The Ear: Hearing 353 Given Bud, Tight Junctions Link the Apical Ends of Adjacent Cells G Protein-Coupled Receptors

The Ear: Hearing 353 given bud, tight junctions link the apical ends of adjacent cells G protein-coupled receptors. In the current model for salty together, limiting movement of molecules between the cells. tastes, Na + enters the presynaptic cell through an apical The apical membrane of a taste cell is modified into microvilli channel and depolarizes the taste cell, resulting in exocytosis to increase the amount of surface area in contact with the en of the neurotransmitter serotonin. Serotonin in turn excites vironment (Fig. 10-16c). the primary gustatory neuron. For a substance (tastant) to be tasted, it must first dissolve Transduction mechanisms for sour tastes are more contro in the saliva and mucus of the mouth. Dissolved taste ligands versial, complicated by the fact that increasing H+, the sour then interact with an apical membrane protein (receptor or taste signal, also changes pH. There is evidence that H+ acts on channel) on a taste cell (Fig. 10-16c). Although the details of ion channels from both extracellular and intracellular sides of signal transduction for the five taste sensations are still contro the membrane, and the transduction mechanisms remain un versial, interaction of a taste ligand with a membrane protein certain. Ultimately, H+-mediated depolarization of the presyn initiates a signal transduction cascade that ends with a series of aptic cell results in serotonin release, as described for salt taste action potentials in the primary sensory neuron. above. The mechanisms of taste transduction are a good example Neurotransmitters (ATP and serotonin) from taste cells ac of how our models of physiological function must periodically tivate primary gustatory neurons whose axons run through cra be revised as new research data are published. For many years nial nerves VII, IX, and X to the medulla, where they synapse. the widely held view of taste transduction was that an individ Sensory information then passes through the thalamus to the ual taste cell could sense more than one taste, with cells differ gustatory cortex (see Fig. 10-4). Central processing of sensory ing in their sensitivities. However, gustation research using mo information compares the input from multiple taste cells and lecular biology techniques and knockout mice [ ~ jJ 292J interprets the taste sensation based on which populations of currently indicates that each taste cell is sensitive to only one neurons are responding most strongly. Signals from the sensory 10 taste. neurons also initiate behavioral responses, such as feeding, and In the old model, all taste cells formed synapses with pri feedforward responses [ ~ fl. 2ll-l-J that activate the digestive mary sensory neurons called gustatory neurons. Now it has system. been shown that there are two different types of taste cells, An interesting psychological aspect of taste is the phe and that only the taste cells for salty and sour tastes (type III nomenon named specific hunger. Humans and other animals or presynaptic cells) synapse with gustatory neurons. The that are lacking a particular nutrient may develop a craving for presynaptic taste cells release the neurotransmitter serotonin that substance. Salt appetite, representing a lack of Na+ in the byexocytosis. body, has been recognized for years. Hunters have used their The taste cells for sweet, bitter, and umami sensations knowledge of this specific hunger to stake out salt licks because (type II or receptor cells) do not form traditional synapses. they know that animals will seek them out. Salt appetite is di Instead they release ATP through gap junction-like channels, rectly related to Na+ concentration in the body and cannot be and the ATP acts both on sensory neurons and on neighboring assuaged by ingestion of other cations, such as Ca2 + or K+. presynaptic cells. This communication between neighboring Other appetites, such as cravings for chocolate, are more diffi taste cells creates complex interactions. cult to relate to specific nutrient needs and probably reflect complex mixtures of physical, psychological, environmental, Taste Transduction Uses Receptors and cultural influences. and Channels The details of taste cell signal transduction, once thought to be CONCEPT CHECK relatively straightforward, are also more complex than scien 14. With what essential nutrient is the umami taste sensation as tists initially thought (Fig. 10-17 e ). The type II taste cells for sociated? bitter, sweet, and umami tastes express different G protein 1S. Map or diagram the neural pathway from a presynaptic taste coupled receptors, including about 30 variants of bitter recep cell to the gustatory cortex. Answers p. 383 tors. In type II taste cells, the receptor proteins are associated with a special G protein called gustducin. Gustducin appears to activate multiple signal transduc THE EAR: HEARING tion pathways. Some pathways release Ca2 + from intracellular The ear is a sense organ that is specialized for two distinct func stores, while others open cation channels and allow Ca 2 + to tions: hearing and equilibrium. It can be divided into external, enter the cell. Calcium signals then initiate ATP release from middle, and inner sections, with the neurological elements the type II taste cells. housed in and protected by structures in the inner ear. The In contrast, salty and sour transduction mechanisms vestibular complex of the inner ear is the primary sensor for both appear to be mediated by ion channels rather than by equilibrium. The remainder of the ear is used for hearing. 