20.7: Physiology of Smell and Olfactory Pathway
Humans detect odors with the help of specialized cells located in the upper part of the nasal cavity, called olfactory receptor neurons (ORNs). ORNs possess hair-like structures called cilia, which are receptive to sensations from the inhaled air. When an odorant molecule binds to a specific receptor on the cell of the cilia, it leads to a series of events that ultimately cause the ORN to send electrical signals to the olfactory bulb in the brain through the olfactory nerves.
The olfactory bulb, located in the front part of the brain, is responsible for processing and recognizing smells. Upon receiving the signals from the ORNs, the olfactory bulb sends information to other parts of the brain, including the amygdala (associated with emotions) and the hippocampus (associated with memory). Integrating smell with other senses helps us better perceive our environment.
The human olfactory system can detect thousands of odors, each with a unique chemical structure. Interestingly, there are no separate receptors for each odorant molecule. Instead, each receptor can detect multiple odors, and the brain interprets the mix of activated receptors to identify the specific odor. Furthermore, the ability to identify smells is influenced by personal experience and cultural factors. We may associate certain smells with particular memories or emotions, leading to a subjective odor perception.
Once an odorant molecule binds to a receptor, it activates a G protein that activates an enzyme called adenylate cyclase. Adenylate cyclase produces a molecule called cyclic adenosine monophosphate (cAMP). The cAMP molecules bind to and open ion channels, allowing positively charged ions like sodium (Na+) and calcium (Ca2+) to flow into the cell. The influx of positively charged ions generates an electrical signal, which travels down the length of the sensory neuron and is transmitted to the olfactory bulb through the olfactory nerve. The signals are integrated and processed in the olfactory bulb, allowing the brain to recognize and distinguish different odors. The olfactory information is sent to other brain parts, including the amygdala (associated with emotions) and the hippocampus (associated with memory).
The olfactory pathway in humans involves inhaling odor molecules into the nose, binding them to specialized receptor cells in the olfactory epithelium. From there, signals are sent to the olfactory bulb, a structure located at the base of the forebrain. The signals are then relayed to two nearby brain regions: the primary and secondary olfactory cortex. The primary olfactory cortex recognizes odors and associates them with memories or emotional responses. In contrast, the secondary olfactory cortex processes sensory information about odors’ intensity, directionality, and duration. Additionally, recent research has shown that some neural pathways from the primary olfactory cortex may even directly connect to other parts of the brain involved in emotion and behavior. This indicates that the sense of smell plays a much more significant role in behavior and emotion than was once thought.
The primary olfactory cortex and areas of the brain responsible for memory are also believed to be involved in pheromone detection. Pheromones are chemical signals secreted by animals (including humans) that influence the behavior or physiology of other members of the same species. In humans, pheromones have been linked to sexual attraction, although this connection is still poorly understood. Recent studies have suggested some components of human sweat may act as pheromones and could be used to communicate emotions or even influence mood. Further research into the role of the olfactory system in behavior and emotion is needed to understand its effects fully.
