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The Journal of Neuroscience, August 9, 2006, 26(32):8267-8277; doi:10.1523/JNEUROSCI.2152-06.2006
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Cellular/Molecular
Background Synaptic Activity Is Sparse in Neocortex
Jack Waters andFritjof Helmchen Abteilung Zellphysiologie, Max-Planck-Institut für Medizinische Forschung, 69120 Heidelberg, Germany
Correspondence should be addressed to Jack Waters, Department of Physiology, Feinberg School of Medicine, Northwestern University, 303 East Chicago Avenue, Chicago, IL 60611. Email: jackwaters{at}northwestern.edu
Neurons are continually exposed to background synaptic activity in vivo. This is thought to influence neural information processing, but background levels of excitation and inhibition remain controversial. Here we show, using whole-cell recordings in anesthetized rats, that spontaneous depolarizations ("Up states") in neocortical pyramidal neurons are driven by sparse, mostly excitatory synaptic activity (less than five inputs per millisecond;
10% inhibitory). The mean synaptic conductance change is small (<10 nS at the soma) and opposed by anomalous rectification, resulting in a net increase in input resistance during Up states. These conditions enhance the effectiveness of each synapse at depolarized potentials. Hence, neocortical networks are relatively quiet at rest, and the effect of synaptic background is weaker than previously thought.
Key words: pyramidal cell; summation; patch clamp; dendrite; synaptic integration; dendrite
Received May 21, 2006; revised June 29, 2006; accepted July 3, 2006.
Correspondence should be addressed to Jack Waters, Department of Physiology, Feinberg School of Medicine, Northwestern University, 303 East Chicago Avenue, Chicago, IL 60611. Email: jackwaters{at}northwestern.edu
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