Somatosensory corticospinal tract axons sprout within the cervical cord following a dorsal root/dorsal column spinal injury in the rat

Abstract

The corticospinal tract (CST) is the major descending pathway controlling voluntary hand function in primates, and though less dominant, it mediates voluntary paw movements in rats. As with primates, the CST in rats originates from multiple (albeit fewer) cortical sites, and functionally different motor and somatosensory subcomponents terminate in different regions of the spinal gray matter. We recently reported in monkeys that following a combined cervical dorsal root/dorsal column lesion (DRL/DCL), both motor and S1 CSTs sprout well beyond their normal terminal range. The S1 CST sprouting response is particularly dramatic, indicating an important, if poorly understood, somatosensory role in the recovery process. As rats are used extensively to model spinal cord injury, we asked if the S1 CST response is conserved in rodents. Rats were divided into sham controls, and two groups surviving post‐lesion for ~6 and 10 weeks. A DRL/DCL was made to partially deafferent one paw. Behavioral testing showed a post‐lesion deficit and recovery over several weeks. Three weeks prior to ending the experiment, S1 cortex was mapped electrophysiologically, for tracer injection placement to determine S1 CST termination patterns within the cord. Synaptogenesis was also assessed for labeled S1 CST terminals within the dorsal horn. Our findings show that the affected S1 CST sprouts well beyond its normal range in response to a DRL/DCL, much as it does in macaque monkeys. This, along with evidence for increased synaptogenesis post‐lesion, indicates that CST terminal sprouting following a central sensory lesion, is a robust and conserved response.

Plasticity in Cervical Motor Circuits following Spinal Cord Injury and Rehabilitation

“…After SCI, recovery or compensatory reaching and grasping is mediated by several spared systems that respond after injury. Sparing and sprouting of the CST and RST are two of the most well-characterized mechanisms involved in regaining reaching and grasping following SCI that are conserved across species [75][76][77][78][79]. In addition, plasticity of primary afferent fibers is also a key contributor to improved function post-injury.…”

Quantitative electrophysiological assessments as predictive markers of lower limb motor recovery after spinal cord injury: a pilot study with an adaptive trial design

“…They also hint at the potential of plasticity within the remaining pathways, since all the animals were able to recover quadrupedal locomotion. Moreover, recent studies showed extensive somatosensory and motor corticospinal sprouting following an incomplete SCI [ 50 – 52 ]. The recovery potential will depend on the ability of remaining neuronal circuitry to generate new and coordinated activity through neuroplasticity and could lead to compensation.…”

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

“…Indeed, lesions to the dorsal column have led to deficits in some or all of the above functions. For example, an incision to the dorsal column at the C1 region in rats produced deficits with object handling in rats, though they quickly learned to compensate with alternate limb or body movements (McKenna & Whishaw, 1999) and may be attributed to corticospinal and somatosensory sprouting (Fisher et al, 2018; McCann et al, 2020). In addition, myelopathy in these two fasciculi owing to a rare complication of intrathecal chemotherapy has been shown to produce lower limb numbness and a pronounced lack of proprioception in the same region (Joseph & Reyes, 2014).…”