Neuroscience for Kids - Chemical Senses 1. Overview of the smell and taste systems Odor and food molecules activate membrane receptors Sensations 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 receptor cells. These cells transmit messages to brain areas where we perceive odors and 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 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 apparently not absolute, for a given membrane receptor will respond to a group of structurally similar compounds. Probably hundreds of odor membrane receptors exist, but many fewer taste receptors, perhaps on the order of 10 or 20 (we only know of about five now). The fact that we can discriminate thousands of smells and tastes is a result of complex substances activating different combinations of odor and taste receptors. Researchers frequently test people or animals with pure chemicals in order to find the best stimulus for a receptor, but in the real world odors and foods consist of many different types of molecules. The neural systems for taste and smell share several characteristics Although the neural systems 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. Further, these two senses both have connections to brain areas 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 about ten days, taste receptor cells die and are replaced by cells that differentiate from a sort of stem cell in the taste bud. More surprising is the story of olfactory sensory cells. These are not epithelial cells as are taste cells, but neurons, which until recently were not known to be generated in adults. (Recent evidence shows that this can happen, even in the brain ). The olfactory sensory neurons are not only replaced every 60 days or so, but each must also grow an axon to the correct place in the brain. Researchers are investigating how taste perception and odor recognition are maintained in the face of this turnover and new axon growth. 2. Odor receptors are ciliated sensory neurons in the upper nasal cavity Humans can detect on the order of 10,000 "odorants," or substances that stimulate the sense of smell, and can detect some of these at concentrations as low as a few parts per trillion. Special olfactory receptor cells, about fifty million of them, line the upper reaches of the nasal cavity in a sheet of olfactory epithelium.

Hair-like cilia dangle from the ends of the cells into the mucous layer covering the epithelium, where odor molecules bind to membrane receptors on the cilia. What is the mechanism for distinguishing aromas? As mentioned in Section 1, membrane receptors contain molecular pockets that accommodate only compounds with certain chemical structures. When an odorant molecule binds to a receptor, an intracellular "second messenger" system (usually using cyclic AMP) is engaged. After several steps, the membrane of the nerve cell propagates an electrical signal along its length and passes it on to the next nerve cell in the pathway. The second messenger system is a signaling mechanism used in many sensory nerve cells as well as in other cells in the body. All vertebrate olfactory receptor neurons examined so far are "generalists." In other words, they respond best to one class of substances, more weakly to another group or two, and not at all to others. Insects and some other animals possess odor "specialists" that respond only to one pure substance, that is, a single molecule. Usually this is a pheromone that is employed in finding a mate.

Researchers expect to find specialist cells in vertebrates as well; these are difficult to find because there probably aren't many of them, and scientists haven't yet identified their specific ligands. 3. Olfactory signals go to two types of areas in the brain Where do odor messages go once they activate the sensory neurons in the nasal cavity? To get to the brain, the axons of the olfactory sensory neurons must get through the skull (see side view of the skull in Figure 1 on the left). The olfactory epithelium lines the bone of the part of the skull just above the nasal passages, and the axons of the neurons pass directly through tiny holes in this bone. Here they enter the first relay station, the olfactory bulbs, one on either side of the bottom surface of the brain (Figure 2, right). The electrical signal generated when an olfactory sensory neuron is activated is passed along to a secondary neuron residing in the bulb, and from here the signal goes by way of the olfactory tract to other brain areas. The olfactory system is often described as the most "primitive" sensory system because of its early phylogenetic development and its connections to older, subconscious portions of the brain.

From the olfactory bulbs, odor messages go to several brain structures that make up the "olfactory cortex," an area that evolved before the cortical areas that give us consciousness. This part of the cortex is on the bottom surface of the brain, with some of the olfactory areas folded under the visible parts. These areas have connections to the limbic system (including the hippocampus, amygdala and hypothalamus), which is important in emotional states and in memory formation (see Figure 3). Thus, a smell frequently activates intense feelings and memories before a person even identifies the odor. Messages also go to conscious cortical areas. After a relay in the olfactory cortex , signals enter a way station called the thalamus, and then travel on to the frontal cortex, where identification and other related thought processes take place. Thus, odor messages go to primitive brain areas where they influence emotions and memories, and to "higher" areas where they modify conscious thought (Fig. 3, below). FIGURE 3. Summary of olfactory pathways. Note that not all intermediate relays are included. 4. Odors map onto specific brain areas Because sight, sound, and touch sensations map in a spatial way onto brain areas, researchers wondered if this happens with odors.

