Figure 1.

Schematic pathway of the main neuronal connections in OB. In the nasal cavity, airflow transports different odorants (represented by different colours) to specific olfactory receptors expressed on olfactory sensory neurons (OSNs) localized in the olfactory epithelium (OE). Olfactory information is then transduced in an electrical signal to later reach the olfactory bulb (OB). Olfactory sensory neurons send their axons through the lamina cribrosa to reach the core of corresponding glomeruli, which consists of spherical structures located in the olfactory bulb. Axons of OSNs, expressing the same odorant receptors, are projected into the same glomeruli, so each glomerulus is linked to a single odorant receptor. Within the glomerular core, OSNs axons make synapses with apical dendrites of projections corresponding to OB neurons (Mitral and Tufted cells). On the surface of glomeruli are located periglomerular cells (PG). These cells project their dendrite into the glomerulus and make reciprocal synapses with projections neurons and some of them with axons of OSNs. Based on their synaptic connections with axons of OSNs, two subtypes of PG cells are described. Type 1 PG cells, belonging to dopaminergic PG cells, are synaptically connected both with axons of OSNs and dendrites of other neurons in the olfactory bulb. On the other hand, type 2 PG cells, belonging to calretinin PG cells, are not synaptically connected with axons of OSNs. The figure shows the three subtypes of DA cells present in the glomerulus area according to Nagayama et al. (2014) and Kosaka et al. (2020). The PG-DAs are represented in green. The large ones that have an axon and dendrites that enter more glomeruli and the small ones that in most cases enter one or a few glomeruli and do not have an axon have been represented. The ET cells and those belonging to the subtype of TH⁺ sSA were represented too.