Fig. 9.

Possible neural circuit to account for the interactions between CS_AP and CS_PA and other forms of cortical inhibition. Neuron P is a large pyramidal cell in the motor cortex with an axon projecting to spinal cord. It receives excitatory (open circles) and inhibitory (filled circles) synaptic inputs, representing I-wave inputs and SICI, respectively. Following recent modelling (Aberra et al. [6]), we assume the lowest threshold sites of activation are presynaptic terminals. The lightning bolts represent the sites activated by subthreshold CS_PA and CS_AP. The former activates excitatory inputs to a GABAa-ergic inhibitory interneuron (B) that causes SICI_CSPA. The latter activates synapses in a more rostral location (e.g., premotor cortex) that activate a different set of GABAa-ergic inhibitory interneurons (A) that produce SICI_CSAP. Neuron B also receives excitatory and inhibitory inputs from SAI and CBI that are responsible for the interaction of these forms of inhibition with SICI_CSPA. However, they have no direct effect on neuron A which mediates SICI_CSAP. Higher intensities of TMS activate excitatory inputs: PA-TMS activates I1-wave input to the proximal portion of neuron P at a site where it is relatively unaffected by dendritic SICI. Later I-wave inputs, activated by PA-TMS or AP-TMS, are shown as targeting dendritic sites near to the SICI inputs.