OUR CHEMICAL SENSES: 2. TASTE

Experiment: How Taste and Smell Work Together

Developed by Marjorie A. Murray, Ph.D.; Neuroscience for Kids Staff Writer

FEATURING: A "CLASS EXPERIMENT"
PLUS: "TRY YOUR OWN EXPERIMENT!"

[Teacher Guide] | [Student Guide]

To view the Teacher Guide and Student Guide, you must have the free Adobe Acrobat Reader.

TEACHER RESOURCE

Index

[Summary] | [Background Concepts] |
[Planning and Teaching the Lab] | [References] | [Science Education Standards]

I. SUMMARY

Students learn how to investigate the sense of taste and then find out how to plan and carry out their own experiments. In the "CLASS EXPERIMENT," students find that the ability to identify a flavor depends on the sense of smell as well as the sense of taste. They learn basic facts about sensory receptors, nerve connections, and brain centers.

In "TRY YOUR OWN EXPERIMENT," students design experiments to further explore the sense of taste. They can extend the Class Experiment by finding the chemical categories that taste receptors can detect without olfactory input.

In other self-directed investigations, they can learn whether mixing substances makes it harder to identify the components, or whether identifying flavors is difficult when other sensory input interferes.

SUGGESTED TIMES for these activities: 45 minutes for introducing and discussing the activity, 45 minutes for the "Class Experiment;" and 45 minutes for "Try Your Own Experiment."

II. BACKGROUND CONCEPTS

1. Overview of the smell and taste systems

Odor and food molecules activate membrane receptors

Reports from our noses and mouths alert us to pleasure, danger, food and drink in the environment. The complicated processes of smelling and tasting begin when molecules detach from substances and float into noses or are put into mouths. In both cases, the molecules must dissolve in watery mucous in order to bind to and stimulate special cells. These cells transmit messages to brain centers where we perceive odors or tastes, and where we remember people, places, or events associated with these olfactory (smell) and gustatory (taste) sensations.

The neural systems for these two chemical senses can distinguish thousands of different odors and flavors. Identification begins at membrane receptors on sensory cells, where odorant or taste molecules fit into molecular slots or pockets with the right "lock and key" fit. This latching together of binding molecule or ligand and membrane receptor leads to the production of an electrical signal, which speeds along a pathway formed by nerve cells (neurons) and their extensions called axons. In this way, information reaches brain areas that perceive and interpret the stimulus.

A membrane receptor will respond to several structurally related molecules

The activation of receptors by discrete chemical structures is not absolute, because a given membrane receptor will accept a number of structurally similar ligands. Nevertheless, we can discriminate many thousands of smells and tastes, even though some chemicals stimulate the same receptor. How are we able to distinguish these? Our ability results from the fact that most substances we encounter are complex mixtures, which activate different combinations of odor and taste receptors simultaneously. Thus, each substance we smell or taste has a unique chemical signature. In the laboratory, researchers frequently test people or animals with pure individual chemicals in order to find the best stimulus for a receptor, but in the real world we seldom encounter these molecules alone.

Although we do have overlap in the response of taste and smell receptors to ligands, scientists have identified quite a number of receptor types. Humans probably have hundreds of kinds of odor membrane receptors, and on the order of 50 to 100 different kinds of taste receptors. It is true that we typically describe only five categories of tastes (see below): this means that each of the categories probably has more than one type of receptor. Further research will show how this puzzle fits together.

The neural systems for taste and smell share several characteristics

Although the neural systems (sensory cells, nerve pathways, and primary brain centers) for taste and smell are distinct from one another, the sensations of flavors and aromas often work together, especially during eating. Much of what we normally describe as flavor comes from food molecules wafting up our noses. Furthermore, these two senses both have connections to brain centers that control emotions, regulate food and water intake, and form certain types of memories.

Another similarity between these systems is the constant turnover of olfactory and gustatory receptor cells. After ten or so days, taste sensory cells die and are replaced by progeny of stem cells in the taste bud. More surprising is the story of olfactory sensory cells. These are not epithelial cells like taste cells, but rather neurons, which are not commonly regenerated in adults (although recent evidence shows that new neurons are produced, even in the brain). Researchers are investigating how taste perception and odor recognition are maintained when cells die and new connections to the nervous system must be generated.

