Neuro-circuits & Interneurons

Interestingly, the static (quantal) and dynamic (depression and facilitation) properties of facilitating synapses from single pyramidal neurons onto interneurons vary across layers126, which might cause targeted inhibitory cells in deep cortical layers to discharge before those in supragranular layers. Such layer-specific differences in the recruitment of interneurons could influence the direction of information flow in the cortical column.


Remarkably, all the synapses formed by one interneuron onto multiple pyramidal neurons show identical synaptic dynamics15. All the synapses from an interneuron onto all targets of the same type (pyramidal neurons in this case) seem to have identical release probabilities and time constants for recovery from synaptic depression and facilitation. This homogeneity principle contrasts sharply with the heterogeneity of glutamatergic synapses formed by a pyramidal neuron onto other pyramidal neurons and also has implications for the forms of learning that might shape these synapses. The absolute strength of these synaptic connections is heterogeneous (probably due to different numbers of synapses and/or postsynaptic receptors), indicating that they could be modified by the relative timing of activity in only one pair of neurons (presynaptic and postsynaptic), but their dynamics are homogeneous, suggesting that these parameters must be modified by the activity patterns of the entire population of postsynaptic pyramidal neurons relative to the single presynaptic interneuron.

Why balance Yang with Yin? Why does excitation need to be balanced with inhibition and why do transient moments of imbalance occur? This is a vast area, which will not be dealt with in this review, except to speculate on two potential reasons. At the level of individual neurons, matching inhibition as a function of stimulus intensity could allow information to be processed and encoded at a higher or lower temporal resolution, depending on the baseline firing rates. This can be achieved by changing the membrane time constant, which changes the time window for temporal integration161 (see also Ref. 162) and by changing the temporal precision of spike generation by adding high-frequency membrane 'noise'163, 164, 165. At this level, balance might be required to normalize the baseline for synaptic integration as a function of activity (to normalize the mutual information between the input channels) and spiking might reflect moments of imbalance (high mutual information between the input channels). At the microcircuit level, a sliding scale between integration and coincidence detection as a function of activity in each neuron could be important to control which neurons synchronize at which frequencies162. Balance might be required to keep all neurons independent (to normalize mutual information across neurons) and oscillations might reflect orchestrated momentary imbalances of groups of neurons (high mutual information between neurons). Needless to say, considerable work is required to test and turn theory into fact.

http://www.nature.com.ezproxy.hsclib.sunysb.edu/nrn/journal/v5/n10/full/nrn1519.html