Figure 1.

A schematic view of the model of corticospinal volley generation based on canonical cortical circuit proposed by Douglas et al. (1989). This model includes the superficial population of excitatory pyramidal neurons of layers II and III (P2-P3), the large pyramidal tract neurons in layer V (P5), and the inhibitory GABA cells [modified from Figure 1.14 in “The Synaptic Organization of the Brain” (Shepherd, 2004)]. Electrical anodal stimulation activates the axons of P5 cells evoking a D wave. Magnetic stimulation with a latero-medial (LM) induced current in the brain produces a direct activation of the axons of corticospinal cells evoking the D wave followed by an I1 wave produced by monosynaptic activation of P5 cells by the axons of superficial pyramidal neurons, at high intensities it also produces a recurrent activity in the circuit composed of the layer II and III and layer V pyramidal neurons together with their connections with local GABAergic interneurons (red ellipse and arrows) evoking late I-waves. Magnetic stimulation with a posterior-anterior (PA) induced current in the brain evokes the I1 wave and, at higher intensities, late I-waves. Magnetic stimulation with an anterior-posterior (AP) induced current in the brain recruits small and delayed descending volleys with slightly different peak latencies and longer duration than those seen after posterior to anterior magnetic stimulation. It is proposed that this more dispersed descending activity is produced by a more complex circuit (green dotted ellipse and arrows) that might include cortico-cortical fibers originating from the premotor cortex and projecting upon the motor cortex circuits generating the I-waves.