Rostral olfactory cortex

Fig. 13. Basic olfactory network. Schematic of the networks linking the olfactory bulb and primary olfactory cortex. Olfactory nerve axons (ON) terminate in the glomeruli (glom) onto mitral (m) and tufted (t) cells which project via the lateral olfactory tract (LOT) to layer la of primary olfactory cortex to terminate on the dendrites of layer Il-III pyramidal (p) cells. Layer 11-111 pyramidal cells in rostral olfactory cortex project to layer Ib in caudal olfactory cortex and vice versa. Olfactory cortical pyramidal cells also send reciprocal projections back to the olfactory bulb. Thus olfactory bulb output is continuously modified by feedback from areas it targets. Inhibitory interneurons in olfactory bulb and olfactory cortex (shown in gray) modulate network function. Neurons in the ipsilateral (AONi) and contralateral anterior olfactory nuclei (AON) link olfactory networks in the two hemispheres via the anterior commissure.

The main olfactory bulb projects to a collection of structures referred to collectively as primary olfactory cortex (De Olmos et al. 1978). These structures may be usefully divided into three groups (A) the anterior olfactory nucleus (Fig. 15) (B) rostral olfac-... 

The main olfactory bulb sends a projection to the entire extent of piriform, peri-amygdaloid and lateral entorhinal cortex (see above. Outputs of MOB). This projection terminates in the superficial half of layer I, layer la. Both mitral and tufted cells project to the rostral parts of AON and piriform cortex while the projection to more caudal parts of olfactory cortex becomes progressively dominated by mitral cells (Schoenfeld and Macrides, 1984). 

Piriform cortex, lateral entorhinal cortex and the transitional cortical areas project heavily back to the olfactory bulb (Figs. 13,14, 18,19). The projections are heavier from the rostral than the caudal parts of primary olfactory cortex in rat and mouse (Shipley and Adamek, 1984). A few cells in the posterolateral and medial cortical amygdaloid areas may project to the MOB (Shipley and Adamek, 1984). These feedback projections to the olfactory bulb arise mainly from pyramidal neurons in layers II and III in primary olfactory cortex. 

Two classes of POC outputs were discussed above the feedback projection back to the olfactory bulb and the association connections between rostral and caudal olfactory cortex. A third class of outputs is treated separately because it represents the projections of POC to brain regions not generally included in the olfactory system per se although their receipt of inputs from POC obviously implicates these POC targets in olfactory function. The extrinsic outputs of POC are both to cortical and subcortical structures (Fig. 19). 

Fig. 35.1 Simplified diagram of chemosensory circuit in amygdala. Vomeronasal input via accessory olfactory bulb (VNO/ AOB) is analyzed in anterior and posterior medial amygdala (MeA, MeP). MeP appears to be inhibited by intercalated nucleus (ICNc) for heterospecific and artificial stimuli. MOE/ MOB Main olfactory epithelium/Main olfactory bulb. ACN Anterior Cortical Nucleus. PC Piriform Cortex. BLA Basolateral amygdala. ICNr rostral part of medial intercalated nucleus. ICNc caudal part of ICN. MPOA Medial Preoptic Area. VMH Ventro-medial hypothalamus...

The indusium griseum (IG) or dorsal hippocampal continuation receives input from but does not project to the olfactory bulb. It is a thin layer of cortex which runs parasagit-tally just dorsal to the corpus callosum. IG has been the subject of debate as to whether it is more related to the hippocampus or olfactory bulb (cf. Wyss and Sripanidkulchai, 1983 Adamek et al. 1984 for further discussion). It now seems clear that IG receives direct inputs to its tiny molecular layer from the olfactory bulb (Wyss and Sripanidkulchai, 1983 Adamek et al. 1984 De Olmos et al. 1978). This input is mainly to the rostral IG with fewer fibers running more caudally. The molecular layer of IG also receives input from the lateral and medial entorhinal cortex (Luskin and Price, 1983b). Since the entorhinal area receives direct olfactory bulb inputs and, in turn, projects to the dentate gyrus of the hippocampus it has been suggested that IG is a phylogenetically old part of the hippocampus that receives direct olfactory information as opposed to most of the hippocampus that receives only indirect olfactory input via the entorhinal area (Adamek et al. 1984). 

Piriform cortex (Fig. 15 C-D 17 A-F), also referred to as pyriform or prepyriform cortex, is a phylogenetically old, paleocortical structure. PC is located along the entire length of the lateral olfactory tract at the ventrolateral convexity of the base of the cortex. PC is thicker and more elaborate caudally than it is rostrally. PC is allocortical, having only two-three cellular layers and is thinner and less complex than the neocortex which has six layers. Haberly and Price (1978b) divided the piriform cortex into 3 layers that are further subdivided on the basis of cytoarchitecture and afferent connections. 

Commissural projections to the contralateral PC originate nearly exclusively from layer II neurons and travel in the anterior commissure [AC]. These projections innervate more posterior parts of the contralateral PC as well as nearby olfactory cortical sites (periamygdaloid cortex, lateral entorhinal cortex, anterior cortical nucleus, nucleus of the lateral olfactory tract) (Haberly and Price, 1978a,b). The caudally-directed commissural projections arise almost entirely from rostral layer lib neurons. However, there are shorter, less extensive commissural projections from caudal PC that target rostrally adjacent regions. This pathway arises mostly from deep layer III neurons although there is a modest contribution from layer II neurons. 

Figure 1. A. Sagittal section of the anterior telencephalon stained with antibody to olfactory marker protein. Ant=anterior AOB, AOB=accessory olfactory bulb, FCx=frontal cortex, MOB=main olfactory bulb, Post=posterior AOB. Rostral is to the left. B. Vomeronasal organ stained by the NissI method. Bv=blood vessel, Lu=lumen, S=septum arrows point to sensory epithelium.

---