The Taste & Brain Philosophy

Taste is a sensory function of the central nervous system.  The receptor cells for taste in humans are found on the surface of the tongue, along the soft palate, and in the epithelium of the pharynx and epiglottis.

The Major Senses

The purpose of the major senses is to detect and discriminate among signals coming from our environment. These signals carry information necessary for us to support our vital functions, such as taste and smell in eating, as well as functions used in communicating with others and in our work, such as sight, touch, and hearing. In addition to the traditional five senses, other senses of which we are not aware are at work within our bodies, such as the sense of balance and the sense of muscle effort, called kinesthesia, and many senses involved in detecting chemical changes in the blood and other tissues.

All of these senses are present at birth in the human. Research on newborn babies has shown that when they are tested with different taste solutions before any exposure to feeding, they show the appropriate facial responses, such as smiling at a sweet taste and grimacing at a bitter taste. Since the higher brain centers, in the neocortex, of a newborn are not yet functional, these experiments have shown that our basic emotional expressions of pleasure and pain are hard-wired into our brain stem circuits from birth.
Each of the different senses has particular sense cells within its particular organs: for taste, taste cells within taste buds in the tongue and back of the mouth.

Our sense of taste is shown in this schematic drawing. It starts with specialized cells on the tongue that send information to the brain’s taste nuclei in the medulla, atop the spinal cord. From these sites, signals go to the amygdala and the thalamus, which in turn alert the portion of the cortex that processes and stores that information. Notice that our taste buds are not the bumps on our tongue but actually line tiny trenches in the surface of the tongue. (Image credit: Kathryn Born)

Exploring Our Senses:

We move through the rough-and-tumble physical world with such ease that it is astonishing to realize the exquisite refinement of each of our sensory systems. Several aspects of sensory systems have been especially studied. One of the most important we are trying to understand is the mechanisms by which the signals from the external world are converted into nerve signals. That is, how can a passing molecule of diesel fuel, for example, start the series of brain cell firings that result in our holding our nose? This process is called sensory transduction. One of the main principles emerging is that transduction begins with the sensory signal acting on a protein that sits in a sensitive part of the membrane of the sensory cell. For example, in the nose, it is a receptor protein that is concentrated in fine hairs that extend from the ends of the sensory cells situated in a patch at the top of the nasal cavity. Research has shown that the sensory protein in the nose belongs to the same family of molecules as rhodopsin. Each protein is adapted to receive its particular sensory signal. They are called G protein-coupled receptors, because a molecule called a G protein (for guanosine triphosphate) must be coupled to them to continue to transmit a signal. When light or an odour activates these receptors, they in turn activate their G proteins.

Researchers have found that activation of a G protein then leads to the production of a small messenger molecule (cyclic adenosine monophosphate [cAMP] or cyclic guanosine monophosphate [cGMP]) that acts on a membrane protein to set up an electrical response in the membrane. Cyclic AMP and cyclic GMP are widespread throughout the body. They are called second messengers because they take the response to the first messenger (the initial signal from outside the cell), amplify it within the cell, and direct their response to an appropriate site within the cell. In the case of sensory cells, this is the electrical response, which in turn generates a discharge of impulses that encodes the strength of the sensory stimulation. Most sensory cells are set at near their physical limits for detecting very weak signals; for instance, the inner ear is set to detect a movement of the tympanic membrane (eardrum) of the width of a hydrogen atom, and the eye is set to detect single photons from starlight on a dark night.

Each type of sensory information has its own area in the cerebral cortex for processing and storage. Vision and hearing take up the most space, smell and taste relatively little. (Image credit: Kathryn Born)

The Senses’ Specialties

It is important that sensory systems not only detect weak signals and determine the strength of a signal but also discriminate between different signals. In taste it involves distinguishing among the basic tastes of sweet, salt, sour, and bitter. Discrimination thus requires populations of sensory receptor cells that can respond to different aspects of the stimuli.

All sensory systems provide for such differently sensitive receptor cells, which give rise to parallel pathways that carry the information to the higher centers where discrimination takes place. These pathways are gathered into nerve tracts that ascend through the lower parts of the brain to the highest centers. Thus, the optic nerves carry information in a highly ordered manner from the retina to a way station called the lateral geniculate body. Smell information is carried in the olfactory nerves to the olfactory bulb, for processing and output to a first cortical station at the base of the brain, for output to the olfactory thalamic nucleus and further relay to the neocortical olfactory area. The taste nerves carry taste information from the tongue and oral cavity to brain stem nuclei for relay to the thalamus andon to the neocortical taste area.

