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Cerebral Cortex Advance Access originally published online on June 4, 2007
Cerebral Cortex 2008 18(2):397-406; doi:10.1093/cercor/bhm074
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© The Author 2007. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Monosynaptic Connections between Pairs of L5A Pyramidal Neurons in Columns of Juvenile Rat Somatosensory Cortex

Andreas Frick1, Dirk Feldmeyer1,2, Moritz Helmstaedter1 and Bert Sakmann1

1 Abteilung Zellphysiologie, Max-Planck-Institut für Medizinische Forschung, D-69120 Heidelberg, Germany, 2 Department of Medicine, Research Centre Juelich, Institute of Neuroscience and Biophysics INB-3, D-52425 Juelich, Germany

Address correspondence to Andreas Frick, PhD, Max-Planck-Institut für Medizinische Forschung, Abteilung Zellphysiologie, Jahnstrasse 29, 69120 Heidelberg, Germany. Email: andreas.frick{at}mpimf-heidelberg.mpg.de.

Layer 5 (L5) of somatosensory cortex is a major gateway for projections to intra- and subcortical brain regions. This layer is further divided into 5A and 5B characterized by relatively separate afferent and efferent connections. Little is known about the organization of connections within L5A of neocortical columns. We therefore used paired recordings to probe the anatomy and physiology of monosynaptic connections between L5A pyramidal neurons within the barrel columns of somatosensory cortex in acute slices of ~3-week-old rats. Post hoc reconstruction and calculation of the axodendritic overlap of pre- and postsynaptic neurons, together with identification of putative synaptic contacts (3.5 per connection), indicated a preferred innervation domain in the proximal dendritic region. Synaptic transmission was reliable (failure rate <2%) and had a low variability (coefficient of variation of 0.3). Unitary excitatory postsynaptic potential (EPSP) amplitudes varied 30-fold with a mean of 1.2 mV and displayed depression over a wide range of frequencies (2–100 Hz) during bursts of presynaptic firing. A single L5A pyramidal neuron was estimated to target ~270 other pyramidal neurons within the same layer of its home barrel column, suggesting a mechanism of feed-forward excitation by which synchronized single action potentials are efficiently transmitted within L5A of juvenile cortex.

Key Words: barrel cortex • cortical connectivity • layer 5A • short-term dynamics • synaptic transmission


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