Relationship between taste and smell

The Connection Between Taste, Smell, and Flavor |

relationship between taste and smell

Chemoreception - Interaction between taste and smell: In humans and other terrestrial vertebrates, odours can reach the olfactory epithelium via the external. When you take away your sense of smell, your brain has a much more difficult time determining the difference between specific tastes. This activity demonstrates that the sense of smell is crucial to determining the flavor of food.

The Difference Between Taste and Flavor

For example, one olfactory receptor protein in rats produces a greater response in the receptor cell when it interacts with an alcohol called octanol eight carbon atoms rather than with an alcohol known as heptanol seven carbon atoms.

Changing one amino acid from valine to isoleucine in the fifth transmembrane domain, which is thought to contribute to the shape of the pocket, alters the receptor protein in such a way that heptanol, instead of octanol, produces the greatest effect.

In mice the equivalent receptor is normally in this form, producing a greater response to heptanol than to octanol. This illustrates the importance of amino acid molecules in determining the specificity of receptor cells.

When a receptor protein binds with an appropriate chemical known as a ligandthe protein undergoes a conformational change, which in turn leads to a sequence of chemical events within the cell involving molecules called second messengers. Second-messenger signaling makes it possible for a single odour molecule, binding with a single receptor protein, to effect changes in the degree of opening of a large number of ion channels. In mammalsfive families of genes encoding chemoreceptor proteins have been identified.

Genes are considered to belong to the same family if they produce proteins in which high proportions of the amino acids are arranged in similar sequences. Two families of genes are associated with taste, one with smell, and two with the vomeronasal system see below Chemoreception in different organisms: There are about 1, genes in the olfactory gene family, the largest known family of genes.

Since each gene produces a different odour receptor protein, this contributes to the ability of animals to smell many different compounds. Animals not only can smell many compounds but can also distinguish between them. This requires that different compounds stimulate different receptor cells.

Taste and Smell

Consistent with this, evidence indicates that only one olfactory gene is active in any one olfactory receptor cell. As a consequence, each receptor cell possesses only one type of receptor protein, though it has many thousands of the particular type on the membrane of the exposed cilia of the cell.

Since each cell expresses only one type of receptor protein, there must be large numbers of cells expressing each type of receptor protein to increase the likelihood that a particular odour molecule will reach a cell with the appropriate receptor protein.

Once the molecule reaches the matching receptor, the cell can respond. A quite different family of genes produces the receptor proteins associated with bitter taste, but this family is much smaller than the olfactory gene family, containing only about 80 different genes. Given the very wide range of chemical structures that produce bitter taste, it is logical that there should be a number of different receptor proteins.

relationship between taste and smell

However, unlike with the olfactory response, animals do not distinguish different bitter compounds. This is because each of the receptor cells stimulated by these compounds produces many different kinds of receptor proteins. Thus, the same cell responds to many different compounds. This does not mean necessarily that all the genes are expressed by all the bitter-sensitive cells. It is probable that the perception of sugars, giving sweet taste, and amino acids, giving umami taste, also depend on protein receptors in the receptor cell membranes.

The mechanism by which inorganic salts are perceived is probably quite different. Because changes in electrical properties of cell membranes depend on ionic movement, cells will be affected by ion concentrations in the medium that bathes them.

It is very likely that when humans and other animals ingest common salt sodium chloridesodium enters the receptor cells directly through sodium channels in the cell membrane.

This has the effect of altering the internal ionic concentration and initiating an electrical signal. Responses to other salts are probably mediated in the same way, and responses to acids sour may be similarly effected by the movement of hydrogen ions.

Acids might also produce their effects by opening ion channels that are sensitive to pH. The gene family that governs the production of olfactory receptors is common to all vertebrates.

relationship between taste and smell

Yet it is well known that mammals differ in the extent to which their behaviour is affected by odours. This is a reflection of the different numbers of olfactory receptor genes that are active. In mice, which have a highly developed sense of smell, most of the approximately 1, olfactory genes are expressed that is, they produce receptor proteins.

But in Old World monkeys and in the great apesgorillaschimpanzeesand humans, as many as 70 percent of the olfactory receptor genes, though still identifiable, are nonfunctional pseudogenes. Evidence indicates that the pool of pseudogenes in humans is increasing, suggesting that, at some time in the future, the human sense of smell will be reduced even further than it is today.

All the olfactory genes of dolphins are nonfunctional. Animal responses to chemicals are greatly affected by chemical concentration. The more sugar present in coffeethe sweeter it tastes, and a smell may be barely perceptible or overpowering. These effects, which are very general and experimentally demonstrated in many animals, arise from the presence of large numbers of molecules at high concentration. As concentration increases, more cells are stimulated and more receptor molecules in a taste or olfactory cell are filled at one time.

The result is that more action potentials nerve impulses are generated by more receptor cells, and the signal reaching the brain is strengthened. It is a common occurrence that, when entering a room, a person may notice a pleasant or unpleasant smell, but within a very short time he can no longer smell it, even though the source of the smell remains. The effect is due to a waning of the response of the receptor cells and is called sensory adaptation.

The cells may adapt completely within a few seconds but become responsive again following an interval without stimulation. Adaptation of taste and olfactory cells occurs in all animals but not in receptor cells of the vomeronasal organ Jacobson organ.

The Connection Between Taste, Smell, and Flavor

Processing olfactory information Although each olfactory receptor cell has only one type of receptor protein, this does not mean that each cell responds to only one chemical. Presumably the receptor site formed by the protein interacts with some specific molecular form, and any chemical that possesses this form in some part of its molecule will stimulate the cell.

Tastants, chemicals in foods, are detected by taste budsspecial structures embedded within small protuberances on the tongue called papillae. Other taste buds are found in the back of the mouth and on the palate.

Taste and Smell

Every person has between 5, and 10, taste buds. Each taste bud consists of 50 to specialized sensory cells, which are stimulated by tastants such as sugars, salts, or acids. When the sensory cells are stimulated, they cause signals to be transferred to the ends of nerve fibers, which send impulses along cranial nerves to taste regions in the brainstem. From here, the impulses are relayed to the thalamus and on to a specific area of the cerebral cortexwhich makes us conscious of the perception of taste.

Airborne odor molecules, called odorants, are detected by specialized sensory neurons located in a small patch of mucus membrane lining the roof of the nose. Axons of these sensory cells pass through perforations in the overlying bone and enter two elongated olfactory bulbs lying against the underside of the frontal lobe of the brain.

An odorant acts on more than one receptor, but does so to varying degrees. Similarly, a single receptor interacts with more than one different odorant, though also to varying degrees.

Therefore, each odorant has its own pattern of activity, which is set up in the sensory neurons. This pattern of activity is then sent to the olfactory bulb, where other neurons are activated to form a spatial map of the odor.