Chapter 13: Gustatory Senses

Introduction to Gustatory Senses

The chemical senses of taste and smell are vital for our survival and provide immense pleasure through the enjoyment of food. This chapter explores taste and the significant role it plays in our lives. From celebrity chefs to the pleasure of fine dining, taste has become a major industry, showcasing our profound connection to the culinary arts.

Basic Physiology of Taste Receptors

Our journey begins with an understanding of taste receptors, which reside in taste buds found in three of the four classes of papillae on our tongues. There are four primary types of papillae: filiform, fungiform, foliate, and circumvallate, each with a unique structure and distribution. All of the papillae other than filiform house taste buds containing taste receptor neurons, the key players in the transduction of taste information. The filiform papillae give the tongue texture (you see these on the central part of your tongue) and may be involved in breaking down food. For cats these same papillae are quite rough and are involved in grooming (if you’ve ever been licked by a cat you may be familiar with the sandpaper feel of a cat’s tongue).

Figure 13.1

Variability in Taste Perception

Taste perception isn't uniform across individuals. Some people are known as "super tasters" and experience taste more intensely due to a higher number of fungiform papillae (these are the small dots you can see on the tip and front parts of the tongue). On the other hand, "non-tasters" have fewer papillae and may not perceive certain tastes as strongly. The story of super tasters begins with a serendipitous discovery by a chemist named Fox in 1932. While preparing a chemical called PTC (phenylthiocarbamide), he noticed that some individuals found it incredibly bitter, while others perceived no taste at all. This chance observation led to the identification of a genetic trait related to taste sensitivity, which divided the population into non-tasters and super tasters. This discovery opened the door to exploring the genetic basis of taste perception.

Taste Pathway to the Brain

To comprehend the neural processing of taste, we follow the journey from the tongue to the brain. Two cranial nerves, the chorda tympani (sends signals from front and sides of tongue) and the glossopharyngeal (sends signals from the back of the tongue), transmit taste information to the brainstem's nucleus of the solitary tract, where the information is structured in a chemotopic map (with salty, sour, bitter, etc represented in different locations). From there, the information is relayed to the thalamus before reaching the primary gustatory cortex in the anterior insula and the frontal operculum. The secondary receiving area, the orbitofrontal cortex, integrates gustatory input with other sensory information, creating the full flavor experience.

The Five Basic Tastes and Beyond

While traditionally considered to have five basic tastes—salty, sour, sweet, bitter, and umami—some researchers suggests that metallic taste may also be a distinct category. These taste qualities result from the activation of specific receptors by various chemicals found in foods and beverages.

Influences on Flavor Perception

Flavor perception is not limited to taste alone. It is significantly influenced by olfaction, as smell and taste work together to create the rich tapestry of flavors we enjoy. Additionally, factors like texture, appearance, and recent food experiences contribute to how we perceive flavor, with the orbital frontal cortex playing a crucial role in integrating these sensory inputs.

Modifiers of Taste Perception

Certain substances can alter taste perception. "Miracle berries" from West Africa, also known as Synsepalum dulcificum, temporarily change the way we taste sourness by activating both sour and sweet receptors on the tongue. Gymnema sylvestre, an herb from India, can block sweetness receptors, making sweet foods taste bland. These modifiers provide unique insights into the complex world of taste perception.

Figure 13.3

Figure 13.4

Conclusions

In this chapter, we explored the fascinating realm of gustatory senses, exploring the intricacies of taste receptors housed within the various papillae on the tongue. Our journey took us through the diverse landscape of taste perception, as we uncovered the intriguing differences between supertasters and nontasters, shedding light on why some individuals experience a richer tapestry of taste sensations than others. We then embarked on a journey through the neural pathways to the brain. Our exploration culminated with a discussion of the ways various substances can wield their influence over our perception of taste, adding depth and complexity to our understanding of this fundamental human sensory experience. As we conclude this chapter, we are left with a heightened appreciation for the intricate interplay between our taste buds, our brains, and external factors that shape our perception of flavor.