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20.3: Smell and Taste in the Brain - Biology

20.3: Smell and Taste in the Brain - Biology


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Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The olfactory bulb is composed of neural clusters called glomeruli, and each glomerulus receives signals from one type of olfactory receptor, so each glomerulus is specific to one odorant. Olfaction is finally processed by areas of the brain that deal with memory, emotions, reproduction, and thought.

Taste neurons project from taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem. From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex.


20.3: Smell and Taste in the Brain - Biology

Taste, or gustation, is a sense that develops through the interaction of dissolved molecules with taste buds. Currently five sub-modalities (tastes) are recognized, including sweet, salty, bitter, sour, and umami (savory taste or the taste of protein). Umami is the most recent taste sensation described, gaining acceptance in the 1980s. Further research has the potential to discover more sub-modalities in this area, with some scientists suggesting that a taste receptor for fats is likely.

Taste is associated mainly with the tongue, although there are taste (gustatory) receptors on the palate and epiglottis as well. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. In the surface of the tongue are raised bumps, called papilla, that contain the taste buds. There are three types of papilla, based on their appearance: vallate, foliate, and fungiform.

The number of taste buds within papillae varies, with each bud containing several specialized taste cells (gustatory receptor cells) for the transduction of taste stimuli. These receptor cells release neurotransmitters when certain chemicals in ingested substances (such as food) are carried to their surface in saliva. Neurotransmitter from the gustatory cells can activate the sensory neurons in the facial and glossopharyngeal cranial nerves.


20.3: Smell and Taste in the Brain - Biology

Smelling and the Nose

We use our nose to smell things. At the top of the inside of our nose are millions of tiny little hairs called cilia. These hairs are connected to smell sensors which send signals to our brain about smell via the olfactory nerve. We smell things when they emit small molecules that float in the air and end up in our nose. We can't see these tiny molecules, but they are there. The reason we sniff is to get more of those molecules up into the top of our nose to where they can attach to the special sensors and determine the smell.

Smelling helps us in many ways. It first makes our food taste better. We can't really taste that many flavors, but with the help of smell we can "taste" thousands of different things. Also, smell helps to warn us from bad things like rotten food or smoke from fire.

Tasting and the Tongue

We use our tongue to taste things. The tongue uses taste buds or sensor cells to determine the type of food and send taste signals back to our brains. The tongue can taste four different flavors: bitter, sour, salty, and (maybe best of all) sweet. It was once thought that each of these tastes came from a different spot on the tongue: sweet from the tip, salty from the sides, sour from the back sides, and bitter from the back. Now scientists say that flavors can be tasted from most any part of the tongue.

Using Smell and Taste Together

As we discussed above, we can taste four distinct flavors. We can also smell over 10,000 smells. When we eat something, the flavor comes from a combination of taste and smell. Sometimes touch or pain can affect the taste as well due to the texture of the food or the hotness of spicy food.


The Five Tastes

Not all chemoreceptors sense the same kinds of chemicals, or tastes. Humans have five kinds of taste buds – salty, sweet, sour, bitter, and the newly recognized, umami. Each of these types has a specific kind of chemosensitive cell inside. It was originally thought that each type of taste bud (and corresponding taste) was found only on a certain region of the tongue. We now know that this is not the case, and that each taste can be detected throughout the surface of the tongue.

    Sweet taste is overwhelmingly pleasant, and signals the presence of carbohydrates in food. Carbohydrates have a very high calorie content, and are therefore an efficient source of energy.

People often crave salty foods as salt is important to body function. Image by Jonathan M.

Cheese and other savory items have a taste called umami. Image by Hubertl.


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Aging and Illness

Because taste and smell receptors are in direct contact with the environment, it’s not surprising that they become blunted over the years. Smell, in particular, typically declines, which can make food less appealing, adversely affecting appetite and sometimes contributing to poor nutrition in the elderly.

Complete loss of the sense of smell, anosmia, afflicts some six million Americans. The causes are varied and sometimes unknown. Marked decline in olfaction may also be a sign of neurological disorders. In fact, it sometimes occurs quite early in Parkinson’s and Alzheimer’s diseases.


In experiments echoing mice behavior, researchers emulate how brains recognize specific smells


The brain has many different parts. The brain also has specific areas that do certain types of work. These areas are called lobes. One lobe works with your eyes when watching a movie. There is a lobe that is controlling your legs and arms when running and kicking a soccer ball. There are two lobes that are involved with reading and writing. Your memories of a favorite event are kept by the same lobe that helps you on a math test. The brain is controlling all of these things and a lot more. Use the map below to take a tour of the regions in the brain and learn what they control in your body.

The brain is a very busy organ. It is the control center for the body. It runs your organs such as your heart and lungs. It is also busy working with other parts of your body. All of your senses – sight, smell, hearing, touch, and taste – depend on your brain. Tasting food with the sensors on your tongue is only possible if the signals from your taste buds are sent to the brain. Once in the brain, the signals are decoded. The sweet flavor of an orange is only sweet if the brain tells you it is.

Frontal Lobe

  • Movement of the body
  • Personality
  • Concentration, planning, problem solving
  • Meaning of words
  • Emotional reactions
  • Speech
  • Smell

Parietal Lobe

Temporal Lobe

Occipital Lobe

Cerebellum

  • Latin for little brain
  • Fine motor (muscle) control
  • Balance and coordination (avoid objects and keep from falling)

Limbic Lobe


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Discussion

What differences did you qualitatively feel between closing your nose while tasting and not closing your nose. Explain the results in the tables and graphs in terms of whether closing the nose (blocking the sensation of smell) had an effect on taste. What does the t-test say about the statistical significance of this difference in the perception of taste? Is the taste really affected or is it flavor? Explain.

Explain some plausible factors that could have influenced your results. Did you unintentionally or intentionally use other senses other than taste and smell to guess? Put this in perspective and discuss their effect on your results and conclusion.


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