Somatotopic arrangement and corticocortical inputs of the hindlimb region of the primary motor cortex in the macaque monkey
Introduction
Representations of the hindlimb, trunk, forelimb and orofacial part are arranged orderly from medial to lateral in the primary motor cortex (MI), which is located in the precentral gyrus of the primate brain. Among these representations, much attention has so far been paid to the forelimb representation, because complex neural mechanisms are required to achieve precise reaching and grasping movements. The forelimb region of the MI has repeatedly been analyzed by examining movements elicited with intracortical microstimulation (ICMS) (Kwan et al., 1978, Murphy et al., 1978, Sessle and Wiesendanger, 1982, Strick and Preston, 1982, Gould et al., 1986, Sato and Tanji, 1989, Stepniewska et al., 1993) and by examining its corticocortical connections with the frontal and parietal lobes (Muakkassa and Strick, 1979, Leichnetz, 1986, Ghosh et al., 1987, Dum and Strick, 1991, Stepniewska et al., 1993, Tokuno and Tanji, 1993).
On the other hand, since hindlimb movements are usually applied for locomotor behavior and postural maintenance, they may require the motor control system that is essentially distinct from that for forelimb movements. Thus, it is interesting to investigate the hindlimb region of the MI and related premotor areas by the use of both electrophysiological and anatomical approaches. The purpose of the present study was twofold: (1) to examine the pattern of the representations in the hindlimb region of the MI by means of ICMS and (2) to analyze the distribution of corticocortical neurons projecting to the hindlimb region of the MI by means of retrograde double labeling with fluorescent dyes.
Section snippets
ICMS mapping
Experiments were performed in seven female Japanese monkeys (Macaca fuscata) weighing 4.2–6.0 kg. The use of the animals in the present study followed the Guidelines approved by the Animal Experiment Committee at the Tokyo Metropolitan Institute for Neuroscience. Each monkey was anesthetized with ketamine hydrochloride (10 mg/kg body wt., i.m.) and sodium pentobarbital (25 mg/kg body wt., i.v.) and received surgery to gain easy access to electrophysiological mapping. Under aseptic conditions,
Somatotopy of the MI
In monkeys L1–3, we performed ICMS mapping throughout the medial MI. Analysis of the spatial distribution of loci where movements were produced by ICMS revealed that hindlimb representations were located medially in the precentral gyrus, while forelimb representations were located more laterally (Fig. 1, Fig. 2, Fig. 3). Trunk representations were found between these two representations. Tail movements were evoked by ICMS in the medial wall of the hemisphere beneath the loci where movements of
Somatotopic organization of the MI hindlimb region
By means of ICMS mapping in rhesus monkeys (Macaca mulatta), Wise and Tanji (1981) have shown the nested distal–proximal organization in the hindlimb region of the MI. In their map, digit representations were surrounded by more proximal representations, as seen in the present study. However, the digit representations were reported to be found in the medial wall of the hemisphere. Their work also showed that the tail movements were evoked in the medial wall of the hemisphere beneath the loci
Acknowledgements
We thank E. Mine and Y. Ikeuchi for technical help. Supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.
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