Scientific Understanding of Consciousness
Motor Cortex Neurons Directly Influence Motor Output
Science 6 November 2015: Vol. 350 no. 6261 pp. 667-670
Corticomotoneuronal cells are “functionally tuned”
Darcy M. Griffin, et.al.
University of Pittsburgh Brain Institute, Center for the Neural Basis of Cognition and Systems Neuroscience Institute, Pittsburgh, PA, USA.
Department of Neurobiology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
Research Service, Department of Veterans Affairs Medical Center, Pittsburgh, PA, USA.
Corticomotoneuronal (CM) cells in the primary motor cortex (M1) have monosynaptic connections with motoneurons. They are one of the few sources of descending commands that directly influence motor output. We examined the contribution of CM cells to the generation of activity in their target muscles. The preferred direction of many CM cells differed from that of their target muscles. Some CM cells were selectively active when a muscle was used as an agonist. Others were selectively active when the same muscle was used as a synergist, fixator, or antagonist. These observations suggest that the different functional uses of a muscle are generated by separate populations of CM cells. We propose that muscle function is one of the dimensions represented in the output of M1.
Even simple movements are produced by complex patterns of muscle activity. For example, during wrist flexion, some muscles function as agonists to generate force in the direction of flexion. Other muscles function as fixators to prevent joint motion in the radial and ulnar direction, and still others serve as antagonists to brake movement and assist in speed control. Movement dexterity depends on the central control over the precise timing and amplitude not only of agonist muscle activity but also of the activity of muscles performing other functions.
We examined the contribution of corticomotoneuronal (CM) cells in the primary motor cortex (M1) to the generation and control of different patterns of muscle activity. CM cells are output neurons in M1 that have monosynaptic connections with motoneurons in the spinal cord. CM cells are located in a distinct caudal portion of M1 that is both phylogenetically and ontogenetically new. We identified 41 CM cells and their target muscles using spike-triggered averaging (SpTA) of electromyographic (EMG) activity from 12 to 13 forearm muscles. We examined the directional tuning of CM cells and their target muscles while a monkey performed wrist movements in eight directions with the limb in three different postures. Twenty CM cells (~49%) were directionally tuned for all three wrist postures. Nearly all of these CM cells (19 of 20) were considered to be “muscle-like,” and none were considered to be “extrinsic-like.” We compared the preferred direction of these CM cells (i.e., the direction of cell’s maximal activity) with that of their target muscles.
The key result of the present study is that for many CM cells there is a major disparity between the cell’s preferred direction and the preferred directions of its target muscles. We interpret this result as indicating that individual CM cells are functionally tuned. Indeed, we provide evidence that some CM cells specifically contribute to the agonist function of a muscle, whereas other CM cells specifically contribute to the synergist, fixator, or antagonist function of the same muscle. From this perspective, the multiple functions of a target muscle are represented by the activity of separate populations of CM cells. The concept of functional tuning is supported by Muir and Lemon’s observation that some CM cells were more active during a precision grip, whereas others were more active during a power grip, even though both types of CM cells facilitated the same target muscles. In their case, like ours, CM cell activity was linked to the functional use rather than the magnitude of muscle activity.
It is possible that functional tuning is an emergent property of New M1. The wrist movements and patterns of muscle activity required by our task are not part of the animal’s natural repertoire. Thus, skilled performance of the task requires an animal to generate new patterns of muscle activity that are acquired through extensive practice. We have previously argued that “the direct access to motoneurons afforded by CM cells enables New M1 to bypass spinal cord mechanisms and sculpt novel patterns of motor output that are essential for highly skilled movements.” Thus, functional tuning of CM cells may emerge as part of the sculpting process during extended practice on the task. In any event, functional tuning, whether explicitly represented or an emergent property, reflects a clear expansion of the known motor dimensions that M1 generates and controls.
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