They have found that in the olfactory bulb, neurons, their cytoplasmic processes, and support cells are not evenly distributed but form clumps called glomeruli. Certain odorants activate only one or a small number of glomeruli, producing a kind of mapping, although it is not a spatial map of the location of odors in relation to the body, but a functional map of odor types. Whether this mapping is continued at higher brain levels is uncertain. A type of spatial mapping also exists in the olfactory epithelium, in the nasal passages. Here, scientists have shown that certain volatile chemicals attach to receptor cells in defined patches of epithelium and not to others. 5. Patterns of neural activity allow us to identify odors To summarize the process of odor detection, let's follow an odorant into the nose. At 7 a.m., molecules from a cup of coffee float into the air and drift into your nose. You breathe deeply, drawing the molecules up into the top of your nasal passages. Several types of molecules are present, and each fits into a slot on a membrane receptor that can accommodate only that class of molecular structures.

As soon as the molecules stick to their receptors, intracellular systems of enzymes and substrates go into gear, quickly causing each cell to produce an electrical signal. The signal flashes through the axon of each olfactory sensory neuron and on to cells in the olfactory bulb. Each type of receptor neuron sends its signal to a specific clump of cells in the olfactory bulb, forming a sort of odor map. From here, the message from each bulb cell zips to several places by way of branches from the cell's axon. Messages to the olfactory cortex give you that "aahhh" feeling, others activate memories of previous cups of coffee, still others wind up stimulating motor areas to cause salivation. The signals travel another pathway to your frontal cortex, which "says" to you: "COFFEE!" The experiences of perceiving and interpreting the coffee odor are the result of activating a pattern of neural components, and in turn, specific memories, feelings, and thoughts. 6. Several mechanisms may contribute to olfactory fatigue Adaptation, or fatigue, to constant stimulation is a general feature of sensory systems.

For instance, the touch receptor cells in the skin adapt to the stimulation of our clothes, a fortunate thing, or we would be distracted by them constantly. Adaptation involves mechanisms at the level of the receptor cell, including the inactivation of ion channels in the membrane that generate the electrical signal. In a simplified explanation, after a stimulus causes a receptor cell to produce an electrical signal, the cell membrane soon stops allowing ions to flow, thus preventing further signals. Removal of the stimulus followed by restimulation activates the process all over again. Researchers have noted that people adapt to odors, such as the smell of tobacco smoke in a room, more quickly than the properties of olfactory receptor cells would predict. Thus, they believe that olfactory fatigue involves some types of central nervous system mechanisms as well as receptor adaptation. Although these brain mechanisms are currently unknown, scientists speculate that inhibitory circuits in the brain "quash" the incoming sensory signals from receptors before they reach conscious levels. 7.

Genes determine the kinds of odor receptors that we have, and experiences shape our perceptions Olfactory abilities vary widely among individuals -- we all know someone who is able to smell things when no one else can, or someone who doesn't seem to mind an unpleasant odor when most people do. Studies have shown that people who are unable to smell one or one class of odors frequently have small genetic differences from the general population. The inability to smell is called "anosmia," and it may be general, or specific for one odor. Temporary general anosmia or "hyposmia" (lessened sense of smell) can result from a cold or certain medicines. "Hyperosmia," a heightened sense of smell, can be a genetic trait. Previous experiences and our physiological states also affect our reactions to odors and our perceptions. The odor of frying trout or hot cocoa may smell wonderful to a hungry camper, but terrible to someone with the stomach flu. A child who remembers her mother sprinkling cinnamon on her little brother's vomit before cleaning it up may never want cinnamon cookies again, even as an adult. Our expectations and beliefs can even affect measurements such as olfactory fatigue times.

Studies have shown that the time for adapting to an odor is significantly different when people believe they are being exposed to a harmless aroma, compared to when they think they are smelling a hazardous substance, even when the odor is exactly the same. 8. Olfactory disorders may be genetic, or may result from illness or injury While genetic differences account for some cases of anosmia or other olfactory disorders as mentioned above, most are caused by illnesses or accidents. 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 fibers or break axons. Patients who receive radiation therapy for cancers of the head or neck often develop changes in their sense of smell. Are these disorders serious? Our sense of smell alerts us to fires, poisonous fumes, leaking gas, and spoiled food. It can stop us from entering a dangerous area or putting something into our mouths that can make us sick. Further, such disturbances can be a signal that some other disease is present, such as Parkinson's disease, hypertension, or diabetes. People should see a doctor if they realize something is wrong with their sense of smell. 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 olfactory system, 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 or Brainstorming 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. For some experiments, such as the olfactory lab, brainstorming is probably best. In these labs, if students investigate the materials before starting the experiments, they will probably identify (or hear others identify) and memorize many of them. Further, in activities to stimulate memories associated with odors, the "surprise" element of a sudden whiff of material is important in generating an interesting experience. Instead of letting students explore, the teacher can simply indicate the lab bench, saying that the containers of odor materials will be available for experiments. When students see that opaque, unmarked containers of materials are available, they can begin to generate questions for investigations, and the teacher can offer more ideas. Class Experiment After students gain an interest in the materials and subject, lead the class into the Class Experiment and help them to formulate the Lab Question.

