Sense of Taste - Introduction

Tutorial 29: Sense of Taste

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

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The sensation commonly called taste is really a combination of both smell and taste. The appropriate term to use for the combined experience of smell and taste as experienced while eating and drinking is flavor. The term "taste" should be restricted to the perceived outcome of stimulating the receptor cells of taste buds located on the surface of the tongue. This sense of taste, or gustation, is similar to the sense of smell (olfaction) in that stimuli interact chemically with the receptors during the encoding process. Tasted substances must dissolve in saliva before they can interact with the taste receptors. Saliva then holds the dissolved chemicals close to the collections of receptor cells, called taste buds. The performance of taste buds deteriorates rapidly in the absence of saliva. The chemical taste receptors are located buried within the "bumps" (called papillae) that cover the surface of the tongue. In addition to increasing the surface area for taste buds, the papillae create an abrasive surface that helps to hold food within the mouth.

Four qualities of taste that have emerged from the research on this system are sweetness, saltiness, bitterness, and sourness. Most vertebrates are sensitive to these four taste qualities, but some species differences in sensitivity do exist. In addition, taste preferences may differ within and across species. For example, various members of the feline family from domestic cats to lions and tigers do not perceive sweetness. Humans and rodents like both sucrose and saccharin, but dogs reject saccharin. Humans differ both in their sensitivity to the taste qualities and in their taste preferences. Taste buds are lost with advancing age, and therefore taste thresholds increase with age. Children, with their highly sensitive sense of taste, are often intolerant of spicy foods. There are also differences in taste preference across adults of similar age. Some find particular tastes offensive, while others do not. It is commonly thought that the four taste qualities are encoded by four unique receptors, and that the relative activity in these receptors results in the ultimate perception of taste. The evidence in favor of this hypothesis is, however, uncertain. A description of recent findings concerning the mechanisms underlying taste perception is provided in the advanced section.

Gustation is needed to monitor the substances gaining entry into the digestive system, an ability that can be essential to survival. Sweetness is clearly linked with the experience of eating food and gaining nutritive sources of energy. Saltiness and bitterness, however, may be linked to the detection of substances of life sustaining and life threatening qualities, respectively. Wounds that cause bleeding may deplete "salty" sodium reserves at a rapid rate. Because this ion is essential to sustain life, it is important to be able to detect ready sources of replacement. On the other hand, substances that taste sour or bitter are most often harmful. For example, the presence of bacteria in food increases the level of acidity and makes it taste sour. Poisonous alkaloids produced by plants taste very bitter. The detection of bitter and sour taste qualities, therefore, serves as a warning against danger. The loss of taste, or ageusia, is a very rare condition. Probably because, as noted below, there are three separate cranial nerve pathways that convey taste information.

Information from the taste buds is conducted to the brain via three of the cranial nerves, the facial (CN VII), glossopharyngeal (CN IX), and the vagus (CN X). The axons of the taste buds synapse in the gustatory (solitary) nuclei of the medulla, then in the ventral posterior nucleus of the thalamus, before the final synapses in primary and secondary gustatory cortex. The majority of taste information is conveyed ipsilaterally, a somewhat unique situation among the sensory systems. Tutorial 29 illustrates and discusses the structures located along the gustatory pathway and the functions of these structures.

Advanced

A recent study provides the psychophysical evidence to support the sensory integration of olfaction and taste in the perception of flavor (Dalton, et. al., 2000). In this study, the researchers demonstrated the enhanced ability to detect the unique smell of cherries and almonds (as conveyed in the chemical benzaldehyde) when accompanied with saccharin, which tastes sweet but has no odor. Thus, the presence of saccharin lowered the detection threshold for benzaldehyde. No similar effect of enhanced detection for benzaldehyde was found with the simultaneous presentation of other tastes, such as distilled water, monosodium glutamete (MSG), or salty. The researchers speculate that the amygdala plays a role in the enhanced detection of flavors that occurs with the pairing of particular tastes and smells. They also suggest that the cultural variation of culinary taste preferences around the world may be modulated by the acquisition of "good" tastes via this learned association of sensations.

The field of gustatory neurobiology has directed considerable effort toward understanding the way taste information is encoded by neural signals (Smith & St John, 1999). Two possible explanations offered are the 'labeled lines' hypothesis and the neuronal pattern hypothesis. The 'labeled lines' hypothesis states that taste receptors are specifically tuned for one taste quality. The neuronal pattern hypotheses states that taste receptors are broadly tuned to respond to different taste qualities, and that specific taste qualities are coded by the pattern of activity across neurons. At this point, evidence suggests that a little of both mechanisms may be at work in the gustatory system. In addition to responding to taste qualities, taste neurons are responsive to stimulus intensity, and often touch and temperature.

The activity of taste neurons is also modulated along the gustatory tract by outside factors. For example, modulators of gustation that inhibit the gustatory nuclei include distention of the stomach, changes in blood levels of insulin and glucose, and aversions to particular tastes. The neuronal responsiveness of gustatory cortical areas appears to be linked more to the "hedonic" (i.e., pleasurable) value of a taste stimulus than to the particular taste quality. These cortical 'hedonic-type' neurons tend to respond to preferred taste stimuli (such as sweet and slightly salty tastes), but not to taste stimuli that are typically avoided (such as bitter, sour, and very salty tastes).

There may be two separate gustatory pathways. The first, lemniscal pathway ascending from the thalamus to gustatory cortex, may be specialized for the identification and discrimination of taste stimuli. Whereas the second ventral forebrain pathway involving projections from the gustatory nuclei of the medulla to the amygdala, hypothalamus, and other limbic structures, may be specialized for the affective and motivational responses made to taste stimuli.

References

Dalton, P., Doolittle, N., Nagata, H. & Breslin, P.A.S., (2000). The merging of the senses: integration of subthreshold taste and smell. Nature Neuroscience, 3, 431-432.

Smith, D.V. & St John, S.J., (1999). Neural coding of gustatory information. Current Opinion in Neurobiology, 9(4), 427-435.

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Tutorial 29: Sense of Taste

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