Two distinct neural systems are used to sense the molecular environment of the world around us: the gustatory system, which mediates taste, and the olfactory system, which serves smell. These systems are among the phylogenetically oldest neural systems of the brain. Compared with those of the other sensory systems, the neural systems for processing chemical stimuli are remarkably different. For example, both taste and smell have ipsilateral ascending projections, whereas those of the other sensory systems are either contralateral or bilateral. Moreover, the primary cortical areas for taste and smell are within limbic system regions, where emotions and their associated behaviors are formed. Information from the other sensory modalities reaches the limbic system only after additional processing stages. Smells and tastes have a particular knack for evocative recall of our dearest memories. Recall Marcel Proust’s vivid description of how a spoonful of tea-soaked madeleine brought back childhood memories.

The gustatory and olfactory systems work jointly in perceiving chemicals in the oral and nasal cavities, a more essential collaboration than that which occurs between the other sensory modalities. For example, even though the gustatory system is concerned with the primary taste sensations—such as sweet or sour—the perception of richer and more complex flavors such as those present in wine or chocolate is dependent on a properly functioning sense of smell. Chewing and swallowing cause chemicals to be released from food that waft into the nasal cavity from the orapharynx, where they stimulate the olfactory system. Damage to the olfactory system, as a result of head trauma—or even the common cold, which temporarily impairs conduction of airborne molecules in the nasal passages—can dull the perception of flavor even though basic taste sensations are preserved. Although taste and smell work together and share similarities in their neural substrates, the anatomical organization of these systems is sufficiently different to be considered separately.

There are classically four taste qualities—sweet, sour, bitter, and salty—and there are corresponding taste receptor cells for each of these modalities. A fifth quality has been proposed, termed savory, which is best associated with a meaty broth because a fifth class of taste receptor cell has been identified, umami (Japanese, flavor). Whereas we may think our gustatory system’s primary function is to identify foods, this is more a role of sights and smells. Rather, the system is exquisitely organized to identify nutrients or harmful agents in what we ingest, in relation to particular physiological processes: sweet and savory are key to maintaining proper energy stores, salty for electrolyte balance, bitter and sour for maintaining pH, and bitter also for avoiding toxins.

Taste is mediated by three cranial nerves, through their innervation of oral structures: facial (VII), glossopharyngeal (IX), and vagus (X). As discussed in Chapter 6, the glossopharyngeal and vagus nerves also provide much of the afferent innervation of the gut, cardiovascular system, and lungs. This visceral afferent innervation provides the central nervous system with information about the internal state of the body.

The Ascending Gustatory Pathway Projects to the Ipsilateral Insular Cortex

Taste receptor cells are clustered in the taste buds, located on the tongue and at various intraoral sites. Chemicals from food, termed tastants, either bind to surface membrane receptors or pass directly through membrane channels, depending on the particular chemical, to activate taste cells. Taste cells are innervated by the distal branches of the primary afferent fibers in the facial, glossopharyngeal, and vagus nerves (Figure 9–2). These afferent fibers have a pseudounipolar morphology, similar to that of the dorsal root ganglion neurons. In contrast to the nerves of the skin and mucous membranes, where generally the terminal portion of the afferent fiber is sensitive to stimulus energy, taste receptor cells are separate from the primary afferent fibers. For taste, the role of the primary afferent fiber is to receive information from particular classes of taste receptor cells and to transmit this sensory information to the central nervous system, encoded as action potentials. For touch, the role of the primary afferent fiber is both to transduce stimulus energy into action potentials and to transmit this information to the central nervous system.

The central branches of the afferent fibers, after entering the brain stem, collect into the solitary tract (Figure 9–2) of the dorsal medulla and terminate in the rostral portion of the solitary nucleus (Figure 9–2, lower inset). Recall that the caudal solitary nucleus is a viscerosensory nucleus, critically involved in regulating body functions and transmitting information to the cortex for perception of visceral information as well as the emotional and behavioral aspects of visceral sensations.

The axons of second-order neurons in the rostral solitary nucleus ascend ipsilaterally in the brain stem, in the central tegmental tract, and terminate in the parvocellular division of the ventral posterior medial nucleus (Figure 9–2). From the thalamus, third-order neurons project to the insular cortex and the nearby operculum, where the primary gustatory cortical areas are located (Figure 9–2). This pathway is thought to mediate the discriminative aspects of taste, which enable us to distinguish one quality from another. The insular cortex projects to several brain structures for further processing of taste stimuli. Projections to the orbitofrontal cortex (see Figure 9–11), as well as the insular and cingulate cortical areas, are thought to be integrated with olfactory information, for the awareness of flavors. In addition, these cortical areas may be important for the behavioral and affective significance of tastes, such as the pleasure experienced with a fine meal or the dissatisfaction after one poorly prepared. A component of the processing of painful stimuli also involves the limbic system cortex, and pain in humans is not without emotional significance.

Although taste and visceral afferent information (see Chapter 6) are distinct modalities and have separate central pathways, the two modalities interact. In fact, linking information about the taste of a food and its effect on body functions upon ingestion is key to an individual’s survival. One of the most robust forms of learning, called conditioned taste aversion, associates the taste of spoiled food with the nausea that it causes when eaten. Another name for this learning is bait shyness, referring to a method used by ranchers to discourage predators from attacking their livestock. In this technique, ranchers contaminate livestock meat with an emetic, such as lithium chloride, which causes nausea and vomiting after ingestion. After eating the bait, the predator develops an aversion for the contaminated meat and will not attack the livestock. People can experience a phenomenon related to conditioned taste aversion, in which they develop an intense aversion to food they ate before becoming nauseated and vomiting, even if the food was not spoiled and the illness resulted from a viral infection. Experimental studies in rats have shown that such interactions between the gustatory and viscerosensory systems, leading to conditioned taste aversion, may occur in the insular cortex.

