Sunday, August 25, 2013

The Special Senses, part 1- the Chemosenses, gustatory and olfactory, aka taste and smell

Smell may be the least neurologically complex system of the sense as the information from the smell sensors in the nose have a direct link to the processor in the brain. Taste is considered one sense but in fact there are 5 specific receptors (located on the tongue in areas) for salty, sweet, tart, sour and what is being called “meat” flavors. These five flavors are given more variety by the sense of smell which distinguishes variations within these five flavors of foods. Often, in high school biology class, teachers do an experiment where they stop the sense of smell and offer a bite of an apple and of an onion to a student. Without smell the student cannot tell which is which as the texture is similar and both are sweet.  Smell is one of the senses that disappears in the aging process and older people often lose interest in most foods, except sweets, because the food becomes bland to them without smell.  Smell is an important first indicator of some dangers as it can detect more remote sensation than taste. To taste something it must touch the tongue but smells waft through the air to your nose.    
The system for smell develops between 16 and 24 weeks of gestation and is in place and ready to function by about 24 weeks gestation. The first taste buds appear at 8-9 weeks gestation with most being present by about 16 weeks gestation. Some refinement continues but most full term babies have a fully developed taste system at birth.
The system of smell is perhaps the strongest sense in infants and babies are able to recognize the smell of their mother’s breast milk from that of another person (the study was done using breast pads) almost from birth. They also respond to strong smells with avoidant movements from an early age. I would avoid strong perfumes and air fresheners around very young infants as these might be unpleasant to their sensitive system. In a study (titled “Natural odour preferences of newborn infants change over time.” by [My paper]H Varendi, R H Porter, J Winberg) it was shown that babies preferred their mother’s unwashed natural breast over the washed and soapy smelling other breast during the first week to 10 days of life.  It has been proposed that smell is the first sense used in socializing as newborns begin to recognize different people by their smell, which allows them to respond socially to those they know and those they do not know from a very early age.

These systems rely on experience after birth to become completely developed. The brain records tastes and smells so that a child can recognize what they mean. Infants can register likes and dislikes from the start. Due to their limited experience, infants are happy with relatively bland food and do not need salt or sugar added to foods to want them. However, they recognize variations in their food and may reject food that tastes
“funny”. Many breast feeding mothers report that their baby is not interested in the breast after they have eaten a certain type of food. Which foods trigger that response are different for each baby but all are strong flavored foods. In my case, it was pickled foods that I had to avoid. This is learned by the baby through experience with foods and by the mother in watching her baby’s reaction to feeding.
On the other hand, older people who have lost some of their sense of smell are best able to taste the flavors that are tasted by the tongue. Sweet is the strongest, being at the front of the tongue, followed by salt. Older adults will often only express interest in sweet foods and will often over salt foods in order to experience variation in tastes. Many older adults will say that food these days doesn’t taste like it used to. For them, it is probably true.
It is thought that these two systems have some protective functions as most really bad smells and bitter tastes are not good for people so your own body can offer some protection against certain poisons. However the system is not foolproof and some unpleasant tasting things are not harmful while some harmful things are not unpleasant. It has been noted that during pregnancy, many women are exceptionally sensitive to smells, which may be a primitive form of guiding them towards the particular foods they need.

In any case, the sense of smell is one of the least studies and most underrated the special senses. Below are some links to references on the sense of smell:

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