Olfactory bulb


The olfactory bulb is a neural structure in the forebrain of vertebrates that is involved in olfaction, or the sense of smell. It transmits olfactory information to the other brain regions including the amygdala, orbitofrontal cortex and hippocampus where it contributes to emotion, memory and learning.
The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway.
Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

Structure

In most vertebrates, the olfactory bulb is the most rostral part of the brain, as seen in rats. In humans, however, the olfactory bulb is on the inferior side of the brain. The olfactory bulb is supported and protected by the cribriform plate of the ethmoid bone, 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.

Layers

The main olfactory bulb has a multi-layered cellular architecture. In order from surface to the center the layers are:
  • Glomerular layer
  • External plexiform layer
  • Mitral cell layer
  • Internal plexiform layer
  • Granule cell layer
The olfactory bulb transmits smell information from the nose to the brain, and is thus necessary for a proper sense of smell. As a neural circuit, the glomerular layer receives direct input from afferent nerves, 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. The glomerular layer of the olfactory bulb is the first level of synaptic processing.
The glomerular layer represents a spatial odor map organized by chemical structure of odorants like functional group and carbon chain length. This spatial map is divided into zones and clusters, which represent similar glomeruli and therefore similar odors. One cluster in particular is associated with rank, spoiled smells which are represented by certain chemical characteristics. This classification may be evolutionary to help identify food that is no longer good to eat. The spatial map of the glomerular layer may be used for perception of odor in the olfactory cortex.
The next level of synaptic processing in the olfactory bulb occurs in the external plexiform layer, between the glomerular layer and the mitral cell layer. The external plexiform layer contains astrocytes, interneurons and some mitral cells. It does not contain many cell bodies, rather mostly dendrites of mitral cells and GABAergic granule cells 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 granule cell layer is the deepest layer in the olfactory bulb. It is made up of dendrodendritic granule cells that synapse to the mitral cell layer.

Function

This part of the brain receives sensations of smell.
As a neural circuit, the olfactory bulb has one source of sensory input, and one output. As a result, it is generally assumed that it functions as a filter, as opposed to an associative circuit that has many inputs and many outputs. However, the olfactory bulb also receives "top-down" information from such brain areas as the olfactory cortex, amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra.
Its potential functions can be placed into four non-exclusive categories:
  • discriminating among odors
  • enhancing sensitivity of odor detection
  • filtering out many background odors to enhance the transmission of a few select odors
  • permitting higher brain areas involved in arousal and attention to modify the detection or the discrimination of odors.
While all of these functions could theoretically arise from the olfactory bulb's circuit layout, it is unclear which, if any, of these functions are performed exclusively by the olfactory bulb. By analogy to similar parts of the brain such as the retina, many researchers have focused on how the olfactory bulb filters incoming information from receptor neurons in space, or how it filters incoming information in time. At the core of these proposed filters are the two classes of interneurons; the periglomerular cells, and the granule cells. Processing occurs at each level of the main olfactory bulb, beginning with the spatial maps that categorize odors in the glomeruli layer.
Interneurons in the external plexiform layer are responsive to pre-synaptic action potentials and exhibit both excitatory postsynaptic potentials and inhibitory postsynaptic potentials. Neural firing varies temporally, there are periods of fast, spontaneous firing and slow modulation of firing. These patterns may be related to sniffing or change in intensity and concentration of odorant. Temporal patterns may have effect in later processing of spatial awareness of odorant. For example, synchronized mitral cell spike trains appear to help to discriminate similar odors better than when those spike trains are not synchronized. A well known model is that the bulbar neural circuit transforms the odor information in the receptors to a population pattern of neural oscillatory activities in the mitral cell population, and this pattern is then recognized by the associative memories of olfactory objects in the olfactory cortex. Top-down feedback from the olfactory cortex to the olfactory bulb modulates the bulbar responses, so that, for example, the bulb can adapt to a pre-existing olfactory background to single out a foreground odor from an odor mixture for recognition, or can enhance sensitivity to a target odor during odor search.

