Sensory (olfactory) neurons are present at the top of the nasal cavity, extending their axons into the cranium. Olfactory signals are the only sensory system to send signals directly to the limbic system, which is integral to memory and emotional functions. In humans, from 100-200 different functional receptor proteins have been identified (there are over 1000 in rodents).
Specific molecules (odorants) bind to receptor proteins and activate neural electrical signals (action potentials). Patterns of olfactory neuron activity can code for complex odors, integrated within the olfactory bulb and temporal cortex. Olfactory neurons will undergo adaptation and decrease signals to the brain with constant exposure to a stimulus.
Humans consistently recognize certain odorants (e.g. spearmint, orange, anise). Specific oils for these are available, and can be prepared as serial dilutions. Students can then test for sensitivities for each by starting with a series at the low end of the concentrations. Odorants can be detected by some sensitive individuals at concentrations below the micromolar range. List the micromolar concentrations of mint and circle the one where you can begin to smell the mint.