Chemical Senses: Smell and Taste

Chemoreceptors The specialized cell receptors found both in the nose and mouth respond to chemicals in solution. Four types of taste receptors have been identified. The olfactory receptors in the nose are believed to be sensitive to a wider range of chemicals than those of the mouth. Receptors for smell and taste complement each other.

Olfactory Receptors & the Sense of Smell The human nose does not have the same acute sense of smell as many other animals, but the nose is still very effective. Persons who develop their sense of smell often can find jobs working as: Tea or coffee blenders Perfumers Wine tasters Taste testers

The Nose Air that enters the nasal cavity, has to make a hairpin turn to enter the respiratory passage below. At the roof of the nasal cavity is a small area (about the size of a stamp) that houses thousands of olfactory receptors. Air that is breathed in through the nose passes over these receptors and the sense of smell is originated.

Olfactory receptor cells are actually neurons (nerve cells) with dendrites that protrude from the nasal epithelium lining the ethmoid bone. These receptors are continually bathed in mucus that is secreted by glands in the nose. The receptors are stimulated when chemicals that make up odors/smells are dissolved in the mucus.

Ethmoid bone Olfactory nerve Olfactory receptor cells

The receptors then transmit the impulses through the olfactory nerve to temporal lobe of the brain. At the brain, smells are interpreted and an “odor snapshot” is made. The olfactory pathways are tied to the emotional part of the brain, so smells become linked to our memories and emotions.

Our reactions to odors are rarely neutral Our reactions to odors are rarely neutral. We either like them or we dislike them. It is possible to become immune to some smells if you are continually exposed to them. It is also possible to suffer olfactory disorders (anosmias) from head injury, from inflammation of the nasal cavity, from old age, or from brain disorders.

Taste Buds & the Sense of Taste The taste buds are receptors found in the oral cavity. They include: Over 10,000 taste buds found on the tongue Those found on the soft palate Those found on the inner surface of the cheek Each of these receptors is specific and can respond to chemicals that dissolve in our saliva.

The surface of the tongue is covered with small peg-like projections called papilla. Each papillae has numerous taste buds lining the sides of each bump. So in order to taste foods with our tongue, the foods must: - first be able to dissolve in saliva - reach the taste buds tucked in between the thousands of papilla.

Cranial Nerves and Taste Two cranial nerves (VII – facial & IX - glossopharygneal) carry taste impulses to the brain. The facial nerve serves the anterior (2/3) part of the tongue. The glossopharyngeal nerve serves the other areas of the mouth.

Traditional belief is that: that tip of the tongue is sensitive to sweet and salty the sides of the tongue is sensitive to sour the back of the tongue is sensitive to bitter. Newer information now includes a 5th taste known as umami. This is a sensitivity to savory foods. Scientists are also looking into adding “metallic” as a 6th taste.

Some will argue that our taste likes and dislikes serve to maintain a homeostatic balance. Liking sweets and salty foods satisfies our need for carbohydrates and minerals. Craving sour and naturally acidic foods (oranges, lemons, tomatoes) provides us with Vitamin C.

An interesting point is that most natural poisons and spoiled foods are bitter so our dislike for bitter can be considered protective. What doesn’t make sense is that these taste receptors are at the back of the tongue and by the time we’ve tasted the substance, we’ve actually swallowed some.

When we taste things, we are determining what tastes good to us or not. Usually foods that do not taste good to us will not be allowed to enter the body. Taste is affected by many factors and it also depends on our sense of smell. In addition, the texture or temperature of a food can also enhance or spoil its taste for us.