(Using the Teacher Demonstration suggested in the Teacher Guide will also help students focus on the subject.) Hand out the Student Guide and any worksheets after brainstorming, so students have a chance to think on their own. (See the accompanying Guides.) "Try Your Own Experiment" For the Try Your Own Experiment activity, 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. 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 olfactory sense activities.

The Benchmarks are now on-line at: 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 olfactory sense 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 olfactory 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. The Sense of Smell: A Powerful Sense The Sense of Smell: A Powerful Sense by Gloria Rodriguez-Gil, M.Ed., California Deaf-Blind Services Educational Specialist Reprinted from reSources , Spring 2004, Volume 11, Number 2 with permission from California Deaf-Blind Services <~cadbs> Abstract: This article is about how the sense of smell works and how this powerful sense may impact programming in the field of deafblindness. Key Words: programming, deafblind, smell, senses Several years ago I was shopping at Macy’s in New York when suddenly I smelled something familiar, and I immediately thought of my childhood doll Lucy.

You see, I had not thought about Lucy for years, much less that Lucy had been my favorite doll back when I was growing up in Spain. Looking around, I realized that I was in the store’s toy section and that I was very close to a stand of dolls. Out of curiosity, I reached out for one of the dolls. On the doll’s box it said: “Made in Spain.” This experience was incredible to me—that something so far back in my memory could be brought to the present by something so fleeting as one smell! Years later I recalled this incident when I learned that the part of the brain responsible for our sense of smell—the limbic system—is related to feelings and memory. In order to make sense of what smell is and how we can think of using it in our work, let’s first explore how the sense of smell is put together. How the sense of smell works The sense of smell, just like the sense of taste, is a chemical sense. They are called chemical senses because they detect chemicals in the environment, with the difference being that smell works at dramatically larger distances than that of taste.

The process of smelling goes more or less like this: Vaporized odor molecules (chemicals) floating in the air reach the nostrils and dissolve in the mucus (which is on the roof of each nostril). Underneath the mucus, in the olfactory epithelium, specialized receptor cells called olfactory receptor neurons detect the odor. These neurons are capable of detecting thousands of different odors. The olfactory receptor neurons transmit the information to the olfactory bulbs, which are located at the back of the nose. The olfactory bulbs has sensory receptors that are actually part of the brain which send messages directly to: The most primitive brain centers where they influence emotions and memories (limbic system structures), and “Higher” centers where they modify conscious thought (neo-cortex). These brain centers perceive odors and access memories to remind us about people, places, or events associated with these olfactory sensations. It is important to add that “Our sense of smell is 10,000 times more sensitive than any other of our senses and recognition of smell is immediate. Other senses like touch and taste must travel through the body via neurons and the spinal cord before reaching the brain whereas the olfactory response is immediate, extending directly to the brain.

This is the only place where our central nervous system is directly exposed to the environment.” (von Have, Serene Aromatherapy) The Sense of Smell and the Limbic System The olfactory bulb is one of the structures of the limbic system and a very ancient part of the brain. As mentioned in the previous description of the olfactory process, the information captured by the sense of smell goes from the olfactory bulb to other structures of the limbic system. The limbic system is a network of connected structures near the middle of the brain linked within the central nervous system. These structures “work together to affect a wide range of behaviors including emotions, motivation, and memory” (Athabasca University-Advance Biological Psychology Tutorials). This system deals with instinctive or automatic behaviors, and has little, if anything, to do with conscious thought or will. The limbic system is also concerned with translating sensory data from the neo-cortex (the thinking brain) into motivational forces for behavior.

The limbic system is centrally involved in the mediation between a person’s recognition of an event, their perception of it as stressful, and the resulting physiological reaction to it, mediated via the endocrine system: Stimuli are processed conceptually in the cortex, and passed to the limbic system where they are evaluated and a motivational response is formulated. What does this has to do with our field? In the field of deaf-blindness, we have always known that many children who are deaf-blind have a very sensitive sense of smell to compensate for their limited use of vision and hearing. Consequently, we have always said that the sense of smell plays a key role in this population for identifying people, places, objects and activities. The following statements are heard frequently in this field: avoid wearing strong fragrances because they can elicit seizures in some children; use the sense of smell to provide additional information (olfactory cues) to the child about what is about to happen to the child, e.g., bringing a bar of soap close to the child’s nose before taking a bath to tell him that soon he will be taking a bath; or to wear the same soft scent every time you work with a particular child so he can recognize who you are by this smell.

