Abstract
Upper motoneurons (UMNs) in motor areas of the cerebral cortex influence spinal and cranial motor mechanisms through the corticospinal tract (CST) and through projections to brainstem motor pathways. The primate corticospinal system has a diverse cortical origin and a wide spectrum of fibre diameters, including large diameter fibres which are unique to humans and other large primates. Direct cortico-motoneuronal (CM) projections from the motor cortex to arm and hand motoneurons are a late evolutionary feature only present in dexterous primates and best developed in humans. CM projections are derived from a more restricted cortical territory (‘new’ M1, area 3a) and arise not only from corticospinal neurons with large, fast axons but also from those with relatively slow-conducting axons. During movement, corticospinal neurons are organised and recruited quite differently from ‘lower’ motoneurons. Accumulating evidence strongly implicates the corticospinal system in the early stages of ALS, with particular involvement of CM projections to distal limb muscles, but also to other muscle groups influenced by the CM system. There are important species differences in the organisation and function of the corticospinal system, and appropriate animal models are needed to understand disorders involving the human corticospinal system.
Highlights
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The term Upper motoneurons (UMNs) cannot be applied to all layer V corticospinal neurons because the corticospinal projection mediates functions other than somatic motor control, including autonomic control and control of peripheral somatosensory input [2]
This should question the use of the term ‘upper motoneuron’ as a synonym for an M1 corticospinal cell, suggesting as it does a fixed and inevitable recruitment of that cell for a given movement, and further suggesting that recruitment of the UMN is a requirement for the subsequent discharge of the lower motoneurons (LMNs)
Summary
Ever since Gowers (1886; see [1]), clinicians have used the term ‘upper motoneuron’ to distinguish clinical disorders affecting supraspinal structures directly influencing motor mechanisms from those affecting ‘lower’ cranial and spinal motoneurons While it continues to be a really useful term in clinical description and diagnosis, the term is perhaps less useful in the context of modern neuroanatomy and neurophysiology, because UMNs in cortical lamina V have an extensive ‘connectome’ which includes their corticospinal (CS) projections, and their extensive collateral projections to midbrain and brainstem motor structures, which in turn give rise to brainstem pathways influencing movement. Estimates based on the comparison of the crosssectional area of the cerebral peduncle with that of the medullary pyramidal tract suggest that they number less than 10% of all corticofugal projections that arise from the cortex [6]
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