taste and smell

taste and smell When we eat or drink we perceive a sensation that most people call ‘taste’. However, we all know that when the nose is blocked, for instance when one has a common cold, the sensation is considerably reduced. This is because it results from a combination of stimulation of chemical receptors (chemoreceptors) in the nose as well as the mouth. The chemoreceptors in the mouth are called gustatory receptors and those in the nose are olfactory receptors. The sensations that result from individual stimulation of these two types of chemoreceptors are respectively taste and smell. Chemoreceptors are not, however, the only sensory receptors involved in the appreciation and discrimination of food and drink. At least two other modalities of sensation affect the overall experience. The smoothness, texture and crunchiness of food are conveyed by mechanoreceptors on the tongue, and throughout the rest of the mouth, including the teeth, and in the pharynx. Thermoreceptors in the mouth also detect the temperature of solids and liquids. Just think of the combination of experiences – touch, fizziness, coolness, acidity and exquisite smells – that make up the experience of drinking champagne. Those who like spices in their food even derive pleasure from the stimulation of receptors normally involved in the sensation of pain (nociception). These nociceptors in the mouth are stimulated by chemicals found in common spices, such as Chilli peppers, and the resulting sensation is referred to as the common chemical sense.

The taste/smell system fulfils two separate physiological roles. Not only does it help us to identify ‘good’ food, containing essential nutrients (salts, carbohydrates, proteins and fats), but it also provides a warning of the unsuitability of harmful and potentially toxic substances by detecting them before they are ingested.

Gustatory receptors

The receptors involved in gustation are found in specialized ‘end-organs’ called taste buds, embedded in the epithelium that covers the surface of the tongue, soft palate, pharynx, larynx and epiglottis. However, they are not uniformly distributed in these regions. The taste buds on the tongue are associated with characteristic ‘papillae’ (from the Latin for pimples), whereas those in the other regions are found on the smooth epithelial surface. In humans, the number of taste buds varies considerably from person to person, with the majority having 2000 to 5000, distributed over the various regions. However, the number can be as low as 500 and as high as 20 000 in some individuals.

The papillae in different regions of the tongue have distinctive shapes and characteristic numbers of taste buds associated with them. Scattered over the main body of the tongue are approximately 200, small, mushroom-shaped (fungiform) papillae, which have, on average, only three taste buds each. Larger (foliate) papillae are found at the back and sides of the tongue. They consist of up to nine folds of epithelium and have as many as 600 taste buds each. Eight to 12 larger mushroom shaped (circumvallate) papillae, each surrounded by a circular trough, lie at the back of the tongue in a V-shaped formation; these have on average 250 taste buds each. Scattered taste buds are also found in the epithelium of the soft palate, pharynx, larynx and epiglottis.

Each taste bud is contacted, at its base, by the terminals of sensory nerve fibres. These taste fibres belong to three different cranial nerves, connected to the brain. The nerve supply for most of the taste buds on the soft palate and towards the front of the tongue come from a division of the VIIth (facial) cranial nerve, called the chorda tympani, because its route to the brainstem passes close to the tympanic membrane in the ear. The IXth (glossopharyngeal) and Xth (vagus) nerves innervate taste buds in the back of the mouth and the pharynx respectively.

Each taste bud contains 50–150 neuroepithelial receptor cells arranged, like segments of an orange, to form a compact, pear-shaped structure, about 70 μm high and 40 μm in diameter. There is a small 2–10 μm opening in the epithelial surface called the taste pore, which allows direct contact between chemicals dissolved in the saliva and the tips of the receptor cells. These exposed parts of the receptor cells are made up of many long corrugated folds in the membrane called microvilli, which provide a greater surface area for contact with the saliva. It is difficult to taste food with a dry mouth. Saliva is essential for normal taste because it acts as both a solvent for the chemicals as well as a transport medium for those chemicals to reach the receptors. A layer of saliva extends into the taste pores and constantly bathes the receptors. The dissolved chemicals diffuse through this thin layer of saliva to reach the microvilli. Reflex secretion of saliva from the salivary glands under the tongue and in the cheeks is stimulated by chewing, taste and smell, to varying degrees. And, as Ivan Pavlov demonstrated in his classical experiments on dogs, the simple form of unconscious learning known as conditioning couples the reflex secretion of saliva to the familiar signs of an impending meal – the sound of a dinner bell, the clatter of crockery, the sight of the food.

The taste bud complex is a dynamic system in which the receptor cells are rapidly turning over. The life-span of an individual receptor cell is about 10 days: cells are continually being born (through the division of epithelial stem cells within the bud), maturing, performing their gustatory function and eventually dying. Even though the receptor cell does not itself have an axon or fibre, the base of the cell has specialized regions that look like the terminals of nerve fibres. The cytoplasm in these regions is packed with tiny spherical vesicles, filled with a chemical transmitter substance, which is released when the potential inside the receptor cell becomes more positive (depolarization). In close association with these regions are the endings of the sensory nerve fibres, making an assembly like a synapse. Each taste bud is innervated by more than one nerve fibre and each single nerve fibre can connect to a number of receptor cells, taste buds and even papillae. This suggests a high degree of convergence of input from taste buds on to the sensory nerve fibres. Because of the rapid turnover of receptor cells, the connections between cells and nerve fibres is constantly changing. The nerves are continuously sprouting new processes, forming new synapses with young cells and retracting synaptic connections with dying cells. At any one time less than a third of the cells in the taste bud are innervated.

