In spite of recent progress in brain sciences, the local circuit of the cerebral neocortex, including motor areas, still remains elusive. Morphological works on excitatory cortical circuitry from thalamocortical (TC) afferents to corticospinal neurons (CSNs) in motor-associated areas are reviewed here. First, TC axons of motor thalamic nuclei have been re-examined by the single-neuron labeling method. There are middle layer (ML)-targeting and layer (L) 1-preferring TC axon types in motor-associated areas, being analogous to core and matrix types, respectively, of Jones (1998) in sensory areas. However, the arborization of core-like motor TC axons spreads widely and disregards the columnar structure that is the basis of information processing in sensory areas, suggesting that motor areas adopt a different information-processing framework such as area-wide laminar organization. Second, L5 CSNs receive local excitatory inputs not only from L2/3 pyramidal neurons but also from ML spiny neurons, the latter directly processing cerebellar information of core-like TC neurons (TCNs). In contrast, basal ganglia information is targeted to apical dendrites of L2/3 and L5 pyramidal neurons through matrix TCNs. Third, L6 corticothalamic neurons (CTNs) are most densely innervated by ML spiny neurons located just above CTNs. Since CTNs receive only weak connections from L2/3 and L5 pyramidal neurons, the TC recurrent circuit composed of TCNs, ML spiny neurons and CTNs appears relatively independent of the results of processing in L2/3 and L5. It is proposed that two circuits sharing the same TC projection and ML neurons are embedded in the neocortex: one includes L2/3 and L5 neurons, processes afferent information in a feedforward way and sends the processed information to other cortical areas and subcortical regions; and the other circuit participates in a dynamical system of the TC recurrent circuit and may serve as the basis of autonomous activity of the neocortex.
![Power-law ISI distributions of in vivo motor cortex neurons in different layers. Juxta-cellular visualization and double-logarithmic plots of the ISI histograms (blue curves) of pyramidal neurons (A) and fast-spiking interneurons (B) recorded in layers 3, 4, 5, and 6 of rat motor cortex. The plots were fitted by neuron-dependent beta-2 distributions (red), and light micrographs display the morphological reconstruction of the eight neurons obtained by DAB (3,3′-diaminodbenzidine)-Nickel staining. The four neurons in (B) expressed parvalbumin (PV), a fast-spiking interneuron specific marker (blue: PV, green: biocytin or Neurobiotin). Inset of each panel represents the ISI distributions constructed from the 1st (black) and 2nd (green) halves of the same spike train. They prove the stationarity of the ISI distributions during the recordings. [Modified from Tsubo et al. (2012)].](https://www.frontiersin.org/_rtmag/_next/image?url=https%3A%2F%2Fwww.frontiersin.org%2Ffiles%2FArticles%2F42082%2Ffncir-07-00085-HTML%2Fimage_m%2Ffncir-07-00085-g005.jpg&w=3840&q=75)
![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.](https://www.frontiersin.org/_rtmag/_next/image?url=https%3A%2F%2Fwww.frontiersin.org%2Ffiles%2FArticles%2F39963%2Ffncir-07-00018-HTML%2Fimage_m%2Ffncir-07-00018-g001.jpg&w=3840&q=75)
