The olfactory system gives humans their sense of smell by collecting odorants from the environment and transducing them into neural signals.

The olfactory system gives humans their sense of smell by inhaling and detecting odorants in the environment. Olfaction is physiologically related to gustation, the sense of taste, because of its use of chemoreceptors to discern information about substances. Perceiving complex flavors requires recognizing taste and smell sensations at the same time, an interaction known as chemoreceptive sensory interaction. However, olfaction is anatomically different from gustation because it uses the sensory organs of the nose and nasal cavity to capture smells. Humans can identify a large number of odors and use this information to interact successfully with their environment.

Olfactory transduction is a series of events in which odor molecules are detected by olfactory receptors. These chemical signals are transformed into electrical signals and sent to the brain, where they are perceived as smells.

Once ligands (odorant particles) bind to specific receptors on the external surface of cilia, olfactory transduction is initiated. In mammals, olfactory receptors have been shown to signal via G protein receptors. This is a similar type of signaling of other known G protein-coupled receptors (GPCR). The binding of an odorant particle on an olfactory receptor activates a particular G protein, which then activates adenylate cyclase, leading to cAMP production. cAMP then binds and opens a cyclic nucleotide-gated ion channel. This opening allows for an influx of both Na⁺ and Ca²⁺ ions into the cell, thus depolarizing it. The Ca²⁺, in turn, activates chloride channels, causing the departure of Cl^–, which results in a further depolarization of the cell.

An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds specific molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium; extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins. It is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia. The receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus.

Pheromones are airborne, often odorless molecules that are crucial to the behavior of many animals. They are processed by an accessory of the olfactory system. Recent research shows that pheromones play a role in human attraction to potential mates, the synchronization of menstrual cycles among women, and the detection of moods and fear in others. Thanks in large part to the olfactory system, this information can be used to navigate the physical world and collect data about the people around us.

Key Points

• Olfaction is a type of chemoreception. Like gustation, this sensory system uses the molecular chemical compounds in substances (in this case, in odorants ) to discern information about the environment.

• The main sensory organ responsible for the human sense of smell is the nasal cavity, which contains olfactory receptors that perform the transduction of odors into neural impulses.

• Human beings can detect a large and diverse number of smells due to the vast number of features and combinations of odor molecules.

• Olfaction is the sense most closely tied to memory because of its close neural connections to areas of the brain responsible for emotion and place memory.

• Pheromones are airborne, often odorless molecules that are crucial to the behavior of many animals.

Key Terms

pheromone: a chemical secreted by an animal, especially an insect, that affects the development or behavior of other members of the same species; functions often as a means of attracting a member of the opposite sex

odorant: any substance that has a distinctive smell, especially one added to another substance (such as household gas) for safety purposes.

gustation: the sense of taste

olfaction: the sense of smell

G protein–coupled receptor: a receptor protein located in the cell membrane that binds extracellular substances and transmits signals from these substances to an intracellular molecule