recap

45.2 recap

All animals receive information about chemical stimuli through chemoreceptors, which have diverse structures and bind to a tremendous variety of ligands. Chemoreceptors are the basis of the sensations of olfaction and gustation and the reception of pheromones. They also monitor some aspects of an animal’s internal environment.

learning outcomes

You should be able to:

  • Describe how a large but limited number of olfactory receptor proteins can discriminate a much larger number of odorant molecules, as well as their concentrations.

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    Explain how the five classes of taste receptors generate action potentials in their sensory neurons.

Question 1

Different odorants have different cell surface receptors, but these receptors activate the same intracellular G protein signaling pathway. How does the olfactory system discriminate different odors?

The odorant receptor neurons (ORNs) that share the same receptors project to the same glomerular cells in the olfactory bulb. Thus the identification of a particular odorant depends on the pattern of connectivity of the ORNs that are activated.

Question 2

How does the olfactory system discriminate between the whiff of a skunk, a repulsive odor of skunk, and the smell of a skunk cabbage?

An olfactory system signals intensity of a stimulus—whiff versus repulsive odor—by the frequency of action potentials it generates. The discrimination between a skunk and a skunk cabbage depends on the mixture of odorant molecules.

Question 3

The concentration thresholds for sensing bitter tastes are lower than for salt or sweet. Offer an evolutionary explanation for why this is so.

For salt, extracellular fluids including saliva have a high concentration of NaCl, therefore a lower sensitivity is appropriate for sensing concentrations above that constitutive level. For sweet, the selective pressure for its evolution was the ability to discriminate between high and low energy foods. A low sensitivity motivates the selection of higher energy foods. Bitter is a characteristic of many protective, hence poisonous, compounds that evolved in plants to discourage predation, so a high sensitivity helps avoid poisoning.

Question 4

With only five classes of taste receptors, how can we discriminate so many different tastes?

With only five classes of taste receptors we can sense a great variety of tastes because (1) some classes have multiple receptor genes; (2) a substance may trigger multiple receptor classes and therefore create a mixed signal; and (3) information from taste receptors is integrated with information from olfactory receptors as well as thermoreceptors (TRP channels) and tactile receptors.

You have now seen how chemoreceptors give rise to the sensations of smell and taste, and how some animals use chemoreception to communicate with others of their species. Next we describe the sensory cells that respond to mechanical forces, including the vibrations you perceive as sound.