All of this is very valuable information. The sense of smell is a strong sense for identification purposes and can have a strong impact in your brain because it is such an integral part of it (to the point that strong chemical smells can definitely elicit seizures). But what about the role the sense of smell plays in relation with children’s moods, levels of arousal, emotions, memories and physical reactions? Now we know that they are connected. Many times we are with a child and we can’t understand what is going on with him. He can’t tell us in a formal way. Maybe he is fussy or crying or smiling and we don’t know why. Why is he having these behaviors? Could it be about something he smells? We don’t know. We definitely know we should be paying more attention to this environmental factor to see if and how this is affecting the child. I still have some questions in relation to the impact of the sense of smell in children who are deaf-blind and whether we can use this sense for our advantage, e.g., the use of consistent olfactory cues that might provide information a child could use to better understand what is happening, or eliciting a specific response from a child using a specific scent.

Are children who are deaf-blind more impacted by the sense of smell than children who have normal vision or hearing? Are children who are deaf-blind with neo-cortex lesions more strongly impacted by the sense of smell because they might depend more on the information processed by the limbic system? Can we help a child become more alert or less overexcited using a particular scent? Can we calm a child using oils that have a calming or soothing scent? Conclusions Even though we don’t know exactly how children who are deaf-blind are impacted through their sense of smell, we know this sense is very strong and basic. When interacting with a child who is deaf-blind we should be aware of the environmental odors that might be affecting the child’s behavior. Ideally we should be pairing an odor with its source so the child can make the connection between what he is experiencing and its concrete referent. As an educational specialist in the area of deaf-blindness, it would be interesting to work with a team that includes a neuroscientist and an aroma-therapist to find ways to use the sense of smell to the benefit of children who are deaf-blind.

The emotional connections and the memories attached to a smell seem to be very personal; it seems to be intrinsically enmeshed with the individual experience. I am certain that if another person had been walking with me that day several years ago at Macy’s, he or she would not have noticed the doll’s smell. And on perceiving the smell, he or she would not have thought of my doll Lucy or felt the same feelings I had with this experience. But strong memories can be encoded and be accessible through the natural workings of the sense of smell. It may be possible to create these links to help open another avenue for communication with children who are deaf-blind. Related links ~cadbs California Deaf-Blind Services Fact Sheet by David Brown, CDBS Education Specialist See CDBS list of Fact Sheets with some topics available in Chinese, Laotian, Spanish, and Vietnamese: <~cadbs/factst.html>; email cadbs@ pacbell.net or call 800-822-7884 Olfactory | Definition of Olfactory by Merriam-Webster See olfactory defined for kids Did You Know?

Olfactory derives from the past participle of the Latin olfacere, which means "to smell" and which was formed from the verb olēre (also "to smell") and facere ("to do"). Olfactory is a word that often appears in scientific contexts (as in "olfactory nerves," the nerves that pass from the nose to the brain and contain the receptors that make smelling possible), but it has occasionally branched out into less specialized contexts. The pleasant smell of spring flowers, for example, might be considered an "olfactory delight." A related word, olfaction, is a noun referring to the sense of smell or the act or process of smelling.

Origin and Etymology of olfactory Latin olfactorius, from olfacere to smell, from olēre to smell + facere to do — more at odor , do First Known Use: circa 1658 OLFACTORY Defined for English Language Learners olfactory adjective ol·fac·to·ry \äl-ˈfak-t(ə-)rē, ōl-\ Definition of olfactory for English Language Learners : of, relating to, or connected with the sense of smell OLFACTORY Defined for Kids The Senses The Mystery of the Senses How our senses really work is still a deep mystery A knowledge of perception not only allows us to understand ourselves better, but is essential to the design of systems that interact with humans. Elsewhere on this site, you will find papers on visual perception and colour , and on a comparison of the auditory and visual senses . In this paper I want to review the other senses, taste, smell and touch, which are really the fundamental and ancient ones, and draw some inferences from the similarities and differences between all the senses. A good review can be found in Reference 2. Seeing and hearing are 'remote' senses that tell us about distant parts of our environment by receiving waves.

The other 'contact' senses all involve physical contact with the things that are sensed. They are chemical and mechanical. The chemical senses are the olfactory (smell) and gustatory (taste) sensations. The olfactory sensors are located in yellow pigmented areas on each side of the inner nose. These areas are about 2.5 cm2 in area each, and contain chemoreceptors, which are nerve cells responding to certain chemicals that are carried to the sensors as gases. The detailed functioning of these cells does not appear to be known. The axons of these nerves apparently are received in olfactory bulbs under the front part of the brain, on both sides. Here there are glomeruli, each receiving signals from some 26 000 receptors. The olfactory bulbs on either side are cross-connected. Finally nerve fibres reach the olfactory areas in the anterior lobes of the brain. The types of olfactory sensations are given as fruity, flowery, resinous, spicy, foul, and burned. The olfactory sense is some 10 000 times as sensitive as taste, and is primarily responsible for the flavours of food.