An intact nerve supply is necessary for the normal function of taste buds. If the nerves are damaged the taste buds degenerate and slough off, and following regeneration of the nerves, the taste buds reappear.

Since the time of Aristotle (384–322 bc) there have been attempts to categorize taste into primary or basic tastes. Although many hundreds of different chemicals can stimulate activity in taste receptor cells, the four basic taste qualities of salt, sour, sweet and bitter have stood the test of time. However, there is still controversy as to whether combination of these four primary tastes adequately describes all gustatory experiences. Metallic and astringent tastes have, in the past, been suggested as primaries, and more recently Japanese researchers have proposed that the characteristic taste of monosodium glutamate (used as a taste enhancer by the food industry) is also a basic taste, with its own receptive mechanism. They have called it “umami” meaning “delicious taste”

. Because of the dual role of gustatory receptor cells, detecting both nutrients and toxins, they must be able to respond, either individually or collectively, to a wide variety of chemicals. These chemicals range from simple ions such as sodium (salt) and hydrogen (sour) to the more complex compounds that give the sensations of sweet (e.g. sugar) and bitter (e.g. quinine). The mechanisms by which the chemical stimuli are translated into electrical events in the receptor cell (transduction) are numerous, varied and complex. The essential process depends on specific interactions between taste substances and specialized protein receptor molecules embedded in the membrane of the receptor cell, which trigger a series of chemical reactions, leading to a change in the flow of ions through pores in the membrane, and hence a change in the electrical potential inside the cell. However, there does not appear to be a unique mechanism for each of the basic tastes: each seems to use several different mechanisms. There may even be similarities of mechanism for different basic tastes. The wayin which we can perceive many subtle tastes and distinguish between different compounds of the same basic taste category might be explained by the multiplicity and specificity of these mechanisms.

The evidence for a particular receptor mechanism is best for sweet sensation. First, certain drugs have specific effects on the detection of sweetness. For instance, after eating a West African fruit called miracle fruit, even quite acidic substances (such as lemon juice), which would normally be sour, taste extraordinarily sweet. Miracle fruit contains a substance that is thought to attach to the protein receptor molecules that detect sweet-tasting substances. A subsequent increase in acidity in the saliva is thought to alter the binding of this substance with the sweet receptor protein such that it stimulates the receptor, like a genuine sweet substance. In contrast, gymnemic acid, found in an Indian plant, Gymnema sylvestre, blocks the sweet receptor in some manner, and abolishes the sensation of sweetness for half an hour or so. Very recently, a gene called T1r2 has been identified in mice, which is selectively switched on in taste bud receptor cells. It turns out that strains of mice that lack sweet taste (they don't prefer sweet food to non-sweet) have a mutation of this gene. There is a very similar gene in human beings.

Researchers have recorded with tiny electrodes from individual nerve fibres innervating the taste buds, in anaesthetized animals. One might have expected that each fibre would respond, with a burst of impulses, when a solution of just one of the primary taste substances was dripped on to the appropriate taste bud or buds. Such selectivity of response is, in fact, very rare. Most nerve fibres respond to two or more of the basic taste stimuli, the magnitude of the response varying from one taste substance to another. In other words, the activity of such a fibre does not provide unambiguous information to the brain about the nature of the stimulus. At some point the brain must perform a comparison between the activity in several different nerve fibres in order to decide what the taste actually is.

The signals from the taste buds are relayed, via a chain of nerve cells and fibres, at various cell stations in the brainstem and thalamus, up to the cerebral cortex. Some experiments in monkeys suggest that nerve cells at higher levels in the taste pathway respond more selectively, with a larger proportion of them essentially responding to only one basic taste. At the first relay in the brainstem almost no neurons respond to one taste, yet in the taste area of the cortex, about 75% of neurons respond to a single taste.

The ‘common chemical sense’ is the sensation caused by the stimulation of free nerve endings by potentially harmful chemicals. The evidence suggests that the free nerve endings are ‘polymodal’ nociceptors (receptive nerve endings that respond to mechanical, thermal and noxious stimulation). Amongst the chemicals that are known to stimulate these receptors, besides noxious, damaging chemicals, are alcohol, menthol, peppermint and capsaicin (chilli pepper).

