||Our sense of smell is stimulated only by gaseous molecules. These may come directly from the air we breathe or may come from volatile substances, released in our mouth, from the food we are eating.
The molecules we perceive as smells are called odorants. Odorant molecules stimulate sensory nerve cells (neurons) at the top of the nasal cavity and these respond by sending impulses to the brain (section 1.1). The sensory nerve cells involved are called receptor cells, their surfaces have regions on them called receptor sites. These detect the odorant molecules and the process triggers a sequence of changes in the cell that eventually generate an electrical signal (figure 2.1).