Motor Control in the Human Spinal Cord

Abstract

Features of the human spinal cord motor control are described using two spinal cord injury models: (i) the spinal cord completely separated from brain motor structures by accidental injury; (ii) the spinal cord receiving reduced and altered supraspinal input due to an incomplete lesion. Systematic studies using surface electrode polyelectromyography were carried out to assess skeletal muscle reflex responses to single and repetitve stimulation in a large number of subjects. In complete spinal cord injured subjects the functional integrity of three different neuronal circuits below the lesion level is demonstrated: first, simple mono- and oligosynaptic reflex arcs and polysynaptic pathways; second, propriospinal interneuron system with their cell in the gray matter and the axons in the white matter of the spinal cord conducting activity between different spinal cord segments; and third, internuncial gray matter neurons with short axons and dense neuron contact within the spinal gray matter. All of these three systems participate continuously in the generation of spinal cord reflex output activating muscles. The integration of these systems and their relative degree of excitation and set-up produces characteristic functions of motor control. In incomplete spinal cord injured patients, the implementation of brain motor control depends on the profile of residual brain descending input and its integration with the functional neuronal circuits below the lesion. Locomotor patterns result from the establishment of a new structural relationship between brain and spinal cord. The functions of this new structural relationship are expressed as an alternative, but characteristic and consistent neurocontrol. The more we know about how the brain governs spinal cord networks, the better we can describe human motor control. On the other hand such knowledge is essential for the restoration of residual functions and for the construction of new cord circuitry to expand the functions of the injured spinal cord.

Gait reveals bilateral adaptation of motor control in patients with chronic unilateral stroke

“…Following this point of view, the increased gait symmetry in faster walking simulates gait in healthy subjects. One explanation may be that preserved central pattern generators in the spinal cord, which may operate in a stereotyped manner under residual supraspinal motor control, try to retain the basic structure of gait and preserve central programming as much as possible (31)(32)(33). This hypothesis is supported by our finding that the asymmetry indices were not related to temporal kinetic gait variables, stroke characteristics, personal demographic data or functional scale.…”

Electrical stimulation of cortex improves corticospinal tract tracing in rat spinal cord using manganese-enhanced MRI

“…In the rat, the tract runs caudally from cortex through the internal capsule, cerebral peduncle, longitudinal pontine fasciculus, pyramid, pyramidal decussation, and descends in the dorsal fasciculus of the SC (Donatelle, 1977;Nudo and Masterton, 1988;Coleman et al, 1997;Dimitrijevic et al, 2005). Current interest in understanding the organization of this long descending pathway remains high because of its disruption as a consequence of spinal cord injury (SCI).…”

Imaging corticospinal tract connectivity in injured rat spinal cord using manganese-enhanced MRI

“…In SCI research, a particular pathway of interest is the corticospinal tract (CST) as it is the major long descending output connecting the cerebral cortex with the motor neurons in the SC [1]. Because, many different types of movements are controlled through the fibers of the CST, understanding how this tract adapts to SCI is the focus of current experimental efforts [2,3].…”