Sensory discrimination generally involves conscious sensory perception. This usually takes place in higher sensory centers within the brain, at the level of the neocortex. The ways in which cortical neurons are able to sort out signals they receive allow the conscious individual to recognize differences in how strong a signal is, how one form of taste, visual, or sensory information varies from another taste, image, or touch. (These differentiations are known as discrimination.) Such physiological processes underlie the larger brain functions: perception, consciousness, memory, and other higher functions.

Differences

The basic functions of high sensitivity for the detection of weak signals, discrimination of increasing stimulus strength, and discrimination between different qualities of a stimulus are present in all humans. However, there can be significant differences. First of all, there are differences during early life. Although the basic sensitivity of the sensory cells appears to be laid down early, it takes time for the central pathways to mature, and the highest centers mature last. Thus the highest levels of sensory perception are generally not reached until the teens and twenties, having been refined by experience, training, and memory.

As we grow older, there are also differences. Hearing begins to fall off during the 40s and 50s, with loss of the highest frequencies first. Smell holds relatively constant until the 60s and then begins a slow decline, which also appears to be true of the sense of taste. Whether the loss is due to damage to the sensory cells or to changes in higher centers is not known.

Our sense of smell differs from other senses, in part because olfactory information passes from the receptors, located in the upper recesses of the nose, to cortical regions without relaying through the thalamus. Some of these cortical regions, however, do connect through the thalamus to the orbitofrontal cortex, a region involved in odour identification. (Illustration by Kathryn Born)

Abnormalities in Taste Perception

Abnormalities in taste perception are common, especially for those who are receiving concomitant radiation therapy to the neck and mouth area. "Taste blindness," or an altered sense of taste, is a temporary condition that occurs because of effects on taste buds that are mostly located in the tongue.

A person's ability to taste and smell may be explained in the simple equation bellow:

The ability to taste = (and is influenced by) one’s ability to smell an aroma

A loss of taste perception makes it more difficult to eat. After a while of not being able to taste or smell foods, your appetite may dwindle, you may feel unenthusiastic about eating which leads to weight loss.

The causes of appetite and taste loss are many. Chemotherapy drugs are known to alter taste and smell by blunting the normal turnover rate of taste and smell receptors on the tongue and in the nasal passages.

In a study of 33 lung cancer patients undergoing chemotherapy, by Duke University Research - a leading center for research in the philosophy of biology, assessed the patients' own perceptions of their taste and smell deficits, then scored their ability to detect and recognise odours and flavours presented to them in a laboratory. Patients who reported the lowest degree of taste and smell ability, and who scored the lowest on the psychophysical measurements, also experienced the most weight loss, body-mass loss and nutritional deficits.

Smell or taste dysfunction can have a significant impact on daily life. Deficits of these senses can adversely affect food choice and intake, and has been implicated in weight loss.

Olfactory Anatomy & Philosophy

FIGURE 2B. Simplified diagram of cortical regions thought to be involved in the processing of olfactory information as it passes from the olfactory epithelium to the brain.

Smell receptors are located within the olfactory neuroepithelium, a region of tissue found over the cribiform plate, the superior septum and a segment of the superior turbinate. The free nerve endings of cranial nerve V are located diffusely throughout the nasal respiratory epithelium, including regions of the olfactory neuroepithelium. It is important to remember the distinctive nature of these two neural systems, because some odorants (e.g., ammonia) are sensed largely by the trigeminal nerve.

Once odorants enter the nose, they must move to the nasal vault and dissolve within the covering mucous layer in order to stimulate the olfactory receptors. Mucous has an important role in dispersing scents to the underlying receptors. The nasal turbinates are also important because they provide moderate resistance and a moist environment, thereby allowing optimal stimulation of olfactory neurons by airborne compounds.

In 2004 the Nobel Prize in Physiology or Medicine was jointly awarded to Richard Axel and Linda Buck, from the Howard Hughes Medical Institute, USA. They were awarded for their discoveries of odarant receptors and the organisation of the olfactory system.

“Most odours are composed of multiple odorant molecules, and each odorant molecule activates several odorant receptors. This leads to a combinatorial code forming an "odorant pattern" – somewhat like the colours in a patchwork quilt or in a mosaic. This is the basis for our ability to recognize and form memories of approximately 10,000 different odours.”

Taste Anatomy and Physiology

Many nerves are responsible for transmitting taste information to the brain (Figure 3). Because of these multiple pathways, total loss of taste (ageusia) is rare. As in the olfactory system, somatosensory sensations (e.g., stinging, burning, cooling and sharpness) can be induced by many foods (e.g., hot peppers) through trigeminal nerve fibers in the tongue and oral cavity.

Taste receptors are found within taste buds located not only on the tongue but also on the soft palate, pharynx, larynx, epiglottis, uvula and first one third of the oesophagus. Taste buds are continually bathed in secretions from the salivary glands, and excessive dryness can distort taste perception.