2. Taste sensory cells are found in taste buds

Looking at your tongue in the mirror, you can see collections of little bumps clustered on the sides and tip. If you stick your tongue out very far, you see larger flattened pegs on the posterior area. These macroscopic structures are papillae, and all over their surfaces are the taste buds, which are in turn made up of several types of cells, including the taste sensory cells. Although an individual taste bud cannot be seen without a microscope (Figure 1, right), it looks something like a balloon with a small opening at the tongue surface: this is the taste pore. Into the pore come food and drink molecules, fitting into membrane receptors located on small finger-like protrusions called microvilli at the tops of taste sensory cells. The microvilli increase the surface area of the cell (see Figure 2).
Figure 1
Image courtesy of Biodidac

Figure 2. A taste sensory cell and the five types of taste receptors. Flavor molecules fit into receptors on the microvilli at the top of the cell, causing electrical changes that release transmitter onto the nerve ending at the bottom of the cell. The nerve carries taste messages to the brain. See text for details on receptor types. Figure courtesy of Dr. Tim Jacob, Cardiff University, Wales

How do these cells begin the process that leads to recognizing tastes? As mentioned in Section 1, the membrane receptors on sensory cells contain molecular pockets that accommodate only compounds with certain chemical structures. According to current research, humans can detect five basic taste qualities: salt, sour, sweet, bitter, and umami (the taste of monosodium glutamate and similar molecules). Investigations of the molecular workings of the first four show that salt and sour receptors are types of ion channels, which allow certain ions to enter the cell, a process that results directly in the generation of an electrical signal.

Sweet and bitter receptors are not themselves ion channels, but instead, like olfactory receptors, accommodate parts of complex molecules in their molecular pockets. When a food or drink molecule binds to a sweet or a bitter receptor, an intracellular "second messenger" system (usually using cyclic AMP) is engaged. After several steps, concluding with the opening of an ion channel, the membrane of the taste receptor cell produces an electrical signal. (The second messenger system is a signaling mechanism used in many sensory nerve cells as well as in other cells in the body.)

Although humans can distinguish only five taste qualities, more than one receptor probably exists for some of these. This is supported by the finding that some people cannot detect certain bitter substances but do respond to others, indicating that only one kind or class of bitter receptor is missing, probably as the result of a small genetic change. (You can demonstrate this with phenylthiourea-impregnated papers in the classroom, as described in the Teacher Guide.)

3. Taste signals go to the limbic system and to the cerebral cortex

Where do taste messages go once they activate the receptor cells in the taste bud? The electrical message from a taste receptor goes directly to the terminal of a primary taste sensory neuron (Figure 2), which is in contact with the receptor cell right in the taste bud. The cell bodies of these neurons are in the brainstem (lower part of the brain, below the cerebrum and their axons form pathways in several cranial nerves. Once these nerve cells get electrical messages from the taste cells, they in turn pass the messages on through relay neurons to two major centers: the limbic system and the cerebral cortex as shown in Figure 3.

The limbic system (which includes the hippocampus, hypothalamus and amygdala) is important in emotional states and in memory formation, so when taste messages arrive here, we experience pleasant, or aversive, or perhaps nostalgic feelings. In the frontal cerebral cortex, conscious identification of messages and other related thought processes take place. The messages from the limbic system and the frontal cortex may be at odds with each other. For example, if you are eating dinner at a friend's home and the first bite of a food item is bitter, you may feel an aversion to eating more. But if you know the food is merely from another culture and not harmful, you may make a conscious decision to continue eating and not offend your hosts. Thus, taste messages go to more primitive brain centers where they influence emotions and memories, and to "higher" centers where they influence conscious thought.

Figure 3. Central taste pathways. (See text for explanation)

4. Patterns of nerve activity encode taste sensations

In other sensory systems, stimulation often activates nerve cells in a spatial pattern that reflects the body area reporting the sensory input. For example, a pointed object touching the thumb activates neurons in a defined patch of the cerebral cortex. When the object touches the pointer finger, neurons right next to the "thumb cortical neuron" are activated; that is, neighborhood relationships are maintained from the skin to the cortex.

But how is taste encoded? How does the brain know that something is sweet? Here we will consider only taste sensations, and not the additional flavors that odors add to an eating experience. Researchers have detected some mapping of tastes is in higher areas such as the taste or gustatory nucleus in the brainstem, the thalamus and the cerebral cortex. The mapping appears to be geographical, as in the touch system-this means that messages from the tip of the tongue go to different areas in the brain than messages from the sides of the tongue. Further, some evidence indicates that cells receiving "sweet" messages in the brainstem may be grouped together, as are cells receiving salt, sour, and bitter messages.

Researchers have also found that while each receptor responds best to one type of taste, say sweet, it can also respond weakly to another, perhaps bitter. This happens because taste cells have more than one type of membrane receptor, not because bitter compounds are squeezing into sweet receptor molecular slots. To complicate matters further, a given nerve axon forms branches in the tongue and sends terminals to several different taste cells. Thus, it will carry different messages to the brain, depending on which of its taste cell subjects are reporting to it at the time. These complicated connections make it likely that neurons in the brain detect a complex taste, such as sweet-sour, or bittersweet, by a pattern of activated sensory nerve axons rather than by an absolute counting of groups of pre-defined sensory and central neurons.

