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 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.
Highlights
OF LOCAL CIRCUIT ANALYSIS IN THE MOTOR-ASSOCIATED AREAS The local excitatory connection of the rodent neocortex has been initially examined by the combination of intracellular recording and focal electrical stimulation (Connors et al, 1982; ChagnacAmitai and Connors, 1989; Sutor and Hablitz, 1989; Silva et al, 1991; Hwa and Avoli, 1992)
Researchers may like to activate neuronal cell bodies and their local axon collaterals in the focal stimulation site, but afferent axons from thalamic nuclei and other cortical areas can be activated. This uncertainty has been removed by the combined technique of intracellular recording and spike-triggered averaging (Thomson et al, 1988), or by the paired intracellular or whole-cell recording technique with intracellular stimulation (Thomson and West, 1993; Markram and Tsodyks, 1996; Buhl et al, 1997; Ohana and Sakmann, 1998; Galarreta and Hestrin, 1999)
Lefort et al (2009) have quantified connectivity maps between excitatory neurons within a barrel column of mouse area S1 by randomly sampling a large number (2550) of excitatory neurons and testing 8895 possible synaptic connections within the column. This multiple whole-cell recording technique with random sampling is effective in mapping the local excitatory connections of the neocortex, no similar studies have been reported in the motor-associated areas yet
Summary
OF LOCAL CIRCUIT ANALYSIS IN THE MOTOR-ASSOCIATED AREAS The local excitatory connection of the rodent neocortex has been initially examined by the combination of intracellular recording and focal electrical stimulation (Connors et al, 1982; ChagnacAmitai and Connors, 1989; Sutor and Hablitz, 1989; Silva et al, 1991; Hwa and Avoli, 1992).
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