Central cord syndrome and upper-limb weakness
Central cord syndrome (CCS) typically affects the upper limbs more severely than the lower limbs. This pattern was traditionally attributed to greater injury to the medial corticospinal tract (CST) fibres, based on the assumption that these medial fibres preferentially innervate upper-limb alpha motor neurons. However, this explanation relied on a somatotopic organisation of the corticospinal tract that recent studies have challenged with recent studies suggesting that CSF does not have somatotopic organisation (Levi & Schwab, 2022; Shakil et al., 2023).
An alternative explanation is that the upper limbs have more direct cortical projections, with large-diameter axons that innervate alpha motor neurons. In CCS, diffuse injury may therefore disproportionately affect these upper-limb fibres. The relative preservation of lower-limb function may also reflect less of the large axon direct cortco spinal nerves to the loweer limbs are affected ( since they are lower for lower limbs) as well as input from descending pathways outside the corticospinal tract, including the rubrospinal and vestibulospinal tracts (Levi & Schwab, 2022; Shakil et al., 2023).
Historically, it was thought that all motor fibres descending from the motor cortex through the corticospinal tract (CST) synapse directly onto alpha motor neurons. However, recent studies suggest that not all corticospinal fibres make direct connections with alpha motor neurons in the ventral horn. Some fibres instead bifurcate to synapse on downstream neurons, while others connect with downstream neurons without bifurcating (Shakil et al., 2023).
Levi, A. D., & Schwab, J. M. (2022). A critical reappraisal of corticospinal tract somatotopy and its role in traumatic cervical spinal cord syndromes. Journal of Neurosurgery: Spine, 36(4), 653–659. https://doi.org/10.3171/2021.7.SPINE21546
Shakil, H., Santaguida, C., Wilson, J. R., Farhadi, H. F., Levi, A. D., & Wilcox, J. T. (2023). Pathophysiology and surgical decision-making in central cord syndrome and degenerative cervical myelopathy: Correcting the somatotopic fallacy. Frontiers in Neurology, 14, 1276399. https://doi.org/10.3389/fneur.2023.1276399