The circuits of the olfactory bulb. The exception as a rule

The connectivity of the neurons of the olfactory bulb is highly idiosyncratic and constitutes an exception to the general plan of how neurons, and especially cortical neurons, construct circuits. The majority of synaptic contacts in the circuits of the cortex are axo-dendritic. In these contacts, the axon is the presynaptic element, which transmits the signal, and the dendrite is the postsynaptic element, which receives the signal. However, the majority of synaptic contacts in the circuits of the olfactory bulb are dendro-dendritic. In fact, most of the neurons of the olfactory bulb lack an axon. Moreover, a high percentage of the dendro-dendritic synapses are reciprocal. This means that the roles of presynaptic and postsynaptic element are not clearly defined, in clear contrast with the universality of unidirectional synaptic transmission in the cortex and elsewhere in the central nervous system. In this review, we analyze and discuss some peculiarities of the circuits of the olfactory bulb.