Olfactory bulb — structure, function and significance

Structure and connections

Incoming signals arrive as bundles of axons from olfactory receptor neurons that converge in spherical units called glomeruli. Principal output neurons — the mitral and tufted cells — send processed information onward to several brain regions. Local interneurons such as periglomerular and granule cells shape activity through inhibition and synaptic plasticity. The olfactory bulb has a relatively simple, layered layout that supports rapid transformations of peripheral input.

Although much of its drive is feedforward from receptors, the olfactory bulb also receives substantial top‑down input from higher centers, which can alter sensitivity and patterning. Anatomical and functional feedback comes from limbic and cortical areas including the amygdala, neocortex, hippocampus, and modulatory brainstem nuclei such as the locus coeruleus and substantia nigra. These pathways enable context, learning and internal state to influence odor processing. Some authors describe the bulb as a dynamic filter that selects and highlights behaviorally relevant signals.

Functions and coding

• Detection and discrimination of odors through spatial patterns of glomerular activation.
• Temporal coding: oscillatory activity and spike timing contribute to how odors are represented.
• Pattern separation and contrast enhancement, improving the brain's ability to tell similar smells apart.
• Plasticity and learning: synaptic changes in the bulb reflect experience and can influence odor memory.

Development, evolution and distinctions

The olfactory bulb develops from the embryonic telencephalon and shows marked variation across vertebrates in size and complexity, often correlating with reliance on smell. Many mammals possess both a main olfactory bulb and an accessory olfactory bulb connected to the vomeronasal system; the latter mediates certain social and pheromonal signals in species that retain it. A notable feature of the mammalian olfactory bulb is ongoing incorporation of new neurons in adulthood, supplied by progenitors from the subventricular zone — a widely studied example of adult neurogenesis.

Clinical and research relevance

Impaired olfactory bulb function underlies hyposmia or anosmia and is often an early indicator of broader neurological disorders. Because of its accessibility and well‑defined circuits, the bulb is a popular model for studying sensory coding, synaptic plasticity and the integration of bottom‑up and top‑down signals. Experimental approaches include electrophysiology, imaging and genetic manipulation to link cellular mechanisms with behavior.