The olfactory bulb is the brain’s first processing station for olfaction, or the sense of smell. It receives signals from sensory neurons in the nose to interpret the chemical world. For many animals, this function is linked to survival, guiding behaviors like feeding, mating, and avoiding danger. The mouse is a primary subject for understanding how the brain decodes odors.
Anatomy of the Mouse Olfactory Bulb
Located at the front of the mouse brain, the olfactory bulb is a multi-layered structure. Its laminar architecture consists of distinct, concentric layers of cells that reflect the organized flow of information. From the outside in, these layers include the glomerular layer, external plexiform layer, mitral cell layer, and granule cell layer. Each layer contains specific types of neurons for processing scent information.
The most superficial layer contains thousands of spherical structures called glomeruli. These are sorting hubs where information about specific odors is initially organized. Each glomerulus receives signals from olfactory sensory neurons that detect the same type of odorant molecule.
Deeper within the bulb are the principal output neurons: the mitral and tufted cells. These cells extend a primary dendrite into a single glomerulus to receive sorted odor information. The granule cells are the most numerous neurons in the bulb and act as modulators, forming connections with mitral and tufted cells.
The Neural Pathway of Smell
The process of smelling begins when chemical molecules enter the nasal cavity and bind to olfactory sensory neurons. Each of these neurons expresses a single type of olfactory receptor, making it sensitive to a specific chemical feature. When a receptor binds to its odorant, it triggers an electrical signal that travels along the neuron’s axon, converting a chemical stimulus into a neural code.
These axons bundle together to form the olfactory nerve, which connects directly to the olfactory bulb. All sensory neurons expressing the same type of receptor send their axons to one or two specific glomeruli. This precise wiring creates a distinct spatial map of odors across the glomerular layer, where each activated glomerulus represents a component of a scent.
From the glomeruli, mitral and tufted cells transmit the processed signal. Their axons form the olfactory tract, which projects to higher-order brain areas without first relaying through the thalamus. The main destination is the piriform cortex for conscious identification of smells. Other projections go to the amygdala, linking smells to emotional responses like fear or pleasure.
The Mouse as a Premier Model for Olfactory Study
The mouse is an important model for studying olfaction because its survival is linked to its sense of smell. Mice rely on odors to navigate, find food, identify mates, and detect predators. This dependence has resulted in a highly developed olfactory system that is accessible for scientific investigation.
The neural circuitry of the mouse olfactory system is well-defined and consistent between individuals. This stereotyped organization provides a reliable framework for researchers to study specific cell types and connections. The consistency allows for experiments that link neural activity to specific olfactory tasks, knowing the findings will be broadly applicable.
The availability of genetic tools for mice has advanced olfactory research. Scientists can engineer mice to express fluorescent markers in specific neuron populations, allowing them to visualize circuits in detail. It is also possible to use optogenetics, a technique that uses light to activate or silence specific cells. These tools enable researchers to test the function of different circuit components and observe the effects on an animal’s perception of smells.
Adult Neurogenesis and Brain Plasticity
The mouse olfactory bulb is one of the few sites in the adult mammalian brain where neurogenesis, the birth of new neurons, occurs throughout life. This provides a constant supply of new cells that integrate into existing circuits. These new neurons, called neuroblasts, originate from stem cells in the subventricular zone and migrate to the olfactory bulb.
This migration occurs through a pathway known as the rostral migratory stream. Upon arrival in the olfactory bulb, the neuroblasts differentiate. The majority become granule cells, which are inhibitory interneurons that modulate mitral and tufted cell activity. A smaller number become periglomerular cells, another interneuron type in the glomerular layer.
The functional purpose of this cellular renewal is still being researched. Evidence suggests these new neurons are involved in olfactory learning and memory. By adding new elements to the circuitry, the brain may enhance its ability to discriminate between similar odors and adapt to new scent environments. This neurogenesis provides a window into how the brain remodels itself in response to experience.