal saliva production include:
- Food: Presence of food in the mouth increases saliva production. The increase can be up to 20-fold in comparison to the baseline level. This is because the presence of physical food in the mouth stimulates taste and tactile sensations that results in parasympathetic activation of the salivary glands.
- Aromas: Even in the absence of food in the mouth, the aroma of delectable foods stimulates production of saliva.
- Thoughts: Saliva production is also stimulated by just thinking about certain foods, even when no food or aroma is physically present.
- Sleep: Saliva production decreases during sleep.
In addition to the above mentioned factors that regulate the normal rate of saliva production, certain pathological factors can increase the rate of saliva production abnormally.
Causes of Excessive Salivation
The quantity of saliva in the mouth at any one time is dependent on the amount of saliva secreted by the salivary glands and the drainage of saliva usually down the throat into the gut. Excessive salivation can be attributed to two conditions:
- Excessive production of saliva by the salivary glands (a condition known as polysialia).
- Block in saliva drainage from the mouth into the gut (often related to swallowing problems, or dysphagia).
In most cases, excessive salivation is not a serious problem since excess saliva can be swallowed. However, it might be an indication of a serious underlying cause. Warning signs include abnormal enlargement of a salivary gland and blood in the saliva.
Read more on bloody saliva.
Increased Saliva Production
- Orodental: Various conditions that affect the teeth and the mouth can lead to excessive saliva production. Examples include ill-fitting dental prosthetics, dysfunction of temporomandibular joint, and involuntary teeth grinding in bruxism.
- Water brash: Water brash refers to sudden episodes of excessive salivation due to regurgitation of gastric contents. Conditions that can cause water brash include inflammation of esophagus (esophagitis), gastroesophageal reflux disease (GERD), and gastritis.
- Inflammation and infection: Certain inflammatory and infectious diseases are also associated with excessive saliva production. Examples include inflammation of the mouth (stomatitis), infections of the upper respiratory tract, syphilis, tuberculosis, rabies, encephalitis, and labyrinthitis.
- Pain: A sudden onset pain in the mouth (stomatodynia) can cause excessive salivation as well.
- Nerve disorders: Since nerve inputs regulate saliva production, it is no surprise that some of the nerve disorders can cause excessive saliva production. Examples include trigeminal neuralgia, geniculate neuralgia, and facial nerve palsy (also known as Bell’s palsy).
- Medications: An overdose of certain drugs (both medicines and illicit drugs) can cause excessive saliva production. Examples include ketamines, nicotine, cholinergics, narcotics, and psychoactive drugs.
- Poisoning: Cases of poisoning have also been associated with increased salivation. Examples include poisoning with arsenic, mercury, mushrooms, and neurotoxins in snake venom.
Impaired Saliva Drainage
- Stroke: Stroke is a condition that arises after blood supply to a part of the brain is either severely reduced or completely blocked. The resulting oxygen deprivation kills that part of the brain, and affect various downstream processes throughout the body.
- Motor neuron disorders: Motor neuron diseases specifically lead to progressive destruction of motor neurons that are responsible for controlling muscular contractions in the body. Some of these disorders lead to dysphagia or difficulty in swallowing.
- Multiple sclerosis: Multiple sclerosis is an autoimmune disease. It is characterized by a destruction of the myelin sheath that envelopes the nerves, leading to disruption of nerve impulses. This can lead to dysphagia.
- Muscular dystrophy: Muscular dystrophy is a group of genetic diseases that progressively destroy the skeletal muscles of the body. Among other signs and symptoms, it can lead to difficulties in breathing and swallowing.
- Myasthenia gravis: Myasthenia gravis is characterized by muscular weakness caused by the loss of the ability of the muscles to respond to neural signals. It is an autoimmune disease.
- Parkinson’s disease: Lack of dopamine neurotransmitter in the brain causes Parkinson’s disease. This affects muscle movements and coordination all over the body.
- Polio: Polio virus destroys the motor neurons in the central nervous system and affects skeletal muscle movements.
- Swelling: Any inflammation and swelling in the mouth and throat could also result in difficulties in swallowing.
- Tumors: Tumors in the mouth and throat may cause obstructions that lead to dysphagia.