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Objective: Spinal cord injury (SCI) often results in debilitating movement impairments and neuropathic pain. Electrical stimulation of spinal neurons holds considerable promise both for enhancing neural transmission in weakened motor pathways and for reducing neural transmission in overactive nociceptive pathways. However, spinal stimulation paradigms currently under development for individuals living with SCI continue overwhelmingly to be developed in the context of motor rehabilitation alone. The objective of this study is to test the hypothesis that motor-targeted spinal stimulation simultaneously modulates spinal nociceptive transmission. Approach: We characterized the neuromodulatory actions of motor-targeted intraspinal microstimulation (ISMS) on the firing dynamics of large populations of discrete nociceptive specific and wide dynamic range neurons. Neurons were accessed via dense microelectrode arrays implanted in vivo into lumbar enlargement of rats. Nociceptive and non-nociceptive cutaneous transmission was induced before, during, and after ISMS by mechanically probing the L5 dermatome. Main results: Our primary findings are that (1) sub-motor threshold ISMS delivered to spinal motor pools immediately modulates concurrent nociceptive transmission; (2) the magnitude of anti-nociceptive effects increases with longer durations of ISMS, including robust carryover effects; (3) the majority of all identified nociceptive-specific and wide dynamic range neurons exhibit firing rate reductions after only 10 min of ISMS; and (4) ISMS does not increase spinal responsiveness to non-nociceptive cutaneous transmission. These results lead to the conclusion that ISMS parameterized to enhance motor output results in an overall net decrease in spinal nociceptive transmission. Significance: These results suggest that ISMS may hold translational potential for neuropathic pain-related applications and that it may be uniquely suited to delivering multi-modal therapeutic benefits for individuals living with SCI.

Spontaneous action potential discharge (spAP) is both ubiquitous and functionally relevant during neural development. spAP remains a prominent feature of supraspinal networks in maturity, even during unconsciousness. Evidence suggests that spAP persists in mature spinal networks during wakefulness, and one function of spAP in this context could be maintenance of a "ready state" to execute behaviors. The extent to which spAP persists in mature spinal networks during unconsciousness remains unclear, and its function(s), if any, are likewise unresolved. Here, we attempt to reconcile some of the questions and contradictions that emerge from the disintegrated picture of adult spinal spAP currently available. We recorded simultaneously from large populations of spinal interneurons in vivo in male rats, characterizing the spatial distribution of spAP in the lumbar enlargement and identifying subgroups of spontaneously active neurons. We find (1) concurrent spAP throughout the dorsoventral extent of the gray matter, with a diverse yet strikingly consistent mixture of neuron types across laminae;(2) the proportion of neurons exhibiting spAP in deeper, sensorimotor integrative regions is comparable to that in more superficial, sensory-dominant regions; (3) firing rate, but not spike train variability, varies systematically with region; and (4) spAP includes multimodal neural transmission consistent with executing a spinally-mediated behavior. These findings suggest that adult spAP may continue to support a state of readiness to execute sensorimotor behaviors even during unconsciousness. Such functionality has implications for our understanding of how perception is translated into action, of experience-dependent modification of behavior, and (mal)adaptative responses to injury or disease.

Spinal cord injury results in multiple, simultaneous sensorimotor deficits. These include, but are not limited to, full or partial paralysis of muscles below the lesion, muscle spasms, spasticity, and neuropathic pain. Bowel, bladder, and sexual dysfunction are also prevalent. Yet, the majority of emerging spinal stimulation-based therapies focus on a single issue: locomotor rehabilitation. Despite the enormous potential of these translational advances to transform the lives of people living with spinal cord injury, meaningful recovery in other domains deemed critical priorities remains lacking. Here, we highlight the importance of considering the diverse patterns of neural transmission that underlie clinically similar presentations when developing spinal stimulation-based therapies. We also motivate advancement of multi-modal rehabilitation paradigms, which leverage the dense interconnectivity of sensorimotor spinal networks and the unique ability of electrical stimulation to modulate these networks to facilitate and guide simultaneous rehabilitation across domains.