Branches of the Facial, Glossopharyngeal, and Vagus Nerves Innervate Different Parts of the Oral Cavity

Taste receptor cells are epithelial cells that transduce soluble chemical stimuli within the oral cavity into neural signals. They are present in complex microscopic sensory organs, called taste buds (Figure 9–3A). Taste receptor cells are short lived; they are regenerated approximately every 10 days. Taste receptor cells are responsive to a single taste quality. In addition to the taste receptor cells, taste buds contain two additional types of cells: basal cells, which are thought to be stem cells that differentiate to become receptor cells, and supporting cells, which provide structural and possibly trophic support (Figure 9–3A). Taste receptor cells have a synaptic contact with the distal processes of primary afferent fibers. A single afferent fiber terminal branches many times, both within a single taste bud and between different taste buds, so that it forms synapses with many taste cells. However, each sensory fiber will contact taste receptor cells that are responsive to a single taste modality.

Taste buds are present on the tongue, soft palate, epiglottis, pharynx, and larynx. Taste buds on the tongue are clustered on papillae (Figure 9–3B), whereas those at the other sites are located in pseudostratified columnar epithelium or stratified squamous epithelium rather than distinct papillae. Taste receptor cells that are located on the anterior two thirds of the tongue are innervated by the chorda tympani nerve, a branch of the facial (VII) nerve. (The facial nerve consists of two separate roots [Figure 9–4], a motor root commonly known as the facial nerve and a combined sensory and autonomic root called the intermediate nerve.)

Taste buds on the posterior third of the tongue, which are located primarily in the circumvallate and foliate papillae (Figure 9–3B), are innervated by the glossopharyngeal (IX) nerve (Figure 9–4). Taste buds on the palate are innervated by a branch of the intermediate nerve. Taste buds on the epiglottis and larynx are innervated by the vagus (X) nerve, whereas those on the pharynx are innervated by the glossopharyngeal nerve. The familiar taste map of the tongue—showing that sweet and salty are sensed in the front of the tongue, sour laterally, and bitter at the back of the tongue—is wrong. Taste buds in all regions are sensitive to the five basic taste attributes.

The cell bodies of the afferent fibers innervating taste cells are located in peripheral sensory ganglia. The cell bodies of afferent fibers in the intermediate branch of the facial nerve are found in the geniculate ganglion. Those of the vagus and glossopharyngeal nerves are located in their respective inferior ganglia. As discussed in Chapter 6, the glossopharyngeal and vagus nerves also contain afferent fibers that innervate cranial skin and mucous membranes; the cell bodies of these afferent fibers are found in the superior ganglia. The afferent fibers of the intermediate branch of the facial nerve enter the brain stem at the pontomedullary junction, immediately lateral to the root that contains somatic motor axons (Figure 9–4). The taste fibers of the glossopharyngeal and vagus nerves enter the brain stem in the rostral medulla.

The Solitary Nucleus Is the First Central Nervous System Relay for Taste

Gustatory fibers innervating the taste buds enter the brain stem and collect in the solitary tract, located in the dorsal medulla. The axons of the facial nerve enter the tract rostral to those of the glossopharyngeal and vagus nerves. After entering, however, the fibers send branches that ascend and descend within the tract, similar to the terminals of afferent fibers in Lissauer’s tract of the spinal cord. The axon terminals leave the tract and synapse on neurons in the surrounding rostral solitary nucleus where second-order neurons (Figures 9–2, 9–5A, and 9–6B) project their axons into the ipsilateral central tegmental tract (Figures 9–5A and 9–6A) and ascend to the thalamus. The trigeminal and medial lemnisci, which carry the ascending somatic sensory projection from the main trigeminal and dorsal column nuclei, are ventral to the central tegmental tract (Figure 9–6A). Recall that the caudal solitary nucleus is important for visceral sensory function. It has a projection to the parabrachial nucleus, a pontine nucleus that is critical for relaying interoceptive information to the hypothalamus and amygdala for controlling various bodily functions, such as autonomic nervous system regulation. Brain stem centers that respond to tastants can be imaged using fMRI. The active region is the rostral pons, where the rostral solitary nucleus is located.

The Parvocellular Portion of the Ventral Posterior Medial Nucleus Relays Gustatory Information to the Insular Cortex and Operculum

Similar to somatic sensations, vision, and hearing, a thalamic relay nucleus receives taste information and projects this information to a circumscribed area of the cerebral cortex. The ascending projection from the rostral solitary nucleus terminates in the parvocellular division of the ventral posterior medial nucleus. This nucleus has a characteristic pale appearance on myelin-stained sections (Figure 9–7A). The axons of thalamocortical projection neurons in the thalamic gustatory nucleus project into the posterior limb of the internal capsule (Figure 9–7A, B) and ascend to the insular cortex and the nearby operculum (Figure 9–8B, C). These are the locations of the primary gustatory cortex. A PET scan of the human brain when sucrose is used as a tastant (Figure 9–8D) reveals activation in the insular and opercular regions. Different nuclei in the ventromedial thalamus receive different inputs and project to different cortical areas. Viscerosensory inputs are processed in adjacent but slightly separated thalamic regions and project to adjoining areas of the insular cortex. Touch and pain also engage different thalamic nuclei and nearby cortical areas in the postcentral gyrus and parietal operculum.