Olfactory system

The sense of smell, known scientifically as olfaction, relies on a dedicated sensory system. This system is one of the special senses, each linked to specific organs. In most mammals and reptiles, there are two distinct olfactory systems: a main one that picks up smells carried through the air, and an accessory one that detects stimuli in fluids. Together with the sense of taste (the gustatory system), these are called the chemosensory system, as both inform the brain about an object’s chemical makeup through a process called transduction.

Peripheral

At the periphery, the system includes the nostrils, the ethmoid bone, the nasal cavity, and the olfactory epithelium—layers of thin, mucus-covered tissue lining the nasal cavity. This epithelial tissue is made up of mucous membranes, olfactory glands, olfactory neurons, and nerve fibers from the olfactory nerves.

Odor molecules can reach the nasal cavity either through the nostrils during inhalation (olfaction) or from the throat when chewing or swallowing pushes air to the back of the nasal cavity (retro-nasal olfaction). Inside, mucus on the cavity walls dissolves these molecules. Mucus also coats the olfactory epithelium, where mucous membranes produce and store it, and olfactory glands secrete metabolic enzymes into it.

Sensory transduction begins when olfactory sensory neurons in the epithelium detect odor molecules dissolved in the mucus. These neurons have cilia—tiny hairs—that contain olfactory receptors.

When a receptor binds to an odor molecule, it triggers an electrical response that travels through the sensory neuron to the olfactory nerve fibers at the back of the nasal cavity. These olfactory nerves and fibers then carry information about the odor from the peripheral system to the central olfactory system in the brain. The epithelium is separated from the brain by the cribriform plate of the ethmoid bone, and the nerve fibers pass through this plate to connect the epithelium to the limbic system at the olfactory bulbs.

Central

Inside the main olfactory bulb, pulses are sent to both mitral and tufted cells. These cells help determine odor concentration based on the timing of when certain neuron clusters fire—a mechanism called a timing code.

They also detect differences between very similar odors and use that data to aid later recognition. Mitral cells have low firing rates and are easily inhibited by neighboring cells, while tufted cells fire at high rates and are harder to inhibit. A mathematical model can partly explain how the bulbar neural circuit transforms incoming odor signals into the responses sent to the olfactory cortex.

The olfactory cortex is housed in the uncus and includes the piriform cortex (posterior orbitofrontal cortex), amygdala, olfactory tubercle, and parahippocampal gyrus. The olfactory tubercle connects to many areas, including the amygdala, thalamus, hypothalamus, hippocampus, brain stem, retina, auditory cortex, and the olfactory system itself, with 27 inputs and 20 outputs in total. Its role, simplified, is to verify that odor signals come from actual odors rather than from irritation of nasal villi, to regulate motor behavior (especially social and stereotypical actions) triggered by odors, to integrate auditory and olfactory information for these tasks, and to transmit positive signals to reward sensors, making it involved in addiction.

The amygdala processes signals from pheromones (same-species), allomones (cross-species), and kairomones (cross-species where the emitter is harmed and the sensor benefits). Due to the evolution of the cerebrum, this processing is secondary and largely unnoticed in human interactions.

Allomones include flower scents, natural herbicides, and toxic plant chemicals. Information for these processes reaches the amygdala indirectly from the vomeronasal organ via the olfactory bulb. Pulses from the main olfactory bulb in the amygdala are used to pair odors with names and to recognize differences between odors.

The bed nuclei of the stria terminalis (BNST) act as a pathway for information between the amygdala and hypothalamus, as well as between the hypothalamus and pituitary gland. Abnormalities in the BNST often lead to sexual confusion and immaturity. The BNST also connect to the septal area, which rewards sexual behavior. Mitral pulses to the hypothalamus promote or discourage feeding, while pulses from the accessory olfactory bulb regulate reproductive and odor-related reflex processes.

The hippocampus, though minimally connected to the main olfactory bulb, receives almost all its olfactory information via the amygdala, either directly or through the BNST. It forms new memories and reinforces existing ones. Similarly, the parahippocampus encodes, recognizes, and contextualizes scenes, and the parahippocampal gyrus contains the topographical map for olfaction.

The orbitofrontal cortex (OFC) is heavily linked to the cingulate gyrus and septal area to mediate positive and negative reinforcement. The OFC represents the expectation of reward or punishment in response to stimuli, and it plays a role in emotion and reward during decision-making. Finally, the anterior olfactory nucleus distributes reciprocal signals between the olfactory bulb and the piriform cortex.