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NEUROSCIENCE |
1 Abteilung Zellphysiologie, Max-Planck-Institut für medizinische Forschung, Jahnstr. 29, D-69120 Heidelberg, Germany
2 Institut für Neurowissenschaften und Biophysik-Medizin, Forschungszentrum Jülich, D-52425 Jülich, Germany
3 Institut für Anatomie und Zellbiologie der Albert-Ludwigs-Universität Freiburg, Albertstraße 17, D-79104 Freiburg i.Br, Germany
Synaptically coupled layer 2/3 (L2/3) pyramidal neurones located above the same layer 4 barrel (‘barrel-related’) were investigated using dual whole-cell voltage recordings in acute slices of rat somatosensory cortex. Recordings were followed by reconstructions of biocytin-filled neurones. The onset latency of unitary EPSPs was 1.1 ± 0.4 ms, the 20–80% rise time was 0.7 ± 0.2 ms, the average amplitude was 1.0 ± 0.7 mV and the decay time constant was 15.7 ± 4.5 ms. The coefficient of variation (C.V.) of unitary EPSP amplitudes decreased with increasing EPSP peak and was 0.33 ± 0.18. Bursts of APs in the presynaptic pyramidal cell resulted in EPSPs that, over a wide range of frequencies (5–100 Hz), displayed amplitude depression. Anatomically the barrel-related pyramidal cells in the lower half of layer 2/3 have a long apical dendrite with a small terminal tuft, while pyramidal cells in the upper half of layer 2/3 have shorter and often more ‘irregularly’ shaped apical dendrites that branch profusely in layer 1. The number of putative excitatory synaptic contacts established by the axonal collaterals of a L2/3 pyramidal cell with a postsynaptic pyramidal cell in the same column varied between 2 and 4, with an average of 2.8 ± 0.7 (n = 8 pairs). Synaptic contacts were established predominantly on the basal dendrites at a mean geometric distance of 91 ± 47 µm from the pyramidal cell soma. L2/3-to-L2/3 connections formed a blob-like innervation domain containing 2.8 mm of the presynaptic axon collaterals with a bouton density of 0.3 boutons per µm axon. Within the supragranular layers of its home column a single L2/3 pyramidal cell established about 900 boutons suggesting that 270 pyramidal cells in layer 2/3 are innervated by an individual pyramidal cell. In turn, a single pyramidal cell received synaptic inputs from 270 other L2/3 pyramidal cells. The innervation domain of L2/3-to-L2/3 connections superimposes almost exactly with that of L4-to-L2/3 connections. This suggests that synchronous feed-forward excitation of L2/3 pyramidal cells arriving from layer 4 could be potentially amplified in layer 2/3 by feedback excitation within a column and then relayed to the neighbouring columns.
(Received 10 January 2006;
accepted after revision 21 June 2006;
first published online 22 June 2006)
Corresponding author D. Feldmeyer: Institut für Neurowissenschaften und Biophysik, AG Zelluläre Neurobiologie-Medizin, Forschungszentrum Jülich GmbH, Leo-Brandt-Straße, D-52425 Jülich, Germany. Email: d.feldmeyer{at}fz-juelich.de
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