354 Chapter 10 Sensory Physiology 0- Sweet, umami, Sour or bitter li gand @ Gustducin 0 1 GPCR /0 Ligands activate the taste cell. Various intracellular pathways are activated. ~) Ca2• signal in the cytoplasm triggers exocytosis or ATP formation . • Serotonin o : - Neurotransmitter or ATP is released. """:--- Primary gustatory --~,. , neurons «) Primary sensory neuron fires and action potentials are sent to the brain. e FIGURE 10-17 Summary of taste transduction. Each taste cell senses only one type of ligand. Receptor cells with G protein-coupled membrane receptors bind either bitter, sweet, or umami ligands and release ATP as a signal molecule. Sodium ion for salt taste enters presynaptic cel ls through ion channels and triggers exocytosis of serotonin. It is unclear whether H+ for sour taste acts intracellularly or extracellularly. The external ear consists of the outer ear, or pinna, and the ing. Colds or other infections that cause swelling can block the ear canal (Fig. 10-18 e ). The pinna is another exampJe of an eustachian tube and result in fluid buildup in the middle ear. If important accessory structure to a sensory system, and it varies bacteria are trapped in the middle ear fluid, the ear infection in shape and location from species to species, depending on known as otitis m edia [oto-, ear + -itis, inflammation + media, the animals' survival needs. The ear canal is sealed at its inter middle] results. nal end by a thin membranous sheet of tissue called the Three small bones of the middle ear conduct sound from tympanic membrane, or eardrum. the external environment to the inner ear: the malleus [ham The tympanic membrane separates the external ear from mer], the incus [anvil], and the stapes [stirrup]. The three the middle ear, an air-filled cavity that connects with the phar bones are connected to one another with the biological equiv ynx through the eustachian tube. The eustachian tube is nor alent of hinges. One end of the malleus is attached to the tym mally collapsed, sealing off the middle ear, but it opens tran panic membrane, and the stirrup end of the stapes is attached siently to allow middle ear pressure to eqUilibrate with to a thin membrane that separates the middle ear from the atmospheric pressure during chewing, swallOwing, and yawn inner ear. The Ear: Hearing 355 waves, but there is no noise unless someone or something is EME RGING CONCEPTS present to process and perceive the wave energy as sound. Sound is the brain's interpretation of the frequency, ampli tude, and duration of sound waves that reach our ears. Our CHANGING TASTE brains translate frequency of sound waves (the number of Sweet receptors respond to sugars, and umami wave peaks that pass a given point each second) into the pitch receptors respond to glutamate, covering two of the of a sound. Low-frequency waves are perceived as low-pitched three major groups of nutritious biomolecules. But sounds, such as the rumble of distant thunder. High-frequency what about fats? For years physiologists thought it waves create high-pitched sounds, such as the screech of finger was fat's texture that made it appealing, but now it appears that the tongue may have receptors for nails on a bl ackboard. long-chain fatty acids, such as oleic acid [ ~r lfl ). Sound wave frequency (Fig. 10-19b) is measured in waves Research in rodents has identified a membr·ane re per second, or hertz (Hz). The average human ear can hear ceptor called CD36 that lines taste pores and binds sounds over the frequency range of 20-20,000 Hz, with the fats. Activation of the receptor helps trigger the most acute hearing between 1000-3000 Hz. Our hearing is not feedforward digestive reflexes that prepare the as acute as that of many other animals, just as our sense of digestive system for a meal. Cur-rently evidence is smell is less acute. Bats listen for ultra-high-frequency sound lacking for a similar receptor in humans, but "fatty" waves (in the kilohertz range) that bounce off objects in the may turn out to be a sixth taste sensation. dark. Elephants and some birds can hear sounds in the infra And what would you say to the idea of taste sound (very low frequency) range. buds in your gut? Scientists have known for years Loudness is our interpretation of sound intensity and is that the stomach and intestines have the ability to influenced by the sensitivity of an individual's ear.

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