As you might expect, researchers are still trying to sort out this system and no final answers are at hand. As researchers have found for the visual and olfactory systems, patterns of axon activation are probably giving us distinct taste sensations.

5. Sensory processing allows us to interpret flavors

To summarize how we perceive and interpret flavors, let's follow some food into your mouth. It's a warm June day and as you drive through the countryside, you see a roadside stand ahead. Stopping, you buy a flat of freshly picked strawberries to make jam, but you grab a few to sample. As you bite into the first one, the tart but sweet juice squishes out and floods your mouth; escaping molecules waft into your nose and assail your odor receptors. Many types of molecules are present, and each fits into a slot on a taste or odor membrane receptor that can accommodate only that class of molecular structures.

As soon as the molecules stick to their receptors, both ion channels and second messenger systems go into gear, quickly causing each stimulated cell to produce an electrical signal. The signals flash through the axons of taste and olfactory sensory neurons and on to cells in the brain. The messages zip to several places by way of axons from secondary or relay neurons. Messages to the limbic system give you that "aahhh" feeling, others activate memories of previous strawberries, warm summer days, and steaming pots of bubbling jam. Still other pathways stimulate motor centers to cause salivation, chewing, and swallowing. The signals to your frontal cortex activate motor neurons that allow you to say, "Wow!" and you turn around to buy a second flat of berries.

The experiences of perceiving and interpreting the strawberry flavor are the result of activating a pattern of neural components, and in turn, a pattern specific memories, feelings, and thoughts.

6. Genes determine the kinds of taste receptors that we have, and experiences shape our perceptions

Taste preferences and perceptions vary widely among individuals-we all know someone who hates bananas, or loves rhubarb, or is unusually fond of chocolate. Studies have shown that people who are unable to perceive one type of taste stimulus frequently have small genetic differences from the general population. Thus, in some cases, foods really do not taste the same to everyone. In fact, researchers have found that some people are "supertasters" to whom sweet things taste much sweeter and bitter things much more bitter than to the average person. These supertasters have more papillae on their tongues than usual, so they probably have more taste receptors.

Other differences in taste perception may be temporary. Temporary general inability to taste foods can result from a cold or certain medicines and usually is caused by the blocking of olfactory rather than taste receptors. (You can test this when you have a bad cold by seeing if you can still perceive that, for example, strawberries still taste sweet and tart, although you cannot discern many other qualities that you usually do.)

Experiences as well as genetics influence our food preferences. Anyone who has become memorably ill after eating a particular food seldom wants to eat it again, perhaps for years or forever. Animal experiments on this aversion phenomenon showed that pairing food of a specific flavor with a mild poison that induced vomiting caused permanent refusal to eat anything with that flavor. Pairing the poison with an auditory tone, however, did not result in aversion to the tone, even though the animal became ill this time as well. Scientists believe the close association of stomach illness with taste and odor is a survival trait that many animals have evolved.

7. Taste disorders may be genetic, or may result from illness or injury

Although genetic differences account for some cases of ageusia (the complete loss of taste), the inability to taste, or other disorders of taste, most are caused by illnesses or accidents. Other taste disorders include hypogeusia-diminished taste sensitivity; hypergeusia-heightened sense of taste; and dysgeusia-distortions in the sense of taste. Small growths in the nasal cavities (polyps), dental problems, hormonal disturbances, or sinus infections, as well as common colds, may cause chemosensory losses. Injury to the head may damage nerve centers or break axons. Patients who receive radiation therapy for cancers of the head or neck often develop changes in their senses of taste and smell.

Are these disorders serious? Our sense of taste can warn us that something we put into our mouths may be spoiled or dangerous. Further, eating is much more than just "food intake" for humans; it is an important part of our social lives and a source of pleasure. People should see a doctor if they realize something is wrong with their sense of taste.

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III. PLANNING AND TEACHING LAB ACTIVITIES

Provide background information

First, prepare students for lab activities by giving background information according to your teaching practices (e.g., lecture, discussion, handouts, models). Because students have no way of discovering sensory receptors or nerve pathways for themselves, they need some basic anatomical and physiological information. Teachers may choose the degree of detail and the methods of presenting the sense of taste, based on grade level and time available.

Offer students the chance to create their own experiments

While students do need direction and practice to become good laboratory scientists, they also need to learn how to ask and investigate questions that they generate themselves. Science classrooms that offer only guided activities with a single "right" answer do not help students learn to formulate questions, think critically, and solve problems. Because students are naturally curious, incorporating student investigations into the classroom is a logical step after they have some experience with a system.

The "Try Your Own Experiment" section of this unit (see the accompanying Teacher and Student Guides) offers students an opportunity to direct some of their own learning after a control system has been established in the "Class Experiment." Because students are personally vested in this type of experience, they tend to remember both the science processes and concepts from these laboratories.

