Interactive reportThe hippocampal lamella hypothesis revisited1
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A preparation for studying connectivity principles
The hippocampal formation is a useful preparation in which to study general neuroscience problems. One reason is that the hippocampal formation demonstrates a number of organizational principles. First, between the narrow strip-formed cortical subdivision of the hippocampal formation, there is a one-directional connectivity between the principal cells of each strip. Second, there is a remarkable stratification of afferent fibres such that their synapses often segregate to specific dendritic
Quantitative connectivity data are wanted
Can the lamella idea survive such a set of well conducted studies? We feel that it can, focussing upon the necessity to apply a numerical analysis of the relevant synaptic connectivity and thus offer a re-interpretation of the reported morphological results. Although these findings do indicate a wide distribution of the CA3-to-CA1 axonal projection, they are not easily quantified. Specifically, this approach can not tell the exact proportion of axonal branches from a single CA3 cell which pass
A new approach
Although we feel that the discrepancy between the results from modern morphological studies and the lamella hypothesis is less than expressed by some authors, we appreciate that more precise data on Schaffer collateral fibres and their properties is desirable. Therefore, we now present a set of new data with direct recording of the density of Schaffer collateral branches and of the excitatory synaptic signals generated by these fibres. These observations are made in a set of horizontal slices
Schaffer collateral signals
Following blockade of excitatory synaptic potentials with cyanoquinoxaline (CNQX, 10 μM) and amino-phosphono-pentanoic acid (AP5, 50 μM), the remaining signal was due to conduction along axons. The signals were usually triphasic positive–negative–positive, although the last component was often quite small (Fig. 1A). Following stimulation of a small group of CA3 cells, axonal action potentials were recorded from several positions, the main subdivisions being the fimbria, the longitudinal
Possible functional consequences of the lamellar organization
The original lamella concept was derived from studies of anaesthetized rabbits. The direction of the engaged axons was estimated from antidromic and orthodromic field potentials. Characteristically, orthodromically activated synaptic potentials were recorded in a nearly transverse strip with a width of a few millimeters. Depending upon the stimulus strength, the number of excited axons would vary and, consequently, the width of the excited strip of tissue.
Recently, Hampson et al. [6], recorded
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Published on the World Wide Web on 12 October 2000.