An Electrophysiological Analysis of the Primary Input to the Cat Prepyriform Cortex

Properties of the lateral olfactory tract (LOT) action potential and cortical evoked potential (EPSP) were studied using discrete stimuli applied to the tract in acute and chronic preparations. Stimulus response curves for both presynaptic (action) and postsynaptic (EPSP) potentials were sigmoidal and linearly related over the entire range. Simulated action potentials were used to determine the diameter to amplitude relationship, an intermediate step for finding the threshold as a function of tract axon diameter. Our results indicate that threshold is proportional to (diameter) -3.4 instead of (diameter) -1 as found by others in frog peripheral nerve. Further work is required to test the validity of this method. The tract membrane time constant determined from strength-duration data was 0.3 msec. Divergent and convergent LOT to cortex pathways have been reported. We found that a given cortical cell could be activated equally well from sites anterior or posterior to the recording electrode. This supports the anatomical work of others showing collaterals leaving the tract to synapse with cortical cells, with the main axon sending collaterals along the length of the tract (divergence). Neurons were also activated with similar thresholds at points across the tract trajectory, indicative of convergence from several axons. Antidromic axons had punctate threshold points. Centrifugal axons originating from cortical neurons were found in the olfactory tract. To show this, middle prepyriform cortical neurons were antidromically driven by peduncular volleys. Antidromic cells had much shorter latencies than orthodromically driven cells (Kolmogorov- Smirnov Test). Further, antidromic cells were found slightly above the superficial pyramidal cell layer or in it, the area thought to be the source of centrifugal fibers because of retrograde degeneration following bulbar lesions. The existence of centrifugal fibers points to the possibility of cortical control over olfactory information processed in the bulb. Orthodromically driven cells were classified into firing pattern groups similar to those described in previous studies. Temporal firing patterns determined by poststimulus time histograms and their integrals were used to further separate these cells into subclasses. Generally, spontaneously active cells were inhibited by LOT volleys, the inhibitory period lengthening with increased stimulus intensity. Two recovery patterns were observed, a gradual smooth recovery to spontaneity and a rebound type. Modifications or additions to wiring diagrams presented by others are not suggested in this study