, 2009 and Gentet et al., 2010); and (3) the inherent bias in extracellular recordings which require neurons to fire action potentials before they can be considered in the data set
(cells that do not fire or fire very rarely cannot be detected). Future experiments must directly investigate whether firing rates (and firing correlations) differ depending upon the behavioral conditions, for example running versus stationary (Niell and Stryker, 2010) and/or the complexity of the sensory input and the environment (multiple whiskers contacting textured objects compared to single whisker contacts with simple objects). Under our recording conditions we find a highly skewed distribution of spiking activity in layer 2/3 barrel cortex neurons during active touch, which leads to an interesting unresolved issue of sparse coding regarding the relative importance of the very few neurons that reliably fire many action potentials compared Lenvatinib ic50 to the very many neurons that fire few action potentials. We found that sparse action potential firing during active touch appeared to be enforced by the hyperpolarized reversal potential of the touch response. Indeed, we found close to linear relationships in individual neurons between PSP amplitude
and precontact membrane potential with reversal potentials usually hyperpolarized with respect to action potential threshold. If the precontact Protease Inhibitor Library order membrane potential is spontaneously depolarized above this reversal potential, then the touch response is hyperpolarizing, therefore in fact playing an inhibitory role
by preventing the membrane potential from reaching action potential threshold. Sitaxentan Each neuron has its own cell-specific reversal potential for the touch response. Importantly, we found a strong positive correlation of the touch-evoked firing probability with the reversal potential (Figure 5F). Indeed, the only neuron in our study that fired reliably during active touch was also the only neuron with a touch reversal potential above action potential threshold. The generally hyperpolarized reversal potentials suggest a prominent inhibitory GABAergic contribution to the active touch responses, since the reversal potential for glutamatergic excitatory postsynaptic potentials is close to 0 mV and the reversal potential for GABAergic inhibitory postsynaptic potentials is generally estimated between −70 and −90 mV. We find that GABAergic neurons are strongly recruited during active touch and they are therefore likely to contribute to driving the hyperpolarized reversal potential of the touch PSP, thus preventing the membrane potential from crossing action potential threshold for most neurons during active touch. Our results are consistent with the simple idea that active touch for a given cell evokes a well-defined mixture of excitatory and inhibitory conductances, which drive the membrane potential toward a specific reversal potential.