Abstract
The last few years have seen major advances in our understanding of the organisation and function of the corticospinal tract (CST). These have included studies highlighting important species-specific variations in the different functions mediated by the CST. In the primate, the most characteristic feature is direct cortico-motoneuronal (CM) control of muscles, particularly of hand and finger muscles. This system, which is unique to dexterous primates, is probably at its most advanced level in humans. We now know much more about the origin of the CM system within the cortical motor network, and its connectivity within the spinal cord has been quantified. We have learnt much more about how the CM system works in parallel with other spinal circuits receiving input from the CST and how the CST functions alongside other brainstem motor pathways. New work in the mouse has provided fascinating insights into the contribution of the CM system to dexterity. Finally, accumulating evidence for the involvement of CM projections in motor neuron disease has highlighted the importance of advances in basic neuroscience for our understanding and possible treatment of a devastating neurological disease.
Highlights
The last few years have seen major advances in our understanding of the organisation and function of the corticospinal tract (CST)
‘New’ M1: the origin of the cortico-motoneuronal output We know from the work of Rathelot and Strick[2,3], using retrograde transneuronal tracers, that in the macaque monkey, CM neurons are found in two main cortical areas: ‘new’ M1 and area 3a, part of primary somatosensory cortex (S1)
This rostral region gives rise to corticospinal projections, projections that do not terminate on motoneurons, and it projects to the pontine nuclei and brainstem centres, giving rise to descending motor pathways, including the reticulospinal tract[4]
Summary
CM system is unique to primates and well developed in humans It provides a fast, direct pathway to motoneurons, those supplying muscles that subserve some of the most characteristic human hand movements, including those for tool use, music making, gesture and communication. CM control signals effectively bypass the more rigid networks provided by spinal segmental connectivity and support a much richer repertoire of grasping movements, based on the combination and recombination of many different CM outputs to hand and digit muscles. A better definition of the molecular and genetic identity of the CM system might provide further clues, including the possibility of a selective CM cell blocker which might give further insights into CM function but the recovery process involving other systems. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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