The challenges of respiratory motor system recovery following cervical spinal cord injury

Gibbs H, McCreedy DA, Alilain WJ, Crone SA. V2a neurons restore diaphragm function in mice following spinal cord injury

The specific roles that different types of neurons play in recovery from injury is poorly understood. Here, we show that increasing the excitability of ipsilaterally projecting, excitatory V2a neurons using designer receptors exclusively activated by designer drugs (DREADDs) restores rhythmic bursting activity to a previously paralyzed diaphragm within hours, days, or weeks following a C2 hemisection injury. Further, decreasing the excitability of V2a neurons impairs tonic diaphragm activity after injury as well as activation of inspiratory activity by chemosensory stimulation, but does not impact breathing at rest in healthy animals. By examining the patterns of muscle activity produced by modulating the excitability of V2a neurons, we provide evidence that V2a neurons supply tonic drive to phrenic circuits rather than increase rhythmic inspiratory drive at the level of the brainstem. Our results demonstrate that the V2a class of neurons contribute to recovery of respiratory function following injury. We propose that altering V2a excitability is a potential strategy to prevent respiratory motor failure and promote recovery of breathing following spinal cord injury.

EphrinB2 knockdown in cervical spinal cord preserves diaphragm innervation in a mutant SOD1 mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by motor neuron loss. Importantly, non-neuronal cell types such as astrocytes also play significant roles in disease pathogenesis. However, mechanisms of astrocyte contribution to ALS remain incompletely understood. Astrocyte involvement suggests that transcellular signaling may play a role in disease. We examined contribution of transmembrane signaling molecule ephrinB2 to ALS pathogenesis, in particular its role in driving motor neuron damage by spinal cord astrocytes. In symptomatic SOD1G93A mice (a well-established ALS model), ephrinB2 expression was dramatically increased in ventral horn astrocytes. Reducing ephrinB2 in the cervical spinal cord ventral horn via viral-mediated shRNA delivery reduced motor neuron loss and preserved respiratory function by maintaining phrenic motor neuron innervation of diaphragm. EphrinB2 expression was also elevated in human ALS spinal cord. These findings implicate ephrinB2 upregulation as both a transcellular signaling mechanism in mutant SOD1-associated ALS and a promising therapeutic target.

EphrinB2 knockdown in cervical spinal cord preserves diaphragm innervation in a mutant SOD1 mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by motor neuron loss. Importantly, non-neuronal cell types such as astrocytes also play significant roles in disease pathogenesis. However, mechanisms of astrocyte contribution to ALS remain incompletely understood. Astrocyte involvement suggests that transcellular