There is strong adaptation, in which one soon becomes accustomed to an odour and unaware of it, as well as masking of one odour by another, the theory of perfume. An increase of some 20% in concentration is necessary to cause a perceptible increase in the strength of the perception. In insects, the olfactory sense seems to be located on the antennae. Snakes and lizards possess a Jacobson's organ in the front of the mouth that is directly connected to the olfactory centre in the brain. The flicking tongue transfers scents to this organ for analysis. Scents seem to have a strong influence on the social interactions of all kinds of animals, from bees and ants to the sexual pheromones of mammals. Birds have a well-developed olfactory sense, which was not appreciated until recently. The scents and colours of flowers are probably not accidental, nor designed for our pleasure, but for very practical purposes of identification by co-operating insects. Dr Le Fanu, in the Sunday Telegraph (21 May 2000) comments on how vigorously an odour calls up memory. Smell is the only sense with direct access to the amygdala, the 'emotional centre' of the brain.

It is a double sense, like seeing and hearing, and the two nostrils control the relative rate of air flow between them to receive slightly different chemical signatures, which allows finer discrimination of odours. The receptors also saturate or accommodate, and this is traded side for side, so that a sensitive receptor is always available. The gustatory sense is mediated by taste buds, small onion-shaped bags on the papillae of the tongue and elsewhere that contain 50 to 75 sensitive cells each. Liquids can pass through a small pore to reach the sensitive cells. Remarkably, the taste-sensitive cells have a limited lifetime, and are constantly being replaced. The kinds of taste sensation are usually termed sweet and sour, located on the tongue, and sour and bitter, located on the roof and back of the mouth. The "tongue map" so often seen is erroneous, a result of the passion in physiologists for creating spatial specialization where there is none ("brain maps" are a similar delusion). The lingual papillae are quite a various gang, including filliform that are touch sensors, and fungiform, foliate and circumvallate, active in taste.

The name of the latter, which are at the rear of the tongue, is misinterpreted in Reference 1 as "wall-like" when really it means "surrounded by a ditch," which they so obviously are. The taste fibres proceed along several pathways to the medulla oblongata or brain stem, then to the thalamus, and finally to the taste area on the anterior cortex. There are synaptic connections between neighbouring cells, as in the case of vision and hearing. The taste sense exhibits adaptation and masking, like the other senses. Taste and smell are not reliable guides to poisons, only to identification of known substances. Some quite innocuous substances taste terrible, while some poisons can taste delightful. Lead acetate, or sugar of lead, tastes pleasantly sweet, but is a powerful cumulative poison. The aromatic compounds benzene and toluene are fragrant, but benzene is dangerously carcinogenic, while toluene is relatively safe. The chemical senses are sometimes associated with vivid mental images and recollections, showing an unexpected connection to higher mental processes.

Reference 1 mentions that it has been found that taste perception is a result of differences in neural stimulation, and also, with some evident surprise, that different perceptions can arise from the same cells. The idea that salt excites a salty receptor that passes a salty signal through a salty nerve to a salty center in the brain recalls the primitive idea of the retinal cell that sends a signal through its nerve to paint its pixel in the brain. It would be thought that these principles, so evident in the visual and auditory senses, could hardly be surprising here. There is even a protein involved in taste that is analogous to the transducin of the visual mechanism. It should not be surprising that all senses have a common origin. Touch is one aspect of the important and varied mechanoreceptive senses. Touch, posture or kinesthetic sense, the vestibular or equilibrium sense, and sound all involve sensitive cells that react to a mechanical stimulus. Deformation of the cell causes a change in electric potentials and the initiation of a nerve impulse. Many of these cells have tactile hairs, such as the hair cells of the semicircular canals and the cochlea. Mammals, notably cats, have vibrissae, 'whiskers', that are very sensitive.

Subcutaneous receptors, another kind of sensor, seem to possess sensitive nerve endings. Clusters of such receptors make up the pain spots that differ in density over the skin. Two prongs 2 mm apart can be separately detected on the fingertip, but not on the back. The tip of the tongue has some 200 such points per square centimetre, and can detect two points only 1 mm apart. Fishes have a lateral line down their sides that is a very sensitive touch organ, capable of detecting pressure pulses before actual contact takes place. Other receptors are sensitive to heat. Many touch organs communicate with ganglia in the spinal cord, and may be part of a reflex arc that does not involve processing by the brain. Although touch may seem to involve less mental processing than the other senses, large volumes of the brain are associated with parts of the body, and touch may play a large role, especially in learning and memory. The sensation of pain is closely related to touch, but is obviously a subjective perception like those of sound and vision, involving higher mental processes and consciousness.

