Anterior Olfactory Nucleus

Characteristics The olfactory system is highly developed in vertebrates, particularly in “macrosmic” mammals (such as many rodents) where olfaction is the primary sensory system used to navigate in the world. As olfactory ability has evolved, there has been a concurrent increase in the size and complexity of cortical structures dedicated to encoding and deciphering olfactory information (▶cortical circuitry). Below is an overview of one of these cortical regions, the ▶anterior olfactory nucleus (AON), which plays a central, though largely uncharted role in olfactory information processing. In vertebrates, odor information is transduced by olfactory receptor neurons that line the nasal cavity (or olfactory rosette in fishes). The information is sent via the olfactory nerve (CranialNerve I) to the olfactory bulb, an evagination of the ventral forebrain that is the rostralmost portion of the telencephalon and is commonly found just behind (or above) the nasal cavity. The general circuitry of the olfactory bulb is similar to that of the retina in that a) incoming sensory information is parsed into separate data streams, and b) it contains two layers of inhibitory interactions to reinforce the differences between these data channels. Axons of the two major output neurons, themitral and tufted cells, travel caudally and ventrally in the bulb, coalescing to form the lateral olfactory tract (LOT) that courses along the ventrolateral surface of the forebrain. The area directly behind the olfactory bulb is often referred to as the ▶olfactory peduncle (or retrobulbar area, Fig. 1). The peduncle contains the anterior olfactory nucleus (AON) as well as two other much smaller regions, the tenia tecta (or dorsal hippocampal rudiment) and the dorsal penduncular cortex. The olfactory peduncle merges caudally with the▶olfactory tubercle on the medial side and with the piriform cortex laterally. The core of the peduncle is formed by a subependymal layer that is continuous with the rostral extension of the lateral ventricle, and comprises the “rostral migratory stream” that provides new interneurons to the olfactory bulb throughout life. Overlying the rostral migratory stream is the anterior or olfactory limb of the anterior commissure, the source of centrifugal fibers, including some from the medial forebrain bundle, to the region.