Modular organization of locomotor networks in people with severe spinal cord injury.

Introduction: Previous studies support modular organization of locomotor circuitry contributing to the activation of muscles in a spatially and temporally organized manner during locomotion. Human spinal circuitry may reorganize after spinal cord injury; however, it is unclear if reorganization of spinal circuitry post-injury affects the modular organization. Here we characterize the modular synergy organization of locomotor muscle activity expressed during assisted stepping in subjects with complete and incomplete spinal cord injury (SCI) of varying chronicity, before any explicit training regimen. We also investigated whether the synergy characteristics changed in two subjects who achieved independent walking after training with spinal cord epidural stimulation. Methods: To capture synergy structures during stepping, individuals with SCI were stepped on a body-weight supported treadmill with manual facilitation, while electromyography (EMGs) were recorded from bilateral leg muscles. EMGs were analyzed using non-negative matrix factorization (NMF) and independent component analysis (ICA) to identify synergy patterns. Synergy patterns from the SCI subjects were compared across different clinical characteristics and to non-disabled subjects (NDs). Results: Results for both NMF and ICA indicated that the subjects with SCI were similar among themselves, but expressed a greater variability in the number of synergies for criterion variance capture compared to NDs, and weaker correlation to NDs. ICA yielded a greater number of muscle synergies than NMF. Further, the clinical characteristics of SCI subjects and chronicity did not predict any significant differences in the spatial synergy structures despite any neuroplastic changes. Further, post-training synergies did not become closer to ND synergies in two individuals. Discussion: These findings suggest fundamental differences between motor modules expressed in SCIs and NDs, as well as a striking level of spatial and temporal synergy stability in motor modules in the SCI population, absent the application of specific interventions.

Ankle muscle tenotomy does not alter ankle flexor muscle recruitment bias during locomotor-related repetitive limb movement in late-stage chick embryos.

In ovo, late-stage chick embryos repetitively step spontaneously, a locomotor-related behavior also identified as repetitive limb movement (RLM). During RLMs, there is a flexor bias in recruitment and drive of leg muscle activity. The flexor biased activity occurs as embryos assume an extremely flexed posture in a spatially restrictive environment 2-3 days before hatching. We hypothesized that muscle afferent feedback under normal mechanical constraint is a significant input to the flexor bias observed during RLMs on embryonic day (E) 20. To test this hypothesis, muscle afference was altered either by performing a tenotomy of ankle muscles or removing the shell wall restricting leg movement at E20. Results indicated that neither ankle muscle tenotomy nor unilateral release of limb constraint by shell removal altered parameters indicative of flexor bias. We conclude that ankle muscle afference is not essential to ankle flexor bias characteristic of RLMs under normal postural conditions at E20.

Differences in flexor and extensor activity during locomotor-related leg movements in chick embryos.

Prior to hatching, chick embryos spontaneously produce repetitive limb movements (RLMs), a developmental precursor to walking. During RLMs, flexor and extensor muscles are alternately active as during stance and swing phases of gait. However, previous studies of RLMs observed that flexor muscles were rhythmically active for many cycles, whereas extensors often failed to be recruited. Thus, we asked if flexor muscles are preferentially recruited during RLMs in chick embryos 1 day before hatching and onset of walking. Using a within-subject design, we compared EMG burst parameters for flexor and extensor muscles acting at the hip or ankle. Findings indicated that flexor burst count exceeded extensor count. Also, flexor muscles were consistently recruited at the lowest levels of neural drive. We conclude that there is a bias favoring flexor muscle recruitment and drive during spontaneously produced RLMs. Potential neural mechanisms and developmental implications of our findings are discussed.

Human Induced Pluripotent Stem Cell-Derived Motor Neuron Transplant for Neuromuscular Atrophy in a Mouse Model of Sciatic Nerve Injury.

IMPORTANCE: Human motor neurons may be reliably derived from induced pluripotent stem cells (iPSCs). In vivo transplant studies of human iPSCs and their cellular derivatives are essential to gauging their clinical utility. OBJECTIVE: To determine whether human iPSC-derived motor neurons can engraft in an immunodeficient mouse model of sciatic nerve injury. DESIGN, SETTING, AND SUBJECTS: This nonblinded interventional study with negative controls was performed at a biomedical research institute using an immunodeficient, transgenic mouse model. Induced pluripotent stem cell-derived motor neurons were cultured and differentiated. Cells were transplanted into 32 immunodeficient mice with sciatic nerve injury aged 6 to 15 weeks. Tissue analysis was performed at predetermined points after the mice were killed humanely. Animal experiments were performed from February 24, 2015, to May 2, 2016, and data were analyzed from April 7, 2015, to May 27, 2016. INTERVENTIONS: Human iPSCs were used to derive motor neurons in vitro before transplant. MAIN OUTCOMES AND MEASURES: Evidence of engraftment based on immunohistochemical analysis (primary outcome measure); evidence of neurite outgrowth and neuromuscular junction formation (secondary outcome measure); therapeutic effect based on wet muscle mass preservation and/or electrophysiological evidence of nerve and muscle function (exploratory end point). RESULTS: In 13 of the 32 mice undergoing the experiment, human iPSC-derived motor neurons successfully engrafted and extended neurites to target denervated muscle. Human iPSC-derived motor neurons reduced denervation-induced muscular atrophy (mean [SD] muscle mass preservation, 54.2% [4.0%]) compared with negative controls (mean [SD] muscle mass preservation, 33.4% [2.3%]) (P = .04). No electrophysiological evidence of muscle recovery was found. CONCLUSIONS AND RELEVANCE: Human iPSC-derived motor neurons may have future use in the treatment of peripheral motor nerve injury, including facial paralysis. LEVEL OF EVIDENCE: NA.