The olfactory bulb, a neural structure in the vertebrate forebrain, is central to the sense of smell (olfaction). In humans, these paired structures are found above each nasal cavity, acting as the brain’s initial processing center for scent information from olfactory receptor neurons. Smell is important for human experience, influencing taste, food choices, social interaction, and serving as a warning system. It also strongly connects to memory and emotion, triggering powerful recollections or feelings.
The Olfactory Bulb’s Core Function
Smell begins when odor molecules enter the nasal cavity. They bind to specialized olfactory receptors on neurons within the olfactory epithelium, generating electrical signals. These signals transmit along axons, which pass through the cribriform plate, a bone separating the nasal cavity from the brain, to reach the olfactory bulb.
Within the olfactory bulb, incoming axons from olfactory receptor neurons converge onto small, spherical glomeruli. Each glomerulus processes input from thousands of receptor neurons, synapsing with principal neurons, primarily mitral cells. This convergence amplifies the signal and begins filtering and pattern recognition.
The olfactory bulb actively processes odor information, refining signals through neural circuits involving interneurons like periglomerular and granule cells. These interneurons sharpen contrast and modulate mitral cell output. Processed information then travels via mitral cell axons to other brain regions, including the olfactory cortex, amygdala, and hippocampus, for further processing related to odor identification, memory, and emotion.
Unveiling the Left Olfactory Bulb’s Distinctiveness
Both olfactory bulbs contribute to smell, but research suggests left and right side specializations, known as lateralization. The olfactory system projects ipsilaterally; left nostril input largely goes to the left olfactory bulb and hemisphere, and vice versa. This arrangement allows investigation into hemispheric differences. Studies indicate the left olfactory bulb may play a more prominent role in emotional odor processing.
The left hemisphere may be more involved in processing hedonic (pleasantness) aspects of odors and their linguistic classification. For instance, odors sniffed through the left nostril are named more correctly, aligning with the left hemisphere’s language specialization. Conversely, the right hemisphere appears more involved in odor recognition memory and processing unpleasant smells. These functional asymmetries suggest a nuanced division of labor between the two olfactory bulbs and hemispheres. Their interplay, possibly via the anterior commissure, is important for comprehensive olfactory perception.
When Olfactory Bulb Function Changes
Damage or suboptimal function of the olfactory bulb can significantly impair smell. Anosmia is complete smell loss, while hyposmia indicates a reduced ability. These conditions arise from causes like head trauma, viral infections, and neurodegenerative diseases.
Head trauma, especially to the front of the head, can damage olfactory nerves passing through the cribriform plate or cause contusions in the olfactory bulbs. Viral infections are also a common cause of olfactory dysfunction, with viruses like influenza, rhinoviruses, and coronaviruses (including SARS-CoV-2) inducing smell loss. In COVID-19, smell loss often results from the virus affecting supporting cells in the olfactory epithelium, causing inflammation and tissue disruption. While many recover, some experience persistent loss or distorted perception (parosmia).
Neurodegenerative diseases also frequently impact olfactory function. Over 95% of individuals with Parkinson’s disease, for example, experience significant olfactory loss, often years before motor symptoms appear. Similarly, olfactory dysfunction, particularly hyposmia, is an early indicator in Alzheimer’s disease, with changes in smell preceding cognitive decline. Impaired smell diminishes food enjoyment, poses safety risks (e.g., gas leaks), and affects quality of life and emotional well-being, sometimes leading to feelings of isolation or depression.