Section Summary

All animals rely on the chemical senses for their ability to survive and proliferate. Gustation alerts us to toxic or hazardous compounds in our foods, while at the same time evaluating their nutritional content. Olfaction works in concert with taste to help us locate and recognize food sources. Olfaction also alerts animals to the presence of both predators and potential mates, increasing survival and reproductive opportunities. Our sense of smell also serves to alert us to dangers in the environment like smoke from a fire. Our brain integrates signals from these and other sensory systems to produce the multi-modal perceptions we describe as flavor.

The anatomical and molecular differences between the vertebrate and insect gustatory systems are vast, but what is truly astounding are the commonalities between animals. Animals are responsive to the same broad classes of biomolecules despite utilizing different molecular mechanisms and sensory organs to detect them. Gustatory sensilla and tongues simultaneously provide both chemosensory and mechanosensory signals, which are co-processed in a primary gustatory relay that mediates critical reflexes involving feeding or swallowing. Fundamentally, the gustatory system and its integration with other sensory systems allows animals to learn associations and alter feeding behaviors based on the nutrient content of the food they encounter. Despite vast differences in body plans, life history, or anatomy, the commonalities of the gustatory systems and the essential behaviors it mediates in animals speaks to the profound importance of “taste.”

Like all sensory systems, the olfactory system is organized in a hierarchy. Chemicals are detected at the periphery and then processed in successive steps to extract relevant information for decision-making and behavioral output. Each of the cell types and neural areas described in this section play an important role in this process; however, as we learned, stimulus processing in the olfactory system is vulnerable to several diseases that can disrupt our sense of smell. While the olfactory system's cellular organization and processing capabilities in humans and other mammals are complex, we are not unique. Other animals like insects share the same capabilities and rely on their sense of smell for survival.

While the somatosensory system mainly detects and discriminates between different types of kinetic energy, it is also capable of detecting chemical stimuli. Chemesthesis is the technical term for the ability of the somatosensory system to respond to chemicals. Most chemesthetic stimuli are produced by plants and activate TRP channels on polymodal nociceptors and produce sensations perceived as changes in temperature or described as irritating, burning, cooling, spicy, or pungent. In addition to stimulating free nerve endings directly, these nerve fibers can also be stimulated by solitary chemosensory cells through cholinergic synapses. SCCs detect growing bacterial infections and recruit innate defenses and trigger an immune response. Increasingly, functional chemosensory receptors are being described outside gustatory and olfactory systems and constitute part of the innate immune system that exists to detect infectious agents.

Chemicals are the building blocks of our world. From the sweet taste of an apple to the fresh smell of a newly cut lawn and even the burn of onion in our eyes, every substance we encounter requires us to interact with chemicals. The chemical senses allow us to build internal representations of the rich chemical environment that comprises our external world. Our repertoire of chemical senses directs us to energy-providing food, alerts us to danger, and helps many animals find mates. While the anatomical, cellular, and molecular features of the chemical senses vary between systems and comparatively between organisms and even individuals of the same species, each system serves a unique purpose in allowing animals to interact with and engage the chemical environment surrounding them.