The human motor cortex can be activated by transcranial magnetic stimulation (TMS) evoking a high-frequency repetitive discharge of corticospinal neurones. The exact physiologic mechanisms producing the corticospinal activity still remain unclear because of the complexity of the interactions between the currents induced in the brain and the circuits of cerebral cortex, composed of multiple excitatory and inhibitory neurons and axons of different size, location, orientation and function. The aim of current paper is to evaluate whether the main characteristics of the activity evoked by single- and paired-pulse and repetitive TMS, can be accounted by the interaction of the induced currents in the brain with the key anatomic features of a simple cortical circuit composed of the superficial population of excitatory pyramidal neurons of layers II and III, the large pyramidal neurons in layer V, and the inhibitory GABA cells. This circuit represents the minimum architecture necessary for capturing the most essential cortical input-output operations of neocortex. The interaction between the induced currents in the brain and this simple model of cortical circuitry might explain the characteristics and nature of the repetitive discharge evoked by TMS, including its regular and rhythmic nature and its dose-dependency and pharmacologic modulation. The integrative properties of the circuit also provide a good framework for the interpretation of the changes in the cortical output produced by paired and repetitive TMS.
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