Research report
Neural activity related to reaching and grasping in rostral and caudal regions of rat motor cortex

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Abstract

The objective of this study was to assess the relation of motor cortical neural activity in the rat to self-paced reach-to-grasp movements. Overall, around 40% of excitatory and 60% of inhibitory modulations in neuronal activity began prior to reach onset. These data are consistent with a role for rat motor cortex in the initiation and control of the reaching movement. In addition, although the reach only lasted a short time, 30% of excitations and inhibitions began while it was in progress. The existence of such modulations occurring during the reach is consistent with previous data showing activity of cortical neurons late in the reach, and suggests a heavy involvement of cortical neurons in controlling the recently described, complex movements associated with grasping that are seen in the rat. These features were broadly similar in neurones from both the caudal and rostral subdivisions of rat motor cortex.

Introduction

Neural activity in the rat motor cortex is known to be correlated with performance of reach-to-grasp movements 5, 8, 23, 40, 41, 50, 51. However, the available studies of reaching performance in which timing was systematically investigated used an analysis in which neural activity changes were examined only in relation to the end of a reach-to-grasp movement. In these studies, neural activity was found to precede the end of the reach by only a short interval. Comparison of these results with data on total reach duration, and the timing of associated modulations in muscle activity 15, 16would suggest that very few, if any, cortical neurons become active prior to the beginning of the reach. This stands in contrast to the vigorous engagement of primate motor neurons prior to forelimb reaching in primates 25, 36. Furthermore, it would be surprising if rat motor cortical neurons were not involved in controlling the transport phase of an aimed reaching movement, since a shoulder representation is found in the motor cortex of the rat 26, 28, and deficits in proximal forelimb movement are seen following cortical lesions 2, 4, 48. The present experiments were therefore designed to investigate the extent to which motor cortex neural activity is associated with the transport phase of the reach, by extending previous work in two ways: firstly, the data were analyzed both in relation to the end of the reach (as in previous studies), and the onset of the reach as determined by video analysis of paw movement. Secondly, the rostral motor area 26, 28, which may not have been examined in previous single unit studies was specifically targeted, in addition to the traditionally recognized limb motor area (caudal motor area).

A preliminary report has appeared in abstract form [14].

Section snippets

Methods

All procedures were approved by the University of Otago Committee on Ethics in the Care and Use of Laboratory Animals, and were in accord with the `Principles of Laboratory Animal Care' (NIH publ. no. 86-23, revised 1985).

General

Rats did not appear to differ in their reaching behaviour following implantation of the electrodes in the cortex, and in no case was shifting of paw preference observed post-operatively. As illustrated in Fig. 1C, histologically confirmed coordinates of rostrally targeted electrodes ranged from AP 3.0 to 4.2, and L 1.3 to 2.5, while caudally the region explored extended from AP 0.7 to 1.5 and L 1.0 to 3.5. Microstimulation, where successful, produced contralateral forelimb movements in both

Rat motor cortex activity related to both onset and end of reach-to-grasp movements

A main finding of this study is that significant numbers of excitatory and inhibitory modulations begin 100–300 ms prior to reach. Reaching movements in rats are kinematically complicated 47, 48and EMG activity in limb muscles with onset highly correlated to reach-onset has been shown to occur over a wide time range, up to 150 ms prior to first detectable paw movement [15]. Thus the single unit timing data reported here is consistent with a role for the rat motor cortex in controlling muscle

Conclusions

The results of the present study show that, as in the monkey, rat motor cortex plays an active role in all stages of skilled forelimb movements. With recent work having established the existence of direct connections from the cortex to the spinal cord motor circuits, including possibly spinal motor neurons, and an increasing awareness of the complexity of fine motor control evident in this species, it is clear that the rat motor cortex can be considered a useful model for investigations of the

Acknowledgements

This research was supported by the University of Otago Medical School Bequest fund, the New Zealand Lottery Grants Board, and the New Zealand Neurological Foundation. Thanks to Mr Greg Perk for assistance with data analysis, and Professor Mario Wiesendanger and Dr Jeff Wickens for comments on the manuscript.

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