There is an important sense of equilibrium that uses the semicircular canals in the ear to detect motions of the head, that partly has an involuntary output, as well as an effect on conciousness. The contact senses are important in the development of the infant, especially the visual sense. I believe that motions of the hand and fingers aid learning and the memory. Touch is also subject to 'illusions' that show mental processing is involved. The best-known is probably the pencil between crossed fingers, that is sensed as two pencils. Temperature is often wrongly judged, the sensation depending on contrasts and comparisons. Objects of different materials but at the same temperature may feel variously cooler or warmer. Our knowledge of all the senses is very incomplete and unsatisfactory, especially with regard to the neural and mental processes that are an essential, perhaps the major, keys to understanding consciousness. Anatomical and physiological knowledge of the structures of the nervous system is detailed and rather complete, but furnishes only the slightest clues to the operation of the senses. Empirical knowledge of how the senses behave is extensive, but it only describes and does not explain. There seem to be few areas of modern science so important and interesting to us in which the fundamental knowledge is so incomplete.

The senses should not be studied in isolation from one another, since there are surprising connections as the result of the mental processes of consciousness. The senses do not interact solely with consciousness, but also with subconscious and involuntary responses to the environment. Most of the sensory cells seem to be descended from ciliated primitive cells that would have been unusually active and became included in associations to take advantage of their responsiveness. The rod and cone cells of the retina have lost all apparent characteristics of these primitive sensory cells except perhaps the overall shape, while the chemical and mechanical sensors retain cilia and hairs because of their functionality. The nerve impulses from these cells do not reach the brain directly, but only through many synaptic connections involving cross connections, coding and processing, that result in complex messages carried by far fewer fibres. These trunk nerves enter intermediate bodies, with connections to both hemispheres of the brain, and pass on their signals to further bundles of fibres to distribute the signals to the cortex and the centres of consciousness, wherever they may be. Sensory perception will not be understood until all these pathways are elucidated. There are no simple senses that directly interact with consciousness. The two hemispheres of the brain appear to share sensory information equally and impartially.

In vision and hearing, both halves of the brain are essential to the complete sensation. There is no support whatsoever for the view that there are two brains with different characteristics as far as the senses are involved. The senses also involve the central and old parts of the brain, the brain stem and its associated regions. Consciousness is probably located here, not in the peripheral cortex that seems chiefly devoted to information storage. Simpler psychologists so want the large cortex to be what confers humanity and consciousness that conflicting evidence is overlooked. The functioning of the brain will never be revealed by the scalpel, balance and microelectrode. The senses cannot be understood except by careful separation of the physical and objective stimulus from the mental and subjective perception. We cannot be directly aware of the properties and qualities of external objects, though our language and thinking often identifies an object with its perception. An object cannot, of itself, be red, nor a solution of sugar sweet: these are essentialy perceptions within ourselves, not properties of matter. All the senses appear to depend more or less on differences between the states of neighbouring sensors.

This is strongest in the visual and aural senses, perhaps weakest in touch, but even here the co-operation of several neighbouring cells is probably necessary to launch a sensation. All senses have the widest dynamic range possible, which is greatest in hearing, and least in touch or taste, made possible by a logarithmic response. All senses communicate only by electrical pulses travelling down nerve axons, and are subject to noise. All senses exhibit adaptation, in which a continued steady stimulus has an effect decreasing with time, as well as masking, in which one stimulus increases the threshold for the detection of another. There is no straightforward, universal connection between the intensity of a stimulus and the strength of its perception. Sensation can judge equality with some precision, but ratios cannot be accurately estimated, even approximately. Another common property of the senses is shown by Fechner's Law, first recognized in vision. The psychophysical quantity brightness is related to the physical quantity energy flux or intensity logarithmically, not linearly. If you cast a shadow on a piece of white paper in the moonlight, then turn on a bright electric light, the shadow will disappear, even though the difference in illumination between the shadow and its surroundings does not change.

Fechner observed that the delicate shadows and contrasts of a cloud were unchanged when the cloud was observed through a dark glass. Experiment showed that a minimum fractional change in intensity was observed as a just detectable change in brightness over a wide range, though failing for illumination that was too strong or too feeble. The old system of classifying stars by magnitude in a uniform series of equal steps of brightness was found to be logarithmic, the ratio of intensities being about 2.5 in each step. The loudness of a sound has a similar relation to the intensity, and remarkably, so does pitch. The even-tempered scale of frequency ratios of 21/12 gives equal steps of pitch, as well as frequency ratios close to those that are found harmonic. Fechner's Law is, of course, approximate and inexact, but still expresses a remarkable property of the senses. The subjective experiences of the consciousness are created by dynamic mental activity based on sensory information combined with memory, and all of these three factors are necessary. This includes things like colours, smells, tones, flavours, recognition of faces, and pain, and indeed the world we see. All the senses are simultaneously in action, and our consciousness is dominated by perceptions.

