Motor Cortex Microcircuits (Frontiers in Brain Microcircuits Series)

170.7K
views
54
authors
18
articles
Editors
4
Impact
Loading...

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.

12,278 views
56 citations
Original Research
27 February 2013
Cell and neuron densities in the primary motor cortex of primates
Nicole A. Young
1 more and 
Jon H. Kaas
The location of the primary motor cortex (M1) on the lateral view of the left cerebral hemisphere in several of the primate species discussed in this paper. The location and internal organization of M1 in New world and Old World monkeys (Gould et al., 1986; Huang et al., 1988; Waters et al., 1990; Huntley and Jones, 1991; Donoghue et al., 1992; Nudo et al., 1992; Kaas, 2012) and prosimian galagos (Wu et al., 2000) consists of a fractured mosaic of smaller areas devoted to particular movements that are dispersed within a larger somatotopic framework of functional movement domains (see Kaas, 2012; Kaas et al., 2012 for review). Figure 1 shows the locations of M1 in prosimian galagos (Wu et al., 2000; Wong and Kaas, 2010), New World owl monkeys (Gould et al., 1986) and squirrel monkeys (Donoghue et al., 1992; Nudo et al., 1992), Old World macaques (Huntley and Jones, 1991; Preuss et al., 1997) and baboons (Waters et al., 1990), and Great Ape chimpanzees (Bailey et al., 1950). Hindlimb movement representations (foot) are located at the most dorsal-medial aspect of M1, and transitions to movements of the trunk of the body (trunk), forelimb and hand (hand), and finally face at the most ventral-lateral aspect of M1. Cortical areas located rostral to M1 include the dorsal (PMd) and ventral (PMv) premotor areas, the supplementary motor area (SMA), and frontal eye fields (FEF). Somatosensory areas of the anterior parietal cortex (3a, 3b, 1, 2) are caudal to M1. The location of the central sulcus (CS) is indicated in macaque, baboon, and chimpanzee.

Cell and neuron densities vary across the cortical sheet in a predictable manner across different primate species (Collins et al., 2010b). Primary motor cortex, M1, is characterized by lower neuron densities relative to other cortical areas. M1 contains a motor representation map of contralateral body parts from tail to tongue in a mediolateral sequence. Different functional movement representations within M1 likely require specialized microcircuitry for control of different body parts, and these differences in circuitry may be reflected by variation in cell and neuron densities. Here we determined cell and neuron densities for multiple sub-regions of M1 in six primate species, using the semi-automated flow fractionator method. The results verify previous reports of lower overall neuron densities in M1 compared to other parts of cortex in the six primate species examined. The most lateral regions of M1 that correspond to face and hand movement representations, are more neuron dense relative to medial locations in M1, which suggests differences in cortical circuitry within movement zones.

13,893 views
69 citations
14,751 views
235 citations
Recommended Research Topics