1. The piriform cortex of the opossum has been studied with intracellular recording and staining techniques. The experiments were designed to investigate the association fiber system, but the results have also revealed new properties of the afferent fiber system from the olfactory bulb and the inhibitory systems within the piriform cortex. 2. Following shock stimulation of the lateral olfactory tract (LOT), the response of pyramidal cells consists of an initial excitatory postsynaptic potential (EPSP) followed by a long-lasting inhibitory postsynaptic potential (IPSP). 3. The LOT-evoked EPSP consists of two components an initial monosynaptic followed by a disynaptic component. The monosynaptic EPSP can be isolated by the use of conditioning LOT shocks to block the IPSP and disynaptic EPSP. The disynaptic EPSP can be demonstrated by cutting LOT fibers at the surface of the cortex to eliminate the monosynaptic EPSP and by the use of bicuculline to block the IPSP. The latency of the IPSP is sufficiently brief so that the disynaptic EPSP is blocked at presumed intrasomatic recording sites unless these experimental manipulations are carried out. 4. In all histologically verified pyramidal cells in both layers II and III in which the appropriate tests were carried out, both mono- and disynaptic EPSP components were present. It was concluded on the basis of anatomical considerations, however, that a small number of pyramidal cells would be expected to receive only a disynaptic EPSP. 5. Evidence that the LOT-evoked disynaptic EPSP is mediated, at least in part, by association axons was provided by direct stimulation of these fibers in layer III and by demonstrating that the EPSP is present distal to cuts that sever LOT axons. 6. Direct stimulation of association axons in layer III evokes both a monosynatpic EPSP and a disynaptic IPSP in pyramidal cells at similar latencies. This IPSP is indistinguishable in properties from that evoked by LOT stimulation. Indirect evidence indicates that it is mediated via both feedforward and feedback mechanisms. In most neurons the association fiber-evoked EPSP is masked by the IPSP in response to single deep shocks but can be demonstrated by blocking the IPSP with a preceding LOT shock or by application of bicuculine. 7. Intracellular injection of horseradish peroxidase (HRP) revealed that pyramidal cell axons give rise to an extensive system of local colaterals with a large number of synaptic terminal-like swellings in layer III. It is postulated that these collaterals synapse on both pyramidal and nonpyramidal cells. At least some of the local collaterals represent the initial portions of association axons since pyramidal cells could be antidromically driven from distant points in the cortex. 8. It is concluded that stimulation of the lateral olfactory tract evoked three excitatory processes in both superficial and deep pyramidal cells via different dendritic segments: a) a monosynaptic EPSP in distal apical segments directly from the LOT fibers, b) a disynaptic EPSP concentrated in basal dendrites from local axon collaterals of pyramidal cells and c) a disynaptic EPSP evoked by long association axons that is concentrated in proximal apical segments. 9. While previous models for the piriform cortex have assumed that the IPSP evoked in pyramidal cells by LOT stimulation is generated by an exclusively feedback mechanisms, the brief latency of this potential suggests that there is also a feedforward component. This hypothesis is supported by recent anatomical findings. 10. Evidence is provided for inhibitory interactions between inhibitory interneurons. This mutual inhibition appears to be stronger following stimulation of LOT fibers than following stimulation of association fibers. 11. New evidence concerning the origin of the different components of the LOT-evoked field potential is discussed. 12. Parallels between the structure and physiology of the piriform cortex and higher order neocortical areas are discussed. 13. Speculations concerning the role of the association fiber system in the olfactory discrimination process are presented.
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