The sense of taste. Gustation is chemically mediated, receptors on the tongue detecting each of four prototypical tastes: sweet, sour, bitter and salt. (MONOSODIUM GLUTAMATE, used to enhance flavour, may be a primary taste called UMAMI.) Taste perception begins at TASTE BUDS on the tongue. Taste buds are responsive to specific primary tastes and are differently distributed across the tongue. The tip of the tongue is most sensitive to sweet and salt, the sides to sour, and the back to bitter. Individual taste buds possess 20–50 RECEPTOR cells clustered together and, in order to be tasted, a molecule must be water-soluble and able to bind to one of these receptors. Different prototypical tastes have different actions at receptors: salts operate most simply, sodium ions entering neurons to generate ACTION POTENTIAL. Sourness is detected by hydrogen ions binding to and blocking potassium channels (though the sourness of acids is not only a function of hydrogen concentration). Sweet substances—typically but not only sugars—bind to receptors and activate SECOND MESSENGERS, principally CYCLIC AMP. Bitter substances—quinine for instance—activate INOSITOL TRIPHOSPHATE second messenger systems.
The dendrites of sensory neurons contact taste buds so that information can be sent to the CNS for processing. The action of molecules at receptor sites on the taste buds stimulates activity in these sensory neurons. Several nerves carry information away from the tongue to the CNS: the CHORDA TYMPANI (part of the FACIAL NERVE) carries information from the front of the tongue; the lingual branch of the GLOSSOPHARYNGEAL NERVE carries information from the back; and the VAGUS NERVE carries taste information from the palate and epiglottis. These nerves converge on the NUCLEUS OF THE SOLITARY TRACT from where information is relayed to the THALAMUS (especially the ventral posteromedial nucleus) directly and via the PARABRACHIAL NUCLEI. The thalamus, which is the major relay station for cortical input, sends axons to the primary GUSTATORY CORTEX located in the frontal insular and opercular cortices. This is the principal taste pathway, but fibres conducting gustatory information also reach the AMYGDALA and LATERAL HYPOTHALAMUS. Given that the amygdala is concerned with the ascription of motivational significance to stimuli (that is, deciding whether or not a stimulus is pleasant or aversive) and the lateral hypothalamus performs complex computations involving gustatory and visceral sensations, the representation of gustatory information in these places is not surprising. Perhaps the most curious aspect of the taste pathway however is that taste is not represented bilaterally in the cortex, unlike the other senses.
While taste is carried within these pathways, the nature of the information changes as progress is made through the system. Within the nerves bringing information from the tongue, and in the nucleus of the solitary tract, neurons will respond to any of the prototypical tastes without discriminating between them. As one moves up the system through the thalamus to the cortex, gustatory neurons become increasingly finely tuned: that is, they begin to show selective responding to specific prototypical tastes until, in the gustatory cortex, neurons respond selectively to one prototypical taste only. The perception of complex tastes emerges from an assembly of neurons: it is a function of distributed processing rather than the property of single neurons.
In humans taste has been brought to a state of great refinement. Lovers of fine food and wine for instance are able to discriminate great subtleties of taste. Two points need to be made about this. First, it is clear that taste operates in close conjunction with other senses, particularly olfaction. Vision also has a significant part to play. So, for instance, if one is blind-folded and deprived of olfactory information (by pegging one’s nose) it is impossible to discriminate between apples and onions. All of the initial, pre-digestive reactions to food and drink (the CEPHALIC REFLEXES OF DIGESTION) are triggered by receptors in the head and mouth: salivation, GASTRIC ACID and INSULIN secretion are initiated before food has passed into the gullet. It is the sight, smell and taste of food which initiates this cascade. The second point is that taste has a long evolutionary history and an important practical purpose. To survive, animals must ingest parts of the environment, but some things ingested will be wholly or partly poisonous. Sweet and salty substances are in general good, but sourness—acidity—is associated with food which has gone bad, and bitterness is associated with toxins. Very simple creatures evolved mechanisms of EMESIS (NAUSEA and VOMITING) to deal with this. Moving up the PHYLOGENETIC SCALE however one finds that animals possess ever more complex systems to determine what is toxic and what is not. This is the origin of the taste system.
