Chemoreception: Smell and Taste

The five special senses—smell, taste, hearing, equilibrium, and vision—are concentrated in the head region. Like somatic senses, the special senses rely on receptors to transform information about the environment into patterns of action potentials that can be interpreted by the brain. Smell and taste are both forms of chemoreception, one of the oldest senses from an evolutionary perspective. Unicellular bacteria use chemoreception to sense their environment, and primitive animals without formalized nervous systems use chemoreception to locate food and mates. It has been hypothesized that chemoreception evolved into chemical synaptic communication in animals.

Olfaction Is One of the Oldest Senses

Imagine waking up one morning and discovering a whole new world around you, a world filled with odors that you never dreamed existed—scents that told you more about your surroundings than you ever imagined from looking at them. This is exactly what happened to a young patient of Dr. Oliver Sacks (an account is in The Man Who Mistook His Wife for a Hat and Other Clinical Tales). Or imagine skating along the sidewalk without a helmet, only to fall and hit your head. When you regain consciousness, the world has lost all odor: no smell of grass or perfume or garbage. Even your food has lost much of its taste, and you now eat only to survive because eating has lost its pleasure.

We do not realize the essential role that our sense of smell plays in our lives until a head cold or injury robs us of the ability to smell. Olfaction allows us to discriminate among billions of different odors. Even so, our noses are not nearly as sensitive as those of many other animals whose survival depends on olfactory cues. The olfactory bulb, the extension of the forebrain that receives input from the primary olfactory neurons, is much better developed in vertebrates whose survival is more closely linked to chemical monitoring of their environment (Fig. 10.13a).

Olfactory Pathways

The human olfactory system consists of an olfactory epithelium lining the nasal cavity, with embedded primary sensory neurons called olfactory sensory neurons. Axons of the olfactory sensory neurons form the olfactory nerve, or cranial nerve I [here]. The olfactory nerve synapses with secondary sensory neurons in the olfactory bulb, located on the underside of the frontal lobe (Fig. 10.13b). Secondary and higher-order neurons project from the olfactory bulb through the olfactory tract to the olfactory cortex (Fig. 10.13a). The olfactory tract, unlike most other sensory pathways, bypasses the thalamus.

This arrangement seems quite simple, but complex processing takes place before signals pass on to the cortex. Evidence now suggests that modulation of incoming sensory information begins in the olfactory epithelium. Additional processing takes place in the olfactory bulb. Some descending modulatory pathways from the cortex terminate in the olfactory bulb, and there are reciprocal modulatory connections within and between the two branches of the olfactory bulb.

Ascending pathways from the olfactory bulb also lead to the amygdala and hippocampus, parts of the limbic system involved with emotion and memory. The link between smell, memory, and emotion is one amazing aspect of olfaction. A special cologne or the aroma of food can trigger memories and create a wave of nostalgia for the time, place, or people with whom the aroma is associated. In some way that we do not understand, the processing of odors through the limbic system creates deeply buried olfactory memories. Particular combinations of olfactory receptors become linked to other patterns of sensory experience so that stimulating one pathway stimulates them all.

The Olfactory Epithelium

Olfactory sensory neurons in humans are concentrated in a 3-cm2 patch of olfactory epithelium high in the nasal cavity (Fig. 10.13a). Olfactory sensory neurons have a single dendrite that extends down from the cell body to the surface of the olfactory epithelium, and a single axon that extends up to the olfactory bulb. Olfactory sensory neurons, unlike other neurons in the body, have very short lives, with a turnover time of about two months (Fig. 10.13c).

Stem cells in the basal layer of the olfactory epithelium are continuously dividing to create new neurons. The axon of each newly formed neuron must then find its way to the olfactory bulb and make the proper synaptic connections. To give us insight into how developing neurons find their targets, scientists are studying how these neurons manage to repeat the same connection each time.

In rodents, an accessory olfactory structure in the nasal cavity, the vomeronasal organ (VNO), is known to be involved in behavioral responses to sex pheromones [here]. Anatomical and genetic studies in humans suggest that humans do not have a functional VNO, but experiments with compounds believed to act as human pheromones support the hypothesis that humans may communicate with chemical signals.

Olfactory Signal Transduction

The surface of the olfactory epithelium is composed of the knobby terminals of the olfactory sensory neuron dendrites, each knob branching into multiple nonmotile cilia (Fig. 10.13c). The cilia are embedded in a layer of mucus that is produced by olfactory (Bowman’s) glands in the epithelium and basal lamina. Odorant molecules must first dissolve in and penetrate the mucus before they can bind to an olfactory receptor protein on the olfactory cilia. Each olfactory receptor is sensitive to a limited range of odorants.

