A Sparse View Of Cortical Activity

This week, Waters and Helmchen examined the ups and downs of neuronal activity, using in vivo whole-cell recording from L2/3 neocortical pyramidal neurons in anesthetized rats. Spontaneous activity in these cells drove 10-20 mV membrane excursions between subthreshold "Up states" and hyperpolarized "Down states." Although depolarizing excitatory synaptic potentials might be expected to open glutamate-activated ion channels and increase membrane conductance (i.e., decrease input resistance), the input resistance of layer 2/3 cells actually increased during Up states. Constant current injections confirmed a voltage-dependent increase in the membrane input resistance with depolarization, attributable to intrinsic membrane channels that cause anomalous rectification. Under the conditions of their experiments, spontaneous activity was 90% excitatory, but sparse. Using a compartmental model of L2/3 cells to control the rate of activity and synchrony of synaptic inputs, the authors predict that as few as 10 or as many as 100 activated synapses are required to fire an action potential.

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Jack Waters and Fritjof Helmchen

Source: News tips from the Journal of Neuroscience

Contact: Sara Harris
Society for Neuroscience
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A Sparse View Of Cortical Activity