FIGURE 3. Anatomy of peripheral taste pathways. Multiple nerves, including cranial nerves VII, IX and X, transmit taste information from the mouth and pharynx to the brain via the brain stem.

How the Taste Bud Translates Between Tongue and Brain

A recent article in the New York Times recently reported that:

Contrary to long-held beliefs, new studies reveal taste buds to be far more than simple conduits that immediately pass on information about sweet, sour, salty and bitter substances to the brain to tell you what you are eating and help you decide whether you want more. Rather, the research has shown that cells in the taste buds communicate with each other, actively accepting, rejecting and modifying taste stimuli through a complicated network of chemical and electrical signals before sending signals to the brain.

As scientists scramble to decipher those signals, they are finding that taste stimuli can affect the taste-bud cells in unexpected ways. The stimuli sometimes exert their effect by interacting with messenger molecules in the taste buds, and they sometimes directly stimulate tiny electrical currents within the cells.

For example, researchers at the Roche Institute of Molecular Biology in Nutley, N.J., recently published their identification of an important protein messenger in taste buds, gustducin, that is activated in response to all sweet and some bitter taste stimuli. Dr. Robert F. Margolskee and his colleagues at the institute said gustducin's role in taste buds was comparable to that of protein receptors called transducins in the eye. Transducins, which are far better studied messenger chemicals, help to translate the light that reaches the retina into messages to be sent to the brain. Gustducin, which is found only in taste buds, acts as an intermediary between the receptor molecule for sweet stimuli and a chain of subsequent steps, finally sending a message to the brain that something sweet has been tasted.

The focus of all this research, the taste cell, is really a modified skin cell inside the mouth. Tens of thousands of taste cells reside in the mouth, but most are concentrated on the tongue in clusters of about 100 cells each that make up the taste buds. At the center of each taste bud is a pore; tasty chemicals fall into the pore, stimulating the taste cells to start their sensory analysis.

There are two kinds of taste cells, receptor cells and basal cells. At the surface of the tongue, each taste receptor cell has a chemically sensitive tip that responds to taste stimuli in the mouth. These cells may "talk" back and forth in chemical-ese before passing their environmentally derived message on to the basal cells at the bottom of the taste bud, which may also talk back and forth with the receptor cells. Once the final message has been ironed out, the basal cells convey it to sensory nerve endings that carry the signal, which eventually is interpreted by the brain.

The taste buds, in turn, are grouped in pink mushroom-shaped bumps called papillae, which can be seen by the naked eye on the surface of the tongue. While most papillae house only two to five taste buds, others in different parts of the tongue contain up to 250. This fact has upheld the simplistic belief that different tastes are perceived solely or most intensely in certain regions of the tongue, such as sweet tastes in the front and bitter in the back.

Much has been learned in the past four or five years about the workings of taste cells through research sponsored largely by the National Institutes of Health and, to some extent, by food and fragrance manufacturers. The cells appear to act as tiny microprocessors.

Until the recent studies, most scientists believed that each taste cell had highly specific receptor molecules on its surface that were sensitive to particular taste stimuli. The stimuli would, in effect, utter a chemical password to enter the cell and the message, as Dr. Roper put it, "got magically transmitted to the brain."

The new studies show that for most taste stimuli, other mechanisms are at work. The chemicals that transmit sensations of salty, sour and bitter tastes affect the workings of so-called ion channels, which normally let sodium into cells and potassium out. Because those ions are electrically charged, the change in their movements sets up a small electrical current in the taste cell. That, in turn, activates a synapse, a communication link between cells.

Many nerves are responsible for transmitting taste information to the brain.

Synapses enable taste cells to communicate with their like-minded neighbours as well as with sensory nerve fibers. A host of messenger chemicals are involved in the transmission of taste signals, but the roles played by the various substances are not yet understood.

Dr. Roper suggested that a substance called serotonin may play a role in appetite and weight regulation at the level of the taste buds, as serotonin is known to do in the brain.

Visible bumps called papillas contain multiple taste buds; in each bud, taste cells surround a pore. The taste receptors Receptor taste cells have chemically sensitive tips; basal cells interact with them and work out a message. For example, some tastes affect the workings of ion channels. Ion movements change the cells' electrical charge, triggering chemical messengers to the brain. Recognition in the brain the end result of taste is a series of tiny electric signals to taste centers in the brain.

The major communication center for brain-body interaction, the hypothalamus works with the pituitary gland to regulate your body’s hormones.

Scientific Papers

What the Tongue tells the Brain about Taste

Understanding the Mechanism of Taste Through Plants - Gymnema Sylvestre

The Health Benefits of Cloves

The Amazing Gymnema Sylvestre