Why is it that our mouth waters at the sight of tasty foods?
Pollachi, Tamil Nadu
The three main salivary glands are the parotid, the submandibular and the submaxillary glands. These major glands and 600 other minor salivary glands secrete about 1.5L of saliva per day. Salivary secretion is under neural control. Stimulation of parasympathetic system causes profuse secretion of watery saliva. The parasympathetic nervous signal is under control of two salivatory nuclei present in medulla oblongata. These centres can be directly excited by taste and touch stimuli of the tongue and other areas in mouth.
Sour taste is one of the powerful stimuli. It produces eight times more secretion than other tastes. In addition, the appetite area located close to anterior hypothalamus responds to signals of smell areas located in cerebral cortex by stimulating salivatory nuclei. Therefore, when a person smells his favourite food salivation is greater than disliked food.
Finally, the reflexes from stomach can also increase salivation, particularly when irritating foods are swallowed. This is the reason for increased salivation when one feels nauseated.
In humans, the sight, smell and even thought of food causes salivation —“makes the mouth water”. This is because the salivary secretion is conditioned. Ivan Pavlov’s (1849–1936) classic experiments of conditioned reflex are worth remembering now.
In his classic experiments, the salivation normally induced by placing meat in the mouth of a dog was studied. A bell was rung just before the meat was placed in the dog’s mouth, and this was repeated a number of times until the animal would salivate when the bell was rung even though no meat was placed in its mouth.
In this experiment, the meat placed in the mouth was the unconditioned stimulus, the stimulus that normally produces a particular response, salivation.
The conditioned stimulus was the bell ringing. After these two stimuli were paired sufficient times, the conditioned stimulus produced the response that was originally evoked by unconditioned stimulus. This is also called classical conditioning.
Salivation by humans on the sight, smell or thought of the tasty food is, therefore, a conditioned reflex. The salivation occurs even though the food is not tasted (unconditioned stimulus). The conditioning towards tasty food is owing to training during our childhood. Such training is not given to us towards insipid and unhygienic food.
Associate Professor, Nephrology, NIMS