How these perceptions arise is completely unknown, a mystery. If it were known, we would be able to describe what red is, how salt tastes, and what a major chord sounds like. All we can do now is assume we all have the same sensations and describe things in terms of them. This is probably a pretty good assumption, and we can even detect slight anomalies between individuals when they occur, but it leaves a nagging doubt and a thirst for knowledge. References D. V. Smith and R. F. Margolskee, Making Sense of Taste in Scientific American, March 2001, p. 32-39. New Encyclopedia Britannica, 15th edition, Vol. 27, pages 115-222, article Sensory Perception. The majority of this long article is devoted to seeing and hearing, and the treatment is complementary to the one presented here. Composed by J. B. Calvert Created 30 March 2000 Olfaction: The Nasal Cavity and Smell About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made.

Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Olfaction: The Nasal Cavity and Smell The olfactory system gives humans their sense of smell by collecting odorants from the environment and transducing them into neural signals. Learning Objective Summarize the structure and function of the olfactory system Key Points Olfaction is a type of chemoreception. Like gustation, this sensory system uses the molecular chemical compounds in substances (in this case, in odorants ) to discern information about the environment. The main sensory organ responsible for the human sense of smell is the nasal cavity, which contains olfactory receptors that perform the transduction of odors into neural impulses . Human beings can detect a large and diverse number of smells due to the vast number of features and combinations of odor molecules. Olfaction is the sense most closely tied to memory because of its close neural connections to areas of the brain responsible for emotion and place memory. Terms Register for FREE to remove ads and unlock more features!

Learn more Full Text The olfactory system gives humans their sense of smell by inhaling and detecting odorants in the environment. Olfaction is physiologically related to gustation, the sense of taste, because of its use of chemoreceptors to discern information about substances. Perceiving complex flavors requires recognizing taste and smell sensations at the same time, an interaction known as chemoreceptive sensory interaction. This causes foods to taste different if the olfactory system is compromised. However, olfaction is anatomically different from gustation because it uses the sensory organs of the nose and nasal cavity to capture smells. Humans can identify a large number of odors and use this information to interact successfully with their environment. The Nose and Nasal Cavity Olfactory sensitivity is directly proportional to spatial area in the nose—specifically the olfactory epithelium, which is where odorant reception occurs. The area in the nasal cavity near the septum is reserved for the olfactory mucous membrane, where olfactory receptor cells are located. This area is a dime-sized region called the olfactory mucosa . In humans, there are about 10 million olfactory cells, each of which has 350 different receptor types composing the mucous membrane.

Each of the 350 receptor types is characteristic of only one odorant type. Each functions using cilia, small hair-like projections that contain olfactory receptor proteins. These proteins carry out the transduction of odorants into electrical signals for neural processing. A cross-section of the olfactory system that labels all of the structures necessary to process odor information. Olfactory transduction is a series of events in which odor molecules are detected by olfactory receptors. These chemical signals are transformed into electrical signals and sent to the brain, where they are perceived as smells. Once ligands (odorant particles) bind to specific receptors on the external surface of cilia, olfactory transduction is initiated. In mammals, olfactory receptors have been shown to signal via G protein. This is a similar type of signaling of other known G protein-coupled receptors (GPCR). The binding of an odorant particle on an olfactory receptor activates a particular G protein (Gαolf), which then activates adenylate cyclase, leading to cAMP production. cAMP then binds and opens a cyclic nucleotide-gated ion channel. This opening allows for an influx of both Na+ and Ca2+ ions into the cell, thus depolarizing it.

The Ca2+ in turn activates chloride channels, causing the departure of Cl-, which results in a further depolarization of the cell. Interpretation of Smells Individual features of odor molecules descend on various parts of the olfactory system in the brain and combine to form a representation of odor. Since most odor molecules have several individual features, the number of possible combinations allows the olfactory system to detect an impressively broad range of smells. A group of odorants that shares some chemical feature and causes similar patterns of neural firing is called an odotope. Humans can differentiate between 10,000 different odors. People (wine or perfume experts, for example) can train their sense of smell to become expert in detecting subtle odors by practicing retrieving smells from memory. Smell and Memory Odor information is easily stored in long-term memory and has strong connections to emotional memory. This is most likely due to the olfactory system's close anatomical ties to the limbic system and the hippocampus , areas of the brain that have been known to be involved in emotion and place memory.

Human and animal brains have this in common: the amygdala , which is involved in the processing of fear, causes olfactory memories of threats to lead animals to avoid dangerous situations. The human sense of smell is not quite as powerful as most other animals' sense of smell, but smell is still deeply tied to human memory and emotion. Pheromones are airborne, often odorless molecules that are crucial to the behavior of many animals. They are processed by an accessory of the olfactory system. Recent research shows that pheromones play a role in human attraction to potential mates, the synchronization of menstrual cycles among women, and the detection of moods and fear in others. Thanks in large part to the olfactory system, this information can be used to navigate the physical world and collect data about the people around us. Olfactory System - How Smell Works | HowStuffWorks How Smell Works Henrik Montgomery/AFP/ Getty Images How does the brain recognize, categorize and memorize the huge variety of odors? In 1991, Richard Axel and Linda Buck published a groundbreaking paper that shed light on olfactory receptors and how the brain interprets smell.

They won the 2004 Nobel Prize in Physiology or Medicine for the paper and their independent research. Axel and Buck discovered a large gene family -- 1,000 genes, or 3 percent of the human total -- that coded for olfactory receptor types. They found that every olfactory receptor cell has only one type of receptor. Each receptor type can detect a small number of related molecules and responds to some with greater intensity than others. Essentially, the researchers discovered that receptor cells are extremely specialized to particular odors. Up Next Break the Cycle of Negative Thinking Axel and Buck also found that each olfactory receptor type sends its electrical impulse to a particular microregion of the olfactory bulb. The microregion, or glomerulus, that receives the information then passes it on to other parts of the brain. The brain interprets the "odorant patterns" produced by activity in the different glomeruli as smell. There are 2,000 glomeruli in the olfactory bulb -- twice as many microregions as receptor cells -- allowing us to perceive a multitude of smells.

An illustration of how receptors function in the olfactory system Image courtesy Nobelprize.org Another researcher, however, has challenged the idea that humans have a large number of receptor types that respond only to a limited number of molecules. Biophysicist Luca Turin developed the quantum vibration theory in 1996 and suggests that olfactory receptors actually sense the quantum vibrations of odorants' atoms . While molecular shape still comes into play, Turin purports that the vibrational frequency of odorants plays a more significant role. He estimates that humans could perceive an almost infinite number of odors with only about 10 receptors tuned to different frequencies. Next, we'll learn about how smells trigger memory and find out how much cognition actually influences perception. Follow the Nose ­The human sense of smell has long been maligned -- its sensitivity is often unfavorably compared to that of animals. Smell even came in dead last in a HowStuffWorks battle of favorite senses. But researchers at the University of California at Berkeley have found that humans actually have sophisticated olfactory capabilities. A group of 32 volunteers were asked to track scents with their noses across a 10-meter (about 33-foot) trail.

The subjects were blindfolded and wore gloves and earplugs to isolate their senses of smell. Two-thirds of the volunteers were able to track the scent and, although they were slower than the tracking dogs , most improved with practice [source: BBC ]. Smell and Memory - How Smell Works | HowStuffWorks Smell and Memory William Thomas Cain/ Getty Images A smell can bring on a flood of memories, influence people's moods and even affect their work performance. Because the olfactory bulb is part of the brain's limbic system, an area so closely associated with memory and feeling it's sometimes called the "emotional brain," smell can call up memories and powerful responses almost instantaneously. The olfactory bulb has intimate access to the amygdala, which processes emotion, and the hippocampus, which is responsible for associative learning. Despite the tight wiring, however, smells would not trigger memories if it weren't for conditioned responses. When you first smell a new scent, you link it to an event, a person, a thing or even a moment. Your brain forges a link between the smell and a memory -- associating the smell of chlorine with summers at the pool or lilies with a funeral.

When you encounter the smell again, the link is already there, ready to elicit a memory or a mood. Chlorine might call up a specific pool-related memory or simply make you feel content. Lilies might agitate you without your knowing why. This is part of the reason why not everyone likes the same smells. Up Next Break the Cycle of Negative Thinking Because we encounter most new odors in our youth, smells often call up childhood memories. But we actually begin making associations between smell and emotion before we're even born. Infants who were exposed to alcohol , cigarette smoke or garlic in the womb show a preference for the smells. To them, the smells that might upset other babies seem normal or even comforting. In the next section, we'll find out how some people use smell's ability to trigger memory. Is that Cheese or Just B.O.? Researchers have found that cognition significantly influences the perception of smell. A psychologist at the University of Oxford labeled an ambiguous Brie-like scent as either "cheddar cheese" or "body odor." Test subjects rated the odor higher when it was labeled cheddar cheese. MRIs even showed more activity in the olfactory region of the brain when subjects believed they were smelling cheese [source: Scientific American ].

Question: Olfactory relates to which of the senses?
Answer: Smell