Lateral inhibition

;External plexiform layer
The interneurons in the external plexiform layer perform feedback inhibition on the mitral cells to control back propagation. They also participate in lateral inhibition of the mitral cells. This inhibition is an important part of olfaction as it aids in odor discrimination by decreasing firing in response to background odors and differentiating the responses of olfactory nerve inputs in the mitral cell layer. Inhibition of the mitral cell layer by the other layers contributes to odor discrimination and higher level processing by modulating the output from the olfactory bulb. These hyperpolarizations during odor stimulation shape the responses of the mitral cells to make them more specific to an odor.
There is little information regarding the function of the internal plexiform layer, which lies between the mitral cell layer and the granule cell layer.
;Granule cell layer
The basal dendrites of mitral cells are connected to interneurons known as granule cells, which may produce lateral inhibition between mitral cells. The synapses between mitral and granule cells are of the rare "dendro-dendritic" class, with both sides of the synapse being 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. As a result of its bidirectionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves, as well as neighboring mitral cells. More specifically, the granule cell layer receives excitatory glutamate signals from the basal dendrites of the mitral and tufted cells. The granule cell, in turn, releases GABA to cause an inhibitory effect on the mitral cell. More neurotransmitter is released from the activated mitral cell to the connected dendrite of the granule cell, making the inhibitory effect from the granule cell to the activated mitral cell stronger than the surrounding 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. This aids in odor discrimination. Other research suggests that the lateral inhibition contributes to differentiated odor responses, which aids in the processing and perception of distinct odors. There is also evidence of cholinergic effects on granule cells that enhance depolarization of granule cells, making them more excitable and increasing the inhibition of mitral cells. This may contribute to an output from the olfactory bulb that would more closely resemble the glomerular odor map.
Olfaction is distinct from the other sensory systems where peripheral sensory receptors have a relay in the diencephalon. Therefore, the olfactory bulb plays this role for the olfactory system.

Accessory olfactory bulb

In vertebrates, 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. The vomeronasal organ sends projections to the accessory olfactory bulb making it the second processing stage of the accessory olfactory system. As in the main olfactory bulb, axonal input to the accessory olfactory bulb forms synapses with mitral cells within glomeruli. The accessory olfactory bulb receives axonal input from the vomeronasal organ, a distinct sensory epithelium from the main olfactory epithelium that detects chemical stimuli relevant for social and reproductive behaviors, but probably also generic odorants. It has been hypothesized that, in order for the vomeronasal pump to turn on, the main olfactory epithelium must first detect the appropriate odor. However, the possibility that the vomeronasal system works in parallel or independently from generic olfactory inputs has not been ruled out yet.
Vomeronasal sensory neurons provide direct excitatory inputs to AOB principle neurons called mitral cells. These are transmitted to the amygdala and hypothalamus and therefore are directly involved in sex hormone activity and may influence aggressiveness and mating behavior. Axons of the vomeronasal sensory neurons express a given receptor type which, differently from what occurs in the main olfactory bulb, diverge between 6 and 30 AOB glomeruli. Mitral cell dendritic endings go through a dramatic period of targeting and clustering just after presynaptic unification of the sensory neuron axons. The connectivity of the vomeronasal sensory neurons to mitral cells is precise, with mitral cell dendrites targeting the glomeruli. There is evidence against the presence of a functional accessory olfactory bulb in humans and other higher primates.
The AOB is divided into two main subregions, anterior and posterior, which receive segregated synaptic inputs from two main categories of vomeronasal sensory neurons, V1R and V2R, respectively. This appears as a clear functional specialization, given the differential role of the two populations of sensory neurons in detecting chemical stimuli of different types and molecular weights. However, it does not seem to be maintained centrally, where mitral cell projections from both sides of the AOB converge. A clear difference of the AOB circuitry, compared to the rest of the bulb, is its heterogeneous connectivity between mitral cells and vomeronasal sensory afferents within neuropil glomeruli. AOB mitral cells connect through apical dendritic processes of the glomeruli formed by afferents of different receptor neurons, thus breaking the one-receptor-one-neuron rule which generally holds for the main olfactory system. This implies that stimuli sensed through the VNO and elaborated in the AOB are subject to a different and probably more complex level of elaboration. Accordingly, AOB mitral cells show different firing patterns compared to other bulbar projection neurons. Additionally, top down input to the olfactory bulb differentially affects olfactory outputs.