 In this video I will describe the sensory transduction mechanisms and sensory pathways for the olfactory modality. Olfaction is commonly referred to as the sense of smell. Airborne molecules known as odorants are inhaled into the nasal cavity where they dissolve in the mucus of the olfactory epithelium on the superior surface of the nasal cavity. Odorants will bind to G-protein coupled receptors on the surface of the dendrites of the olfactory receptor neurons. There are approximately a thousand distinct types of olfactory receptors that each express a different G-protein coupled receptor. The olfactory receptor sensory transduction mechanism involves the odorant activating a G-protein coupled receptor. This G-protein coupled receptor will then stimulate the enzyme adenolyl cyclase to produce the second messenger molecule, cyclic AMP. As the cyclic AMP concentration of the cytosol increases, cyclic AMP will bind to a cyclic nucleotide-gated ion channel, stimulating the opening of that ion channel, which enables sodium to enter the cell, and this sodium influx causes depolarization that stimulates action potentials in the olfactory receptor neuron. The axons of olfactory receptor cells travel in the olfactory nerve, cranial nerve number one, in order to enter the cranium and form synapses with mitral cells in the olfactory bulbs of the brain. Each mitral cell receives information from only one of the a thousand distinct types of olfactory receptors, so there are approximately a thousand distinct types of mitral cells in the olfactory bulbs. The axons of mitral cells then travel in the olfactory tract to reach the primary olfactory cortex in the medial temporal lobe of the cerebrum, where the process of perception for the olfactory modality begins. The perception of an odor results from the specific combination of olfactory receptors and mitral cells that are activated by a chemical. One molecule may activate multiple receptors. This produces a combinatorial code that enables us to perceive an enormous variety of more than 10,000 distinct odors.