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Olfaction and Gustation

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Systems Biology

The senses of olfaction and gustation, or smell and taste, are intertwined and rely on chemoreceptors which respond to certain chemicals. Olfactory neurons in the olfactory epithelium (located in the nasal cavity) have a dendrite extending into the nasal cavity with chemoreceptors, specifically G protein-coupled receptors. These neurons synapse in the olfactory bulb, and their bundle of axons forms the olfactory nerve. The olfactory signals are then sent to the olfactory cortex of the temporal lobe via the olfactory tract.

For gustation, taste buds are found on the sides of raised papillae on the tongue and house taste cells containing chemoreceptors. These receptors bind with dissolved tastants in saliva. There are five main categories of taste: sour, salty, bitter, sweet, and umami or savory. Sour and salty foods release hydrogen and sodium ions respectively, both of which bind ionotropic receptors. Conversely, bitter, sweet, and umami tastes are facilitated by G protein-coupled receptors. Taste cells synapses with taste neurons that travel to the brain via cranial nerves VII, IX, or X.

Lesson Outline

<ul> <li>Olfaction and gustation: The senses of smell and taste</li> <ul> <li>They both rely on chemoreceptors</li> <ul> <li>Chemoreceptors bind specific chemicals</li> <li>Two main varieties: ionotropic receptors (open ion channels) and metabotropic receptors (start a G protein cascade)</li> </ul> </ul> <li>Olfaction: The sense of smell</li> <ul> <li>The nasal cavity - trap smell molecules</li> <li>The olfactory chemoreceptors - housed in olfactory receptor cells in the nasal cavity</li> <li>Olfactory neurons - extend through the olfactory epithelium</li> <li>Olfactory transduction - initiated by the binding of an odorant to a chemoreceptor</li> <li>Olfactory neurons use G protein-coupled receptors</li> <li>Olfactory neuron's axon extends to the olfactory bulb</li> <li>The olfactory bulb - synapses with other neural cells and sends combined signal via the olfactory tract to the olfactory cortex in the temporal lobe</li> <li>Olfaction - the only sensory system whose neurons do not go through the thalamus</li> </ul> <li>Gustation: The sense of taste</li> <ul> <li>Taste buds - clusters of taste receptor cells along the sides of the papillae</li> <li>Taste cells - contain taste chemoreceptors</li> <li>Taste chemoreceptors - come in contact with dissolved tastants in saliva</li> <li>Five main tastes: sour, salty, bitter, sweet, and umami</li> <li>Chemoreceptors for taste: ionotropic (for sour and salty foods) and metabotropic (for bitter, sweet, and umami tastes)</li> <li>Taste cells synapse with taste neurons in the tongue</li> <li>Three cranial nerves (IX, X, or VII) transport taste signals to the brain</li> </ul> <li>Connection between olfaction and gustation</li> <ul> <li>Chewing food releases chemicals that bind to olfactory receptors and enhance the taste of food</li> </ul> </ul>

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What roles do chemoreceptors play in olfaction and gustation?

Chemoreceptors are specialized sensory cells that detect chemical substances in the environment and relay that information to the brain. In olfaction, olfactory receptor cells in the nose detect odor molecules, while in gustation, taste bud cells in the mouth respond to taste stimuli. Both types of chemoreceptors transmit signals through cranial nerves to the respective regions in the brain, where these signals are processed and interpreted as smell and taste sensations.

How do olfactory receptor cells function and communicate with olfactory neurons?

Olfactory receptor cells are specialized neurons found in the olfactory epithelium in the nasal cavity. These cells contain G protein-coupled receptors (metabotropic receptors) that bind to specific odor molecules. Upon binding, a cascade of events occurs, leading to the production of cyclic AMP, which further opens ion channels and generates an action potential. This electrical signal is then relayed to the olfactory bulb via the axons of the olfactory receptor cells, which collectively form the olfactory nerve (cranial nerve I). The signal is then transferred to the olfactory cortex in the temporal lobe, ultimately resulting in the perception of smell.

What is the role of ionotropic and metabotropic receptors in taste sensation?

Ionotropic and metabotropic receptors play crucial roles in transducing chemical signals from taste stimuli into neuronal signals that can be processed by the brain. Ionotropic receptors such as the transient receptor potential (TRP) channels are directly gated by certain taste molecules and lead to depolarization of the taste cells upon binding. Metabotropic receptors, on the other hand, are G protein-coupled receptors that initiate a cascade of events upon binding to taste molecules, leading to the generation of secondary messengers and ultimately, cell depolarization. Both ionotropic and metabotropic receptor-mediated pathways contribute to the complexity and diversity of taste sensation and discrimination.

How are taste buds organized, and how do they transmit information to the brain?

Taste buds are specialized structures consisting of several types of taste cells that detect different taste stimuli (sweet, sour, salty, bitter, and umami). They are mainly situated on the tongue, palate, and epiglottis. Each taste bud has a small taste pore opening on the surface that allows taste molecules dissolved in saliva to enter and interact with the taste cells. Taste cells make synaptic contacts with afferent sensory fibers of the cranial nerves, primarily the facial (cranial nerve VII), glossopharyngeal (cranial nerve IX), and vagus (cranial nerve X) nerves, which transmit the taste information to the brainstem and then to higher brain centers for processing and interpretation.

How do G protein-coupled receptors contribute to the perception of smell and taste?

G protein-coupled receptors (GPCRs) play a significant role in the chemosensory perception of both olfaction and gustation. In olfaction, GPCRs in olfactory receptor cells bind to odor molecules, resulting in the production of secondary messengers and generation of action potentials that are transmitted to the brain via olfactory neurons. In gustation, GPCRs are involved in detecting sweet, bitter, and umami taste stimuli. Upon binding to taste molecules, GPCRs activate intracellular pathways, leading to the generation of secondary messengers that ultimately trigger cell depolarization and signal transmission to the brain. Overall, GPCRs act as essential molecular components that allow organisms to perceive diverse chemical stimuli and differentiate a wide range of smells and tastes.