The Chemical Senses of Taste and Smell

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13 The Chemical Senses of Taste and Smell

Chapter Outline

The Perception of Flavor Involves Gustatory, Olfactory, Trigeminal, and Other Inputs
Taste Is Mediated by Receptors in Taste Buds, Innervated by Cranial Nerves VII, IX, and X

Taste Receptor Cells Are Modified Epithelial Cells with Neuron-like Properties
Second-Order Gustatory Neurons Are Located in the Nucleus of the Solitary Tract
Information about Taste Is Coded, in Part, by the Pattern of Activity in Populations of Neurons
Olfaction Is Mediated by Receptors That Project Directly to the Telencephalon

Olfactory Receptor Neurons Utilize a Large Number of G Protein–Coupled Receptors to Detect a Wide Range of Odors
Olfactory Information Bypasses the Thalamus on Its Way to the Cerebral Cortex
Conductive and Sensorineural Problems Can Affect Olfactory Function
Multiple Flavor-Related Signals Converge in Orbital Cortex

The Perception of Flavor Involves Gustatory, Olfactory, Trigeminal, and Other Inputs

When we eat food or drink a beverage, we have a unified perception, centered on the tongue, of some mixture of flavors. However, this unified perception actually results from the CNS combining multiple kinds of information—somatosensory inputs reflecting things like temperature, texture, and fizziness, as well as inputs about the chemical makeup of the food from three different sources. Chemicals dissolved in the mouth stimulate taste buds, providing gustatory information. Simultaneously, vapors emanating from the food reach olfactory receptor cells through either the nostrils or the oropharynx (THB6 Figure 13-8, p. 330). Finally, some dissolved chemicals or vapors stimulate trigeminal and glossopharyngeal endings in the epithelial lining of the oral and nasal cavities, providing information about things like spiciness and pungency. (The latter common chemical sense persists even in the absence of taste buds and olfactory receptors.)

Taste Is Mediated by Receptors in Taste Buds, Innervated by Cranial Nerves VII, IX, and X

Key Concept

The tongue is covered by a series of papillae, some of which contain taste buds.

The surface of the tongue is covered by a series of bumps and folds (papillae), which are the homes of taste buds. Fungiform papillae scattered over the anterior tongue typically contain a few taste buds each. Foliate papillae, folds along the sides of the posterior tongue, contain dozens of taste buds each. Circumvallate papillae are arranged in a V-shaped row about two thirds of the way back on the tongue. They are few in number (8-9) but contain hundreds of taste buds each, accounting for about half of all the taste buds on an average tongue.

Circumvallate and most foliate papillae are innervated by the glossopharyngeal nerve (IX), fungiform and anterior foliate papillae by the facial nerve (VII). The vagus gets into the act by innervating the few taste buds farther back in the pharynx (probably more important for things like coughing when something nasty gets back there than for the perception of taste). This innervation is distinct from that taking care of touch, pain, and temperature in the mouth (Fig. 13-1).

image

Figure 13-1 Innervation patterns of taste buds and lingual epithelium.

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

Key Concept

Taste receptor cells utilize a variety of transduction mechanisms to detect sweet, salty, sour, and bitter stimuli.

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+

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