Taste Receptor Cells Are Modified Epithelial Cells with Neuron-like Properties

Each taste bud (THB6 Figure 13-2, p. 325) is an encapsulated collection of taste receptor cells, supporting cells, and stem cells that give rise to new receptors (taste receptor cells only live for a week or two). An opening at the lingual surface of each bud lets dissolved chemicals in to contact the apical ends of the taste receptor cells.

Taste receptor cells, unlike most other receptors, are not neurons but rather are modified epithelial cells. Nevertheless, they have some very neuron-like properties: they make depolarizing receptor potentials (and many even make action potentials), which in turn increase the release of neurotransmitter (in this case, the principal transmitter is ATP) onto the peripheral processes of cranial nerve fibers (Fig. 13-2). Some taste receptor cells release ATP at typical chemical synapses. Others use an unusual Ca²⁺-independent mechanism in which big voltage-gated channels open and intracellular ATP just spills out into extracellular space.

Figure 13-2 Transmission from taste receptor cells to peripheral endings of cranial nerves VII, IX, and X.

Also unlike other receptors, taste receptor cells collectively use several different transduction mechanisms. These mechanisms range from very simple ones in which the Na⁺ ions in salty foods enter the cell directly through cation channels, to more complex ones in which sweet or bitter substances initiate G protein–coupled processes.

Second-Order Gustatory Neurons Are Located in the Nucleus of the Solitary Tract

Afferents that innervate taste buds reach the brainstem with the facial nerve (from the anterior two thirds of the tongue), the glossopharyngeal nerve (from the posterior third of the tongue), and the vagus nerve (from the epiglottis and esophagus). Like other visceral afferents (see Fig. 12-10) they too travel within the brainstem in the solitary tract and end in the nucleus of the solitary tract, mostly in more rostral portions (Fig. 13-3).

Figure 13-3 CNS taste connections. (Further connections with the hypothalamus and amygdala not indicated.)

Gustatory neurons in the nucleus of the solitary tract participate in feeding (e.g., salivating, swallowing) and protective activities (e.g., coughing). They also convey information to the thalamus and from there to gustatory cortex in the insula (conscious awareness of taste) and indirectly to the hypothalamus and amygdala (pleasantness or unpleasantness of taste). Unlike other ascending sensory pathways to the thalamus, the gustatory pathway is uncrossed.

Information about Taste Is Coded, in Part, by the Pattern of Activity in Populations of Neurons

We usually think of sensory receptors as being good at signaling the location of a stimulus, but this doesn’t work for taste buds. When we eat or drink, dissolved chemicals and vapors get distributed widely in the mouth and nose, so the stimulated taste buds and olfactory receptors can’t provide accurate information about the location of the stimulus; we use somatosensory cues instead to decide this. The major task of the gustatory and olfactory systems is to analyze the composition of mixtures of potentially thousands of chemicals. In principle, we could do this by having one kind of taste receptor specifically tuned to the taste of radishes, another to rutabagas, etc., but this would require an unwieldy number of receptor types. So instead we compare the levels of activity in populations of receptors, each population responsive to more than one kind of chemical. This is a lot like comparing the levels of activity in just three kinds of cones (see Chapter 17) to identify hundreds of colors.

Olfactory Receptor Neurons Utilize a Large Number of G Protein–Coupled Receptors to Detect a Wide Range of Odors

The olfactory system, like the gustatory system, compares the levels of activity in populations of receptors, each population responsive to more than one kind of chemical. We can discriminate many more kinds of odors than tastes, and corresponding to this there are numerous kinds of olfactory receptors. However, all of them use closely related receptor proteins and the same G protein–coupled transduction mechanism (THB6 Figure 13-12, p. 334).

Olfactory Information Bypasses the Thalamus on Its Way to the Cerebral Cortex

The olfactory system continues to break the rules by projecting to cerebral cortex without first relaying in the thalamus (Fig. 13-5). The fibers of the olfactory tract, which arises in the olfactory bulb, end in anterior temporal cortex (piriform cortex and nearby areas), as well as in the amygdala and in areas at the base of the brain (anterior perforated substance). Piriform and nearby cortex, however, is not neocortex like the cortex that covers most of the cerebral hemispheres (see Chapter 22); it has a simpler structure and is referred to as paleocortex. There is an additional olfactory area in neocortex, in the orbital cortex of the frontal lobe, where olfactory information converges with projections from gustatory cortex. In this case the rules are followed more closely: Information from piriform cortex reaches the orbital olfactory/gustatory area via a relay in the dorsomedial nucleus of the thalamus (as well as via direct projections). As in the case of gustatory pathways, all of these olfactory connections are mostly uncrossed.

Figure 13-5 Olfactory CNS connections.

Conductive and Sensorineural Problems Can Affect Olfactory Function

Olfaction can be impaired both by processes that prevent things from reaching olfactory receptors (conductive deficit) and by damage in the nervous system (sensorineural deficit). We have probably all experienced conductive deficits during the stuffiness of colds or allergic conditions. Sensorineural deficits are a common result of head trauma, for example because of a shear injury to olfactory nerve fibers as they pass through the ethmoid bone. Some degenerative conditions, notably Parkinson’s and Alzheimer’s diseases, can also affect the olfactory system. A selective sensorineural olfactory deficit does not involve the common chemical sense, so sensitivity to things like menthol and ammonia may be unaffected. Hence, it is important to use things like coffee, vanilla, or floral extracts to test olfaction.