Local connections of excitatory neurons in motor-associated cortical areas of the rat

Abstract

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 motorassociated 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 corelikeTC 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, theTC 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 theTC recurrent circuit and may serve as the basis of autonomous activity of the neocortex.

Non-invasive brain stimulation as a tool to study cerebellar-M1 interactions in humans

“…We know that facilitatory effects can have a rapid onset, which is consistent with the known duration of the rising phase of a cortical (extrastriate and thalamocortical) excitatory post-synaptic potentials (EPSP, 1–2 ms: [12–14]). There are no comparable ways to estimate how rapidly removal of ongoing facilitation could take effect.…”

Direction- and distance-dependent interareal connectivity of pyramidal cell subpopulations in the rat frontal cortex

“…Therefore, when both L5a COM and CPn cells in M2 project to another cortical area, two types of M2 L5 activity may be transferred independently into the local circuit of the target cortex. L5a CPn cells in the source area send axon collaterals to L1a in the iCC target area, where they interact with axon collaterals in L1a arising from L5 CPn cells in the target area (Thomson and Bannister, 2003), as well as with thalamocortical innervations relaying basal ganglia outputs that heavily terminate in L1a (Kuramoto et al, 2009, 2011; Rubio-Garrido et al, 2009; Kaneko, 2013). Similarly, axon collaterals of L5a COM cells in the source area would interact with those in the target area at L1b and L2/3.…”

Learning in the Rodent Motor Cortex

“…The differential projection patterns of these two thalamic recipient zones suggest that information from the basal ganglia and cerebellum may take different routes through the motor cortex and partake in different types of computation. The motor cortex is therefore in a prime position to integrate motor signals corresponding to particular functional roles, for example, using basal ganglia information to guide movement selection and cerebellar information to initiate or shape ongoing movements (Kaneko 2013). Inputs from other cortical areas also arrive with characteristic laminar profiles.…”

“…These patterns of connectivity may represent parallel pathways to drive motor cortex output neurons in deep layers. One route originates from the thalamus and posterior cortical areas and arrives via the highly recurrent superficial layers of the motor cortex, whereas the other arrives directly from frontal cortical areas and the thalamus, bypassing the superficial layers (Hooks et al 2013, Kaneko 2013). The deep corticofugal neurons are themselves laminarly organized, with intratelencephalic-type neurons distributed from layers 5a to layer 6, pyramidal tract–type neurons restricted to layer 5b, and corticothalamic-type neurons restricted to layer 6 (Harris & Shepherd 2015).…”

“…Regarding afferents, investigators are greatly interested in determining how, for example, inputs from the basal ganglia– and cerebellum-recipient zones of the thalamus are routed through the motor cortex, how the motor cortex uses those inputs to guide movement, and how those connections change with learning (Kaneko 2013). Likewise, the relationship between activity of the motor thalamus and movement and how it may change with learning are largely unknown (Goldberg et al 2013, Sommer 2003).…”