Olfactory receptors

The human olfactory organ, the olfactory epithelium or mucosa, is a sheet of cells 100 –200μm thick, situated high in the back of the nose cavity and on the thin bony partition (the central septum) of the nasal passage. The olfactory system responds to airborne, volatile molecules that gain access to the olfactory epithelium with the in-and-out airflow through and behind the nose. The odour molecules are distributed over the receptor sheet in an irregular pattern by the turbulence of the airflow set up by the turbinate bones in the side walls of the nose. The molecules diffuse through the surface layer of mucus and stimulate the olfactory receptors. Hydrophilic (water-soluble) molecules dissolve readily in the mucus, but the diffusion of less soluble molecules is assisted by ‘odour binding proteins’ in the mucus. These odour binding proteins are also thought to assist in removing odour molecules from the receptor cells. The mucus layer moves across the surface of the olfactory mucosa at 10 to 60 mm per minute toward the nasopharynx (the continuous of the nasal cavity backwards and downwards to link to the pharynx. This flow of mucus (which is increased and becomes more watery in such conditions as infection of the nasal cavity and hay fever), also assists in the removal of odours after they have been sensed.

The olfactory epithelium contains specialized, elongated nerve cells (olfactory receptors). These cells have very thin fibres that run upwards in bundles through perforations in the skull (the cribriform plate) above the roof of the nasal cavity, below the frontal lobes of the brain. These bundles of nerves constitute the Ist cranial nerve, the olfactory nerve. They extend only a very short distance, ending in the olfactory bulbs, which are a pair of swellings underneath the frontal lobes.

The other end of each olfactory receptor, pointing down into the nasal cavity, is extended into a long process, ending in a knob carrying several hairs (cilia) between 20 and 200 μm in length. These cilia are bathed in a thin (35 μm-thick) layer of mucus, secreted by specialized cells in the olfactory epithelium, in which the molecules of odorous substances dissolve. In the membrane of the cilia are olfactory receptor proteins, which interact with the smelly molecules, and initiate a cascade reaction inside the cell that leads to a change in the rate of impulses (action potentials) passing along the nerve fibre.

Human beings are able to distinguish 10 000 or more different odours. There have been valiant attempts to classify these into a smaller number (usually 10–20) basic or primary smells, comparable to the four or so primary tastes, but no scheme is universally accepted. The human nose (not to mention that of a sniffer dog) can be incredibly sensitive to very low concentrations of odorous substances. Certain male moths use similar receptor cells on their antennae to detect even single molecules of a pheromone secreted by female moths.

Individual olfactory receptor neurons fire off spontaneously at between 3 and 60 impulses per second. When stimulated with particular odours they increase their firing frequency. Each receptor cell responds, but not equally, to many different types of odour. As in the gustatory system, the successive nerve cells in the pathway become more selective, each responding to fewer odours. Interestingly, despite the poor selectivity of individual receptor cells, different regions of the olfactory sheet (consisting of hundred or thousands of receptor cells) are maximally responsive to particular odours. The overall pattern of activity in the olfactory epithelium can be mapped with electrical recording methods (electro-olfactogram) or other techniques for detecting active regions. Each distinctive odour produces its own ‘fingerprint’ of activity across the epithelium. This mapping is thought to reflect the patterns of expression (activation) of genes that make the receptor proteins in the receptor cell membranes. A huge family of odour receptor genes exists in the mouse, perhaps as many as 5% of all the genes.

The spatial coding of odour quality is transmitted to the first relay of the olfactory pathway, the olfactory bulb. There is a loose topographical projection from the receptor sheet to the bulb, where the axons form synapses with neurons called mitral cells. The olfactory bulb contains a complex network of nerve cells and is responsible for a considerable amount of sensory processing. Hence, neurons in the olfactory bulb respond with one distinct temporal pattern of impulses to one odour and different patterns to another smells. The mitral cells send their fibres into the olfactory tracts, which run backwards. Some end in the thalamus, which in turn sends fibres up to the olfactory cortex. The neurons of the olfactory cortex are still not highly specific for particular odours. Other fibres of the olfactory tract have direct connections to areas of the limbic system around the region of the hypothalamus. Since the limbic system is thought to be responsible for regulating emotions, this might explain the fact that smells can evoke strong feelings of enjoyment or aversion (the hedonic component of sensation).

Unlike other stimuli, olfactory stimuli are not very time-dependent. The effects of visual, tactile and auditory stimulation follow the stimulus immediately, whereas some olfactory stimuli, such as those left by animals when marking their territory, remain when the animal is long gone. In this way olfactory stimuli, and their behavioural and social effects, can have more lasting consequences.

The olfactory system occupies a smaller fraction of the brain in humans than in many other species, and this is part of the evidence for the commonly-held belief that people are generally inferior in their sense of smell. Studies in other animals, from insects to hamsters to monkeys, have revealed the importance of olfaction for many aspects of behaviour, especially reproduction. For example, male rhesus monkeys use smell to sense the hormonal status of females (ovulating or not), with a marked effect on their level of sexual activity. But even in humans, there is growing evidence that olfaction (mainly unconscious) is important in such functions as sexual preference, and recognition of other people.

R. W. A. Linden

See also brain; cerebral cortex; limbic system; nose; pheromones; sensation; sensory receptors; tongue.