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Received December 14, 2004
Revised January 10, 2005
Accepted after revision January 27, 2005
1 Dept of Neurobiology and The Center for Neural Computation, The Hebrew University, Jerusalem
2 Computation and Neural Systems Program, California Institute of Technology
3 Dept of Neurobiology and The Center for Neural Computation, The Hebrew University, Jerusalem
4 Dept of Neurobiology, The Hebrew University, Jerusalem
* To whom correspondence should be addressed. E-mail: yarom{at}vms.huji.ac.il.
Neurons are noisy elements. Noise arises from both intrinsic and extrinsic sources, and manifests itself as fluctuations in the membrane potential. These fluctuations limit the accuracy of a neuron's output but have also been suggested to play a computational role. We present a detailed study of the amplitude and spectrum of voltage noise recorded at the soma of layer IV-V pyramidal neurons in slices taken from rat neocortex. The dependence of the noise on holding potential, synaptic activity and Na+ conductance is systematically analyzed. We demonstrate that voltage noise increases non-linearly as the cell depolarises (from 0.19 mV SD at -75 mV to 0.54 mV SD at -55 mV). The increase in voltage noise is accompanied by an increase in the cell impedance, due to voltage-dependence of Na+ conductance. The impedance increase accounts for the majority (70%) of the voltage noise increase. The increase in voltage noise and impedance is restricted to the low-frequency range (0.2-2 Hz). At the high frequency range (5-100 Hz) the voltage noise is dominated by synaptic activity. In our slice preparation, synaptic noise has little effect on the cell impedance. A minimal model reproduces qualitatively these data. Our results imply that ion channel noise contributes significantly to membrane voltage fluctuations at the subthreshold voltage range, and that Na+ conductance plays a key role in determining the amplitude of this noise by acting as a voltage-dependent amplifier of low-frequency transients.
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