Olfactory receptors are G protein-linked membrane receptors [here]. Olfactory receptor genes form the largest known gene family in vertebrates (about 1000 genes, or 3–5% of the genome), but only about 400 olfactory receptor proteins are expressed in humans. The combination of most odorant molecules with their olfactory receptors activates a special G protein, Golf, which in turn increases intracellular cAMP. The increase in cAMP concentration opens cAMP-gated cation channels, depolarizing the cell. If the graded receptor potential that results is strong enough, it triggers an action potential that travels along the sensory neuron’s axon to the olfactory bulb.

What is occurring at the cellular and molecular levels that allows us to discriminate between thousands of different odors? Current research suggests that each individual olfactory sensory neuron contains a single type of olfactory receptor that responds to a limited range of odorant molecules. The axons of cells with the same receptors converge on a few secondary neurons in the olfactory bulb, which then can modify the information before sending it on to the olfactory cortex. The brain uses information from hundreds of olfactory sensory neurons in different combinations to create the perception of many different smells, just as combinations of letters create different words. This is another example of population coding in the nervous system [p. 315].

Concept Check

  1. Create a map or diagram of the olfactory pathway from an olfactory sensory neuron to the olfactory cortex.

  2. Create a map or diagram that starts with a molecule from the environment binding to its olfactory receptor in the nose and ends with neurotransmitter release from the primary olfactory neuron.

  3. The dendrites are which part of an olfactory sensory neuron?

  4. Are olfactory neurons pseudounipolar, bipolar, or multipolar? [Hint: See Fig. 8.2, p. 230.]

Taste Is a Combination of Five Basic Sensations

Our sense of taste, or gustation, is closely linked to olfaction. Indeed, much of what we call the taste of food is actually the aroma, as you know if you have ever had a bad cold. Although smell is sensed by hundreds of receptor types, taste is currently believed to be a combination of five sensations: sweet, sour (acid), salty, bitter, and umami, a taste associated with the amino acid glutamate and some nucleotides. Umami, a name derived from the Japanese word for “deliciousness,” is a basic taste that enhances the flavor of foods. It is the reason that monosodium glutamate (MSG) is used as a food additive in some countries.

Each of the five currently recognized taste sensations is associated with a physiological process. Sour taste is triggered by the presence of H+ and salty by the presence of Na+. The concentrations of these two ions in body fluids are closely regulated because of the roles they play in pH balance and extracellular fluid volume. The other three taste sensations result from organic molecules. Sweet and umami are associated with nutritious food. Bitter taste is recognized by the body as a warning of possibly toxic components. If something tastes bitter, our first reaction is often to spit it out.

Taste Pathways

The receptors for taste are located primarily on taste buds clustered together on the surface of the tongue (Fig. 10.14a). One taste bud is composed of 50–150 taste receptor cells (TRCs), along with support cells and regenerative basal cells. Taste receptors are also scattered through other regions of the oral cavity, such as the palate.

For a substance (tastant) to be tasted, it must first dissolve in the saliva and mucus of the mouth. Dissolved taste ligands then interact with an apical membrane protein (receptor or channel) on the taste receptor cell (Fig. 10.14b). Interaction of the taste ligand with a membrane protein initiates a signal transduction cascade that ends with release of chemical messenger molecules from the TRC. The details of signal transduction for the five taste sensations are still controversial, due partly to the fact that some of the mechanisms appear to be different in humans and mice, the primary model organism for mammalian taste research.

Chemical signals released from taste receptor cells activate primary sensory neurons (gustatory neurons) whose axons run through cranial nerves VII, IX, and X to the medulla, where they synapse. Sensory information then passes through the thalamus to the gustatory cortex (see Fig. 10.3). Central processing of sensory information compares the input from multiple taste receptor cells and interprets the taste sensation based on which populations of neurons are responding most strongly (another example of population coding). Signals from the sensory neurons also initiate behavioral responses, such as feeding, and feedforward responses [here] that activate the digestive system.

Taste Transduction Uses Receptors and Channels

The details of taste receptor cell signal transduction, once believed to be relatively straightforward, are more complex than scientists initially thought. Sweet, bitter, and umami tastes are associated with activation of G protein-coupled receptors. In contrast, salty and sour transduction mechanisms both appear to be mediated by ion channels.

Taste buds contain four morphologically distinct cell types designated I, II, and III, plus basal cells. Type I cells are glia-like support cells. Type II cells, or receptor cells, and type III cells, or presynaptic cells, are taste receptor cells.

Each taste receptor cell is a non-neural polarized epithelial cell [here] tucked down into the epithelium so that only a tiny tip protrudes into the oral cavity through a taste pore (Fig. 10.14a). In a given bud, tight junctions link the apical ends of adjacent cells together, limiting movement of molecules between the cells. The apical membrane of a TRC is modified into microvilli to increase the amount of surface area in contact with the environment.

