Contents

Anatomy

In most vertebrates, the olfactory bulb is the most rostral (forward) part of the brain. In humans, however, the olfactory bulb is on the inferior (bottom) side of the brain. The olfactory bulb is supported and protected by the cribriform plate which in mammals, separates it from the olfactory epithelium, and which is perforated by olfactory nerve axons. The bulb is divided into two distinct structures, the main olfactory bulb, and the accessory olfactory bulb.

The main olfactory bulb has a multi-layered cellular architechture. In order from the surface to the center of the bulb the layers are

Glomerular layer

External plexiform layer

Mitral cell layer

Internal plexiform layer

Granule cell layer

The glomerular layer receives direct input from olfactory nerve, made up of the axons from approximately ten million olfactory receptor neurons in the olfactory mucosa, a region of the nasal cavity. The ends of the axons cluster in spherical structures known as glomeruli such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor. Glomeruli are also permeated by dendrites from neurons called mitral cells, which in turn output to the olfactory cortex. Numerous interneuron types exist in the olfactory bulb including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells.

The accessory olfactory bulb, which resides on the dorsal-posterior region of the main olfactory bulb, forms a parallel pathway independent from the main olfactory bulb. It recives axonal input from the vomeronasal organ, a distinct sensory epithelium from the main olfactory epithelium that detects pheremones, among other chemical stimuli. Like the main oflactory bulb, axonal input to the accessory olfactory bulb forms synapses with mitral cells within glomeruli. However, mitral cells in the accessory olfactory bulb project their axons to targets in the amygdala and hypothalamus where they may influence aggressive and mating behavior.

Function

How the olfactory bulb influences animal behavior is a topic of much research and hot debate.

Mitral cells are connected by interneurons known as granule cells, which by some theories produce lateral inhibition between mitral cells. It is not clear what the functional role of lateral inhibition would be, though it may be involved in boosting the signal-to-noise ratio of odor signals by silencing the basal firing rate of surrounding non-activated neurons. The synapse between mitral and granule cells is of a rare class of synapses that are "dendro-dendritic" which means that both sides of the synapse are dendrites that release neurotransmitter. In this specific case, mitral cells release the excitatory neurotransmitter glutamate, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid (GABA). As a result of its bi-directionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves (auto-inhibition), as well as neighboring mitral cells (lateral inhibition).

Evolution

The olfactory bulb arises from a very old part of the brain. Comparison between the antennal lobe in species such as the fruit fly and the olfactory bulb in vertebrates reveal many structural similarities.