The olfactory bulb (OB) is the brain’s primary processing center for the sense of smell, or olfaction. This structure converts raw chemical signals from the nose into organized neural information the rest of the brain can interpret.
The olfactory bulb is integral to how humans and animals detect, recognize, and react to the vast array of odors in the environment. Its unique circuitry and direct connections to higher brain centers are important for sensory perception.
Physical Structure and Location
The olfactory bulb is a paired, bulb-like neural structure situated on the underside of the forebrain, resting just above the nasal cavity. It lies on the inferior surface of the frontal lobe, where it is protected by the cribriform plate of the ethmoid bone. This bony plate is perforated by numerous tiny holes through which nerve fibers pass, connecting the nasal tissue to the brain.
In humans, the bulb is relatively small compared to other brain structures. It is organized into multiple distinct layers, which allows it to perform the initial stages of sensory analysis before transmitting the refined signal further into the brain.
Receiving and Initial Signal Input
The process of smelling begins when odor molecules bind to specialized Olfactory Receptor Neurons (ORNs) located within the olfactory epithelium, a patch of tissue high inside the nasal cavity. These ORNs generate electrical signals that travel as olfactory nerve fibers. The fibers pass through the holes in the cribriform plate to enter the olfactory bulb.
The olfactory pathway is unique among the senses because it bypasses the thalamus, the primary sensory relay station for vision, hearing, and touch. Olfactory signals proceed directly from the nose to the olfactory bulb, and then to the primary olfactory cortex. This direct route contributes to the speed and intensity with which smells can trigger immediate responses, unlike other sensory inputs that must first be gated through the thalamus.
Refining and Processing Odor Information
The olfactory bulb functions as a sophisticated signal processor, transforming the crude sensory input into a meaningful odor code. Axons from ORNs converge onto discrete, spherical structures within the bulb called glomeruli. Each glomerulus serves as a functional sorting station, receiving input exclusively from ORNs that express the same type of odorant receptor.
Within the glomeruli, the sensory input is transmitted to the principal output neurons: the Mitral and Tufted cells. These cells are responsible for carrying the processed odor information out of the bulb to other brain regions. This conversion from many receptor inputs to a focused output creates a spatial map of odor activity across the bulb’s surface.
The bulb sharpens the odor signal through lateral inhibition, mediated by various interneurons. Highly activated glomeruli suppress the activity of surrounding, less-activated glomeruli. This contrast enhancement mechanism helps the brain distinguish between complex odors or those that are chemically similar, refining the perception.
The Olfactory Bulb’s Influence on Cognition and Behavior
After processing, the refined odor information leaves the olfactory bulb primarily via the Mitral and Tufted cells, traveling along the lateral olfactory tract. This information is then distributed to several higher brain centers, linking the sense of smell directly to memory, emotion, and instinctual behavior.
Direct Brain Connections
The bulb projects to the piriform cortex, which is considered the primary olfactory cortex and is responsible for odor identification. Signals are also sent directly to the amygdala, a region central to processing emotions. This connection explains why certain smells can immediately trigger strong emotional responses, such as fear or comfort. Information is also routed to the entorhinal cortex, which provides a pathway to the hippocampus, integrating smell with the formation and retrieval of long-term memories.
Adult Neurogenesis
The olfactory bulb has a unique capacity for adult neurogenesis, the continuous creation of new neurons throughout life. New cells, primarily inhibitory interneurons, migrate from the subventricular zone and integrate into the existing circuits. This ongoing renewal is thought to play a role in the bulb’s ability to adapt to new olfactory experiences and optimize the learning of new smells.