Olfaction, or smell, is the detection of airborne molecules using specialized receptors in the nasal cavity.
The nose is the sensory organ for the sense of smell, also known as olfaction. Specifically, the olfactory chemoreceptors are on the superior wall of the nasal cavity. The olfactory chemoreceptors are in bipolar sensory neurons that descend through the cribriform plate of the ethmoid bone. Their axon terminals are superior to the bone, and the cell body and dendrites are inferior. The dendritic ends are knob-like, covered with cilia, and suspended in the fluid-mucus environment. The cilia, also known as olfactory hairs, serve to increase the surface area of the membrane, and as a result allow for greater concentration of chemoreceptors. These neurons project into an epithelial layer of cells referred to as supporting cells. The supporting cells, along with olfactory glands, produce mucus. The basal surface of the epithelium has olfactory stem cells that replace damaged bipolar cells. The bipolar cells that respond to odorant stimuli send electrochemical signals to neurons called mitral cells in the olfactory bulbs of the olfactory nerve. When inhaled air moves across the mucous membranes of the nasal cavity, odorants—substances that have a smell—dissolve in the mucus and bind to proteins on the chemoreceptor. Each bipolar neuron contains one of ~400 types of olfactory receptor proteins. Each receptor can bind to multiple types of odorants, and each odorant can bind to multiple receptors. This creates a combination of signals that allows humans to distinguish a high number of scents, once estimated at 10,000 but with recent studies supporting estimates closer to a trillion scents. When the odorant binds to the protein receptor, a cascade of events occurs involving a G-protein and ending with the opening of ion channels, which initiates an action potential. These bipolar cells then activate mitral neurons located in the olfactory bulb, and impulses travel via the olfactory tracts to the olfactory cortex of the cerebrum.