Project5

Neural encoding of limb movement

In contrast to their broadly depressant actions on sensory systems, the actions of the monoamines on motor systems are generally facilitatory, but also differentiable. For example, noradrenaline promotes self-reinforcing positive feedback in spinal motor circuits while serotonin promoted negative feedback (Machacek & Hochman 2006).

We published an “Innovative Methodology” paper (Hayes et al 2009) on a newly-developed isolated in vitro rat spinal cord with intact hindlimbs freely stepping on a custom-built treadmill. It combines the neural accessibility of in vitro preparations with modulatory influence of sensory feedback from physiological hindlimb movement. A currently submitted manuscript provides the first-ever intracellular neuronal recordings during mechanically-unrestrained locomotion. Using additional pioneering techniques in another submitted manuscript, we investigated how hindlimb mechanics influence sensory input during locomotion. We found that stance-phase force on the opposite limb strongly and linearly encoded the magnitude and timing of afferent presynaptic inhibition in the swinging limb, thus binding interlimb sensorimotor states by adjusting sensory inflow to the swing limb based on forces generated by the stance limb. These studies suggest that stroke or spinal cord injury rehabilitative approaches that involve loading the unaffected limb may provide a novel means of reducing spasticity and hyperreflexia in the affected limb.

Remarkably, the neonatal rat spinal cord can be surgically isolated, even with hindlimbs attached, and preserved for many hours in an oxygenated saline bath. Serotonin activates the spinal locomotor circuitry, enabling an in vitro study of spinal cord locomotor mechanisms. One goal is to identify pharmacotherapeutic strategies that complement known restrictions in the regenerative repair process to enable a limited reconnection from brain command pathways to activate the locomotor central pattern generator (CPG).

In collaboration with Young-Hui Chang, Heather Hayes has developed innovative methodologies to study the kinematics of a 'walking' spinal cord maintained in a dish (Hayes et al 2009). We have begun to identify sensory and spinal cord mechanisms that regulate ongoing limb movements, and found that ground contact of the contralateral limb is a powerful source of presynaptic inhibition of sensory input to the opposite limb.

walking cord

Overhead view of the in vitro spinal cord-hindlimb preparation (SCHP) with exposed spinal and each intact hindlimb free to walk on a separate 2D force platform. Inlay shows the recording configuration. Dorsal root potentials (DRPs) were recorded near the dorsal root entry zones of L2 and L5 dorsal roots using glass suction electrodes.

Sagittal view of hindlimb-force platform interaction and wheatstone bridge circuitry. Strain produced by strain gauges SG1 and SG2 of Sensor 1 and fed into the wheatstone bridge circuit. Strain sensed by Sensor 2 is fed into a separate but identical wheatstone bridge circuit. Output voltages are amplified by a DC amplifier and then converted to vertical and fore-aft forces in offline analysis.

Response to contralateral and ipsilateral plate removals. Ipsilateral and contralateral DRPs (iL2 DRP, cL2 DRP, blue) are shown with ipsilateral ventral root activity (iL2 VR, black), contralateral force (red), and ipsilateral force (green). Gray boxes highlight the period of contralateral (a) and then ipsilateral (b) plate removals. Note that there was not a significant change in locomotor frequency before, during, or after plate removal. (a) When the contralateral plate is removed, reducing contralateral limb loading, the ipsilateral L2 DRP is nearly abolished. The DRPs return as soon as the contralateral plate is restored. This result demonstrates that sufficient contralateral limb loading is required to generate the large DRPs typically seen during nonfictive locomotion. The contralateral L2 DRP is largely unaffected by the plate removal, as its opposite force remains. (b) When the ipsilateral plate is removed, the contralateral DRP is greatly reduced while the ipsilateral DRP is largely unchanged. Note that small contralateral DRPs remain, which are likely centrally-generated as in fictive locomotor literature or generated by residual ipsilateral and/or contralateral input. Yet, the largest contralateral force-sensitive component of the DRP requires significant contralateral force.