Consequences of correlated synaptic bombardment on the responsiveness of neocortical pyramidal neurons
Introduction
The companion paper [7] has established that the highly fluctuating background activity present in vivo necessary implies the presence of correlations in the release of excitatory and inhibitory synapses. We have analyzed here consequences of this type of activity on the response of pyramidal neurons.
The first consequence of correlated synaptic activity was that the neuron did not respond reliably, thus leading to a probabilistic description of its response. This finding is consistent with in vivo observations. Computing the probability of firing in different conditions showed another consequence: correlated synaptic bombardment enhances the response of the cell, contrary to results previously reported [1], [2], [8], [9].
We begin by showing the probabilistic response of the cell, then we show how to compute such a probability and finally, we illustrate the enhancement of responsiveness induced by correlated synaptic activity.
Section snippets
Methods
The model used in this paper was described in the companion paper and in Ref. [6]. An AMPA-mediated synaptic input of uniform density of conductance in all the dendrites was simulated in addition to the background activity. This second population of synapses is used to stimulate the neuron without disrupting the background activity.
Results
In layer VI pyramidal neuron used in the simulations, synaptic background activity is simulated by Poisson-distributed random release events at glutamatergic and GABAergic synapses [6], [7]. The frequency of release is 1 Hz at glutamatergic synapses and 5.5 Hz at GABAergic synapses.
When the release events are uncorrelated, the model does not demonstrate the typical characteristics of in vivo recordings. The membrane potential undergoes only small fluctuations and no action potentials are
Discussion
Neocortical pyramidal neurons receive 5000–60 000 synapses [4], [5], 70% of them being from other neocortical neurons. It is therefore possible that such an intense connectivity leads to correlations in the afferent inputs. It has indeed been shown that there is an average correlation of 0.12 measured between pairs of neurons in the cerebral cortex of behaving monkeys [10]. There is, however, no proof that the correlations are induced specifically by the structure of the neocortex. This could be
Acknowledgements
Research supported by grants from the Medical Research Council of Canada (MT-13724) and the National Institutes of Health (R01-NS37711). H.K. has been supported by NSERC Canada. N.H. acknowledges a fellowship from Centre de Recherches en Neurobiologie.
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