Use "Explore Time" before experimenting

To encourage student participation in planning and conducting experiments, first provide Explore Time or Brainstorming Time. Because of their curiosity, students usually "play" with lab materials first even in a more traditional lab, so taking advantage of this natural behavior is usually successful. Explore Time can occur before the Class Experiment, before the "Try Your Own Experiment" activity, or both, depending on the nature of the concepts under study.

Explore before the Class Experiment

To use Explore Time before the Class Experiment, set the lab supplies out on a bench before giving instructions for the experiment. Ask the students how these materials, along with the information they have from the lecture and discussion, could be used to investigate the sense of taste. Give some basic safety precautions, then offer about 10 minutes for investigating the materials. Circulate among students to answer questions and encourage questions. After students gain an interest in the materials and subject, lead the class into the Class Experiment with the Teacher Demonstration and help them to formulate the Lab Question. Wait until this point to hand out the Student Guide and worksheets, so students have a chance to think creatively. (See the accompanying Guides.)

Explore before "Try Your Own Experiment"

To use Explore Time before Try Your Own Experiment, follow the procedure above, adding the new materials for student-generated experiments. Let the students suggest a variety of ideas, then channel their energies to make the lab manageable. For example, when a number of groups come up with similar ideas, help them formulate one lab question so that the groups can compare data. The goal is to encourage students to think and plan independently while providing sufficient limits to keep the classroom focused. The Teacher and Student Guides contain detailed suggestions for conducting good student-generated experiments.

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IV. REFERENCES and SUGGESTED READING

  1. Kandel, E.R., Schwartz, J.H., and Jessell, T.M. (Eds.) (2000). Principles of Neural Science. Fourth ed., New York: McGraw Hill.

  2. Shepherd, G.M. (1994). Neurobiology, Third ed. Oxford: Oxford University Press.

  3. Monell Chemical Senses Center

  4. That's Tasty and Simple Taste Experiments from Neuroscience for Kids

  5. The Chemoreception Web

  6. Anatomy and physiology of taste

  7. Stick out your tongue and say Aah! from KidsHealth

  8. Discovery of Bitter Taste Receptor Gene (ref 1) and Discovery of Bitter Taste Receptor Gene (ref 2)

V. MEETING SCIENCE EDUCATION STANDARDS

By reaching Project 2061 Benchmarks for Science Literacy, students will also fulfill many of the National Science Education Standards and individual state standards for understanding the content and applying the methods of science. Because the Benchmarks most clearly state what is expected of students, they are used here. Below is a list of Benchmarks that can be met while teaching the taste sense activities. The Benchmarks are now on-line at: The Benchmarks are now on-line at: http://www.project2061.org/tools/benchol/bolframe.htm

The Benchmarks are listed by chapter, grade level, and item number; for instance, 1A, 6-8, #1 indicates Chapter 1, section A, grades 6-8, benchmark 1.

The process of inquiry used in the sense of taste activities will help students reach the following summarized Benchmarks:

  • 1A, 6-8, #1: When similar investigations give different results, the scientific challenge is to judge whether the differences are trivial or significant, and it often takes further studies to decide.

  • 1B, 6-8, #1: Scientific investigations usually involve the collection of relevant evidence, the use of logical reasoning, and the application of imagination in devising hypotheses and explanations to make sense of the collected evidence.

  • 1B, 6-8, #2: If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables.

  • 12A, 6-8, #2: Know that hypotheses are valuable, even if they turn out not to be true.

  • 12A, 6-8, #3: Know that often, different explanations can be given for the same evidence, and it is not always possible to tell which one is correct.

  • 12C, 3-5, #3: Keep a notebook that describes observations made, carefully distinguishes actual observations from ideas and speculations about what was observed, and is understandable weeks or months later.

  • 12D, 6-8, #2: Read simple tables and graphs and identify the relationships they reveal.

  • 12D, 6-8, #3: Locate information in reference books, newspapers and magazines, compact discs and computer databases.

    The neuroscience content in the taste sense activities and background material will help to meet the following benchmarks:

  • 5C, 6-8, #1: All living things are composed of cells. Different body tissues and organs are made up of different kinds of cells. The cells in similar tissues and organs in other animals are similar to those in human beings.

  • 5C, 6-8, #2: Cells repeatedly divide to make more cells for growth and repair.

  • 6A, 6-8, #1: Like other animals, human beings have body systems for the coordination of body functions.

  • 6C, 6-8, #1: Organs and organ systems are composed of cells and help provide all cells with basic needs.

  • 6D, 6-8, #4: Attending closely to any one input of information usually reduces the ability to attend to others at the same time.

  • 6D, 6-8, #3: Human beings can detect a tremendous range of visual and olfactory stimuli.

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