Sweet, Bitter, and Umami Tastes

The type II taste receptor cells respond to sweet, bitter, and umami sensations. These cells express multiple G protein-coupled receptors (GPCR) on their apical surfaces (Fig. 10.14b). Sweet and umami tastes are associated with T1R receptors with different combinations of subunits. Bitter taste uses about 30 variants of T2R receptors.

The type II cell receptors activate a special G protein called gustducin, which in turn activates multiple signal transduction pathways. Some of these pathways release Ca2+ from intracellular stores, while others open cation channels and allow Ca2+ to enter the cell. Calcium signals then initiate ATP release from the type II cells.

ATP in type II cells is not released through secretory vesicles. Instead it leaves the cell through gap junction-like channels. ATP then acts as a paracrine signal on both sensory neurons and neighboring presynaptic cells. This communication between neighboring taste receptor cells creates complex interactions.

Sour Taste

The type III presynaptic cells respond to sour tastes. Models of transduction mechanisms for sour tastes are complicated by the fact that increasing H+, the sour taste signal, also changes pH. There is evidence that H+ acts on ion channels of the presynaptic cell from both extracellular and intracellular sides of the membrane. The intracellular pathways remain uncertain. Ultimately, H+-mediated depolarization of the presynaptic cell results in serotonin release by exocytosis. Serotonin in turn excites the primary sensory neuron.

Salt Taste

The cells responsible for salt taste have not been definitively identified, but some evidence suggests that salt reception may reside in the type 1 support cells. Signal transduction for salt taste in humans is equally unclear, complicated by the fact that mice have two different mechanisms but humans appear to have only one. In the current model for salty taste, Na+ enters the taste receptor cell through an apical ion channel, such as the epithelial Na+ channel (ENaC, pronounced ēē-knack). Sodium entry depolarizes the cell, setting off a series of events that culminate with the primary sensory neuron firing an action potential.

The mechanisms of taste transduction are a good example of how our models of physiological function must periodically be revised as new research data are published. For many years, the widely held view of taste transduction was that an individual taste receptor cell could sense more than one taste, with cells differing in their sensitivities. However, gustation research using molecular biology techniques and knockout mice currently indicates that each taste receptor cell is sensitive to only one taste.

Nontraditional Taste Sensations

The sensations we call taste are not all mediated by the traditional taste receptors. For years, physiologists thought fat in the diet was appealing because of its texture, and food experts use the phrase “mouth feel” to describe the sensation of eating something fatty, such as ice cream, that seems to coat the inside of the mouth. But now it appears that the tongue may have taste receptors for fats.

Research in rodents has identified a membrane receptor called CD36 that lines taste pores and binds fats. Activation of this receptor helps trigger the feedforward digestive reflexes that prepare the digestive system for a meal. Currently evidence is lacking for a similar receptor in humans, but “fatty” may turn out to be a sixth taste sensation. Other candidates for new taste sensations include carbonation (dissolved CO2) and Ca2+, another essential element obtained through the diet.

Some additional taste sensations are related to somatosensory pathways rather than taste receptor cells. Nerve endings in the mouth have TRP receptors and carry spicy sensations through the trigeminal nerve (CN V). Capsaicin from chili peppers, menthol from mint, and molecules in cinnamon, mustard oil, and many Indian spices activate these receptors to add to our appreciation of the food we eat.

And what would you say to the idea of taste buds in your gut? Scientists have known for years that the stomach and intestines have the ability to sense the composition of a meal and secrete appropriate hormones and enzymes. Now it appears that gut chemoreception is being mediated by the same receptors and signal transduction mechanisms that occur in taste buds on the tongue. Studies have found the T1R receptor proteins for sweet and umami tastes as well as the G protein gustducin in various cells of rodent and human intestines.

An interesting psychological aspect of taste is the phenomenon named specific hunger. Humans and other animals that are lacking a particular nutrient may develop a craving for that substance. Salt appetite, representing a lack of Na+ in the body, has been recognized for years. Hunters have used their knowledge of this specific hunger to stake out salt licks because they know that animals will seek them out. Salt appetite is directly related to Na+ concentration in the body and cannot be assuaged by ingestion of other cations, such as Ca2+ or K+. Other appetites, such as cravings for chocolate, are more difficult to relate to specific nutrient needs and probably reflect complex mixtures of physical, psychological, environmental, and cultural influences.

Concept Check

  1. With what essential nutrient is the umami taste sensation associated?

  2. Map or diagram the neural pathway from a presynaptic taste receptor cell to the gustatory cortex.