Primary olfactory cortex

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- 

---

FIGURE 1.6 Modes of cell m igration to the cerebral cortex. Schematic drawing of the cerebral cortical wall of the rodent at about embryonic day 13.5. The arrows indicate the main routes of cell migration. Abbreviations CP, cortical plate Hi, hippocampal formation IZ, intermediate zone LGE, lateral ganglionic eminence MGE, medial ganglionic eminence MZ, marginal zone POC, primary olfactory cortex V, lateral ventricle VZ, ventricular zone Adapted from (de Carlos et al. (1996). 

Architecture of the MOB (A, B) and primary olfactory cortex (POC, C). A and B Coronal section (Nissl stain) of the rat MOB at low (A) and high (B) magnifications. C Coronal section through the rat brain showing several structures of the POC. Abbreviations ac, anterior commissure Acb, nucleus accumbens AONvp, anterior olfactory nucleus, ventroposterior division DP, dorsal peduncular cortex IL, infralimbic cortex Pir, piriform cortex SEL, subependymal layer Scale bar in A and C = 1 mm. Modified from Handbook of Chem. Neuroanat. Integrated Sys. CNS, Vol. 12, Part III, Chapter III, The Olfactory System, M. Shipley etal., pp. 469-573,1996, with permission from Elsevier... 

The basic circuitry of the MOB. Axons of ORNs travel in the ONL and synapse in the GL on the dendrites of mitrai ceiis (MC), tufted ceiis (externai tufted ceii, ET middie tufted ceii, MT), and generic juxtagiomeruiar (JG) neurons, which include perigiomeruiar ceiis (PG), ET ceiis, and short axon ceiis (SA). SA ceiis interconnect different giomeruii. There are serial and reciprocal synapses between the apicai dendrites of mitral/tufted cells and the processes of JG neurons. Superficial tufted cells (ST) are located in the superficial EPL or at the GL-EPL border. The lateral dendrites of mitral/tufted cells form serial and reciprocal synapses with the apical dendrites of granule cells (GC) in the EPL. GCs are located in the GCL and the MCL. The axons of mitral/tufted cells project locally to GCs (not shown) and also to primary olfactory cortex via the lateral olfactory tract (LOT). The bulb also contains other populations of interneurons neurons, including the van Gehuchten cells (VG) within the EPL... 

Major connections of the main olfactory system. Axons of MOB mitral/tufted cells (circles in the EPL and MCL, respectively) project as the LOT to synapse in a number of structures collectively referred to as primary olfactory cortex (POC). Centrifugal inputs to MOB include feedback projections from POC as well as inputs from subcortical forebrain and brainstem neuromodulatory cell groups. Abbreviations AON, anterior olfactory nucleus DP, dorsal peduncular cortex Ent, entorhinal cortex IG-AHC, indusium griseum-anterior hippocampal continuation LC, locus coeruleus NDB, nucleus of the diagonal band PeCo, periamygdaloid cortex PC, piriform cortex RN, raphe nuclei (dorsal and median raphe) TT, taenia tecta Tu, olfactory tubercle... 

Johnson DM, Illig KR, Behan M, Haberly LB. 2000. New features of connectivity in piriform cortex visualized by intracellular injection of pyramidal cells suggest that primary olfactory cortex functions like association cortex in other sensory systems. J Neurosci 20 6974-6982. 

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.

MOB projects to several structures of the ipsilateral hemisphere, including the superficial plexiform layer of the anterior olfactory nucleus, piriform, periamygdaloid and lateral entorhinal cortices, taenia tecta, the anterior hippocampal continuation, in-dusium griseum and the olfactory tubercle (Figs. 14, 18, 19). Collectively, the regions directly innervated by the output of the MOB have been referred to as primary olfactory cortex (De Olmos et al. 1978). Most of these projections have been reported in several species (for review, cf. Shipley and Adamek, 1984). A recent study has also shown a... 

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. 

Price, J.L. (1985) Beyond the primary olfactory cortex Olfactory-related areas in the neocortex, thalamus and hypothalamus. Chem. Senses, 10(2), 239-258. 

---