The most ancient type of sensory detection is chemoreception. All organisms respond to chemical cues in their environment. Even the earliest branching groups of Bacteria and Archaea have protein receptors in their cell membranes that respond to molecules in the environment.

In animals, chemoreceptors are sensory receptor cells that respond to molecules that bind to specific protein receptors on the cell membrane (Fig. 36.2a). Many animals detect food in their environment by sensing key molecules such as oxygen (O₂), carbon dioxide (CO₂), glucose, and amino acids. Female mosquitoes track CO₂ levels to detect prey for blood meals, and coral polyps respond to simple amino acids in the water, extending their bodies and tentacles toward areas of greater concentration to feed. Other arthropods, such as flies and crabs, have chemosensory hairs on their feet. These animals taste potential food sources by walking on them. Salmon rely on chemoreception to detect chemical traces of the home waters of the river where they hatched, and where they will return to mate and spawn.

Chemoreception underlies the sense of smell and taste. Consider taste as an example. In most cases, the binding of molecules to a protein receptor on a taste receptor causes the protein receptor to change conformation. That change in conformation in turn triggers the opening of Na+ channels through G protein signal transduction pathways (Chapter 9). The influx of Na+ ions depolarizes the receptor cell. In the case of taste receptors, no action potential fires, but the depolarization travels far enough down the receptor’s short axon to trigger the release of neurotransmitters. Other chemoreceptors, such as those involved in the sense of smell or located on antennae to detect chemicals in the air, are sensory neurons that do fire action potentials.

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