32: Smell and Taste: The Chemical Senses

Introduction

• A Large Number of Olfactory Receptor Proteins Initiate the Sense of Smell

  • Mammals Share a Large Family of Odorant Receptors

  • Different Combinations of Receptors Encode Different Odorants

• Olfactory Information Is Transformed Along the Pathway to the Brain

  • Odorants Are Encoded in the Nose by Dispersed Neurons

  • Sensory Inputs in the Olfactory Bulb Are Arranged by Receptor Type

  • The Olfactory Bulb Transmits Information to the Olfactory Cortex

  • Output from the Olfactory Cortex Reaches Higher Cortical and Limbic Areas

  • Olfactory Acuity Varies in Humans

• Odors Elicit Characteristic Innate Behaviors

  • Pheromones Are Detected in Two Olfactory Structures

  • Invertebrate Olfactory Systems Can Be Used to Study Odor Coding and Behavior

  • The Anatomy of the Insect Olfactory System Resembles That of Vertebrates

  • Olfactory Cues Elicit Stereotyped Behaviors and Physiological Responses in the Nematode

  • Strategies for Olfaction Have Evolved Rapidly

• The Gustatory System Controls the Sense of Taste

  • Taste Has Five Submodalities or Qualities

  • Taste Detection Occurs in Taste Buds

  • Each Taste Is Detected by a Distinct Sensory Transduction Mechanism and Distinct Population of Taste Cells

  • Sensory Neurons Carry Taste Information from the Taste Buds to the Brain

  • Taste Information Is Transmitted from the Thalamus to the Gustatory Cortex

  • Perception of Flavor Depends on Gustatory, Olfactory, and Somatosensory Inputs

  • Insect Taste Organs Are Distributed Widely on the Body

• An Overall View

Through the senses of smell and taste we are able to perceive a staggering number and variety of chemicals in the external world. These chemical senses inform us about the availability of foods and their potential pleasure or danger. Smell and taste also initiate physiological changes required for the digestion and utilization of food. In many animals the olfactory system also serves an important social function by detecting pheromones that elicit innate behavioral or physiological responses.

Although the discriminatory ability of humans is somewhat limited compared with that of many other animals, odor chemists estimate that the human olfactory system may be capable of detecting more than 10,000 different volatile chemicals. Perfumers who are highly trained to discriminate odorants can distinguish as many as 5,000 different types of odorants, and wine tasters can discern more than 100 different components of taste based on combinations of flavor and aroma.

In this chapter we consider how odor and taste stimuli are detected and how they are encoded in patterns of neural signals transmitted to the brain. In recent years much has been learned about the mechanisms underlying chemosensation in a variety of animal species. Certain features of chemosensation have been conserved through evolution, whereas others are specialized adaptations of individual species.

A Large Number of Olfactory Receptor Proteins Initiate the Sense of Smell

Odorants—volatile chemicals that are perceived as odors—are detected by olfactory sensory neurons in the nose. The sensory neurons are embedded in a specialized olfactory epithelium that lines part of nasal cavity, approximately 5 cm² in area in ...