Smell and Taste Receptors: What is It We Smell? How Taste Works?
Smell and Taste Receptors: What is it We Smell? How Taste Works?
By Mr Ghaz, January 11, 2011
Smell and Taste Receptors: What is it We Smell? How Taste Works?
The sense of small is probably the oldest and the least understood of our five senses. During evolution it has kept its connections with the parts of the brain which grew to be the sorting house for our emotional responses, intimately linking the odors of things to our emotions.
Our sense of smell also plays an important role in sexual attraction, although this has become considerably muted during human evolutionary development. Its most important roles are those of a warning system and information gatherer: warning us of danger and giving us valuable information bout the outside world.
The close link between the sense of taste and the sense of smell is something we are not always aware of. Only when we have a cold do we realize that not only can we not smell thing, but the taste of food has also vanished.
As with many organs in the body the smelling apparatus is duplicated, each circuit acting independently.
The sensory receptors for smell are found in the roof of the nasal cavity, just beneath the frontal lobes of the brain. This is called the olfactory area and is tightly packed with millions of small cells, the olfactory cells. Each olfactory cell has about a dozen fine hairs-cilia-which project into a layer of mucus. The mucus keeps the cilia moist and acts as a trap for odorous substances, while the cilia effectively enlarge the area of each olfactory cell and so increase our sensitivity to smells.
It is not clearly understood how the minute amounts of chemical substances which give us smells trigger off the olfactory cells, but it is thought that these substances dissolve in the mucous fluids, stick on to the cilia and then cause the cells to fire off electrical signals.
Olfactory nerve fibers channel these signals across the bone of the skull to the two olfactory bulbs in the brain, where the information is gathered, processed and then passed through a complicated circuitry of nerve endings to the cerebral cortex. Here the message is identified and the smell becomes a conscious fact.
The exact molecular mechanism of the sense of smell is still largely unknown. Quite how the receptor cells can detect thousands of different odors and distinguish minute differences between them remains a mystery.
What is it We Smell?
To be smelly a substance must give off particles of the chemical of which it is made. This type of substance is, in general, chemically complex. Simple chemical substances-such as salt- do not have a smell, or have only a faint trace.
The particles of a substance must remain in the air in gas form in order to be swept into the nostrils and to the mucus surrounding the cilia. Once there, they must be able to dissolve in the mucus for the smelling apparatus to detect them.
Those substances that give off gas easily-such as petrol-are usually very smelly as high concentrations of the chemicals are able to reach the cells.
Wetness also heightens smells. As the water evaporates from the substance, it carries particles of the substance into the air. Perfumes are structured in such a way that they are chemically complex and give off gas easily.
Smell, Emotions and Memory
The part of the brain that analyzes messages coming from the receiver cells in the nose is closely connected with the limbic system, that part of the brain that deals with emotions, mood and memory. It is called the primitive brain, sometimes even the ‘smelling brain’. The connection explains why smells are richly endowed with emotional significance. The smell of fresh rain on a summer’s day usually makes people fell happy and invigorated; it may also evoke pleasant memories. The smell of fresh-baked bread may bring on instant pangs of hunger, while the scent of perfume may bring anticipation of sexual pleasure.
Conversely, unpleasant smells-such as rotten eggs-produce revulsion, and even nausea. But there are exceptions. The extremely unpleasant smell of a ripe Gorgonzola cheese actually attracts fervent fans; the smellier the better!
Certain smells will bring memories of long forgotten special occasions flooding back. This is because we tend to remember those things which have special emotional significance, since the areas of the brain which process memories and which are essential in their recall, are also closely linked to the limbic system, and which, in turn, is linked to the centers in the brain for the sense of smell.
The sense of taste is the crudest of our five senses. It is limited in both range and versatility and presents us with lens information concerning the world about us than any other sense. In fact, this sense’s exclusive sole is that of selector and appreciator of food and drink, a role that is considerably aided by the more sensitive sense of smell. This sense adds color to the four basic tastes which our taste buds can recognize. Consequently, the loss of the sense of taste-for whatever reason-is less of a problem than the loss of the sense of smell.
Like smell, the taste mechanism is triggered of by the chemical content of substances in food and drink. Chemical particles are picked up in the mouth and converted into nerve impulses which are transmitted by nerves to the brain, where they are then interpreted.
The taste buds are at the heart of this system. Studding the surface of the tongue are many small projections, called papillae. Inside these are the taste buds. An adult has about 9000 taste buds, mainly on the upper surface of the tongue, but there are also some on the palate and even the throat.
Each taste bud consists of groups of receptor cells, and each of these has fine hair-like projections-called mierovilli-sticking out into the surface of the tongue through fine pores in the surface of the papilla. At the opposite end to this, the receptor cells link up with a network of verve fibers. The design of this network is complex, as there is a great deal of interlinking between nerve fibers and receptor cells. Two different nerve bundles, which make up the facial nerve and the glossopharyngeal nerve, carry the impulses to the brain.
The taste buds respond to only four basic tastes: sweet, sour, salt and bitter; and the receptor sites for these tastes are located on different parts of the tongue. The buds that respond to sweet are at the tip of the tongue, while those specializing in salt, sour and bitter are located progressively further back.
Quite how the taste buds respond to the chemicals in the food and initiate the nerve impulses to the brain is not fully understood, but in order to be tasted, the chemicals must ne in liquid form. Dry food gives very little immediate sensation of taste, and only acquires its taste after being dissolved in saliva.
At present, it is believed that the chemicals in the food alter the electrical charge on the surface of the receptor cells, which in turn cause a nerve impulse to be generated in the nerve fibers.
Taste Analysis of Taste
The two nerves carrying taste impulses from the tongue (the facial nerve or the glossopharyngeal nerve) first pass to the specialized cells in the brain stem. This area of the brain stem also acts as the first stop for other sensations coming from the mouth. After initial processing in this brain stem center, the taste impulses are transferred in a second set of fibers across to the other side of the brain stem and ascend to the thalamus. Here, there is another relay, where further analysis of the taste impulses is carried out before information is passed to the part of the cerebral cortex participating in the actual conscious perception of taste.
The cortex also deals with other sensations-such as texture and temperature – coming from the tongue. These sensations are probably mixed with the basic taste sensations from the tongue, and so produce the subtle sensation with which we are familiar when we eat.
This analysis, carried out in the lower part of the parietal lobe in the cortex, is further influence by smell information being analyzed in the nearby temporal lobe. Much of the refinements of taste sensation are due to smell sensations.
Compared to another sensation (in particular smell) our taste sense is not very sensitive. It has been estimated that a person needs 25,000 times as much of a substance in the mouth to taste it as is needed by the smell receptors to smell it. However, despite this, the combination of the four types of taste buds responding to the basic tastes of salt, sour, bitter or sweet enable a wide range of sensations to be determined as the brain analyzes the relative strength of the basic flavors. Some of the stronger tastes, such as the ‘hot’ flavor of spicy food come about through stimulation of pain-sensitive nerve endings in the tongue.