The chemical senses are olfaction (smell) and gustation (taste). Olfaction and gustation originate from chemical particles interacting with sensory cells. In olfaction, these particles are called “odorants” and they bind to the membrane of receptor proteins, which triggers a cascade of events leading to action potentials spreading through central olfactory pathways. These pathways extend to different areas of the forebrain, like the olfactory cortex. One thing that is unique about the olfactory cortex, as well as the auditory cortex, is that these processing hubs receive information directly from their respective sensory organ without first being processed by the thalamus.
Taste receptor cells, also called taste buds, are the interface between food and our sense of taste. They are not neurons, but have some similar structures such as synapses, which transfer information to gustatory afferent axons. Depolarization of receptor cell membranes triggers the opening of calcium-gated channels. The calcium goes into the cytoplasm, which causes different transmitter molecules to be released based on what type of receptor cell was activated. Adenosinetriphosphate (ATP) is released by sweet, bitter, and umami taste cells and serotonin is released by sour and salty taste cells.
One question that came up while I was reading the book (and eating tacos) is this: if you had local anesthetic on your tongue, could you still taste spiciness from capsaicin? If your tongue was numbed, part of the multi sensory components of taste would be blocked, so the texture and accompanying sense of pain would presumably not be present, but then would you still be able to perceive the “temperature” that makes something seem spicy, and the various arrangements of salty/umami/sweet that gave the rest of the chili pepper flavor? Would it still be phenomenologically spicy?
All the sensory systems require some kind of mapping. In olfaction, it is hypothesized that neurons that respond to specific odorants are spatially arranged, and that action potential timing may be important for some scents. In gustation, the combined activation of cells give rise to specific tastes; the auditory system has frequency based mapping at the inner ear; touch receptors are represented in the motor cortex based on relative density; and eyes hold maps of visual space.