Fundamentals of Neuroscience/Taste and Smell

Type classification: this is a lesson resource.

Subject Classification: this is a neuroscience resource.

Goals

• To learn how sensory systems detect chemicals in the environment
• To learn how olfactory and gustatory signals are created by these systems, and how they interact

Smell

The olfactory system. Odorants enter the nose where they stimulate smell receptors in the olfactory bulb (receptors of different sensitivities are depicted in different colors).

What links the seemingly disparate senses of taste and smell so closely is that both are, at their most basic level, triggered by the same environmental cues. Both smell receptors and taste receptors are attenuated to detect certain chemicals, and then relay information about the chemical environment to the brain. In the case of olfaction, these chemical molecules are suspended in air and thus labeled as odorants.

After an odorant molecule enters through the nose, it reacts with olfactory receptors attached to a brain structure called the olfactory bulb, which connects to a large number of brain processing regions, including the limbic system. There are several hundred unique types of olfactory receptors, each with a slightly different configuration and thus a slightly different affinity for bonding with any given odorant. Any given conscious perception of smell is the result of an odorant activating a wide yet specific pattern of receptor types at different levels. As a simplified example, the smell of oatmeal can be thought of as the product of the oatmeal releasing a certain chemical odorant (or more accurately, a complex and characteristic fingerprint of diverse chemical odorants) that activates 70% of receptor I cells, 20% of receptor II cells, etc. such that whenever the brain detects that 70% of its receptor I and 20% of its receptor II etc. cells have been activated, it experiences the distinct scent of oatmeal.

Taste

A single taste bud in-bedded in the tongue

The taste equivalent to airborne odorants are called tastants, and are found as the chemicals contained in food (or at least anything ingested as food). Taste receptors are found exclusively on the tongue, and like their olfactory counterparts they exist in specific configurations or types, giving rise to a gradation of taste experience. Thus when distinguishing between tastes the same general principle applies: a given tastant will have a greater affinity for certain receptors over others, and pattern of activation that results conveys the individual taste perception.

Not only are the sensory mechanism for smell and taste similar, but in terms of the processing regions of the brain they are highly interconnected. Perhaps this is no better seen than in how a food's physical taste and smell combine and intermingle to give rise to the sensation of flavor. Both also form the basis of powerful psychological associations, such as visceral reactions one has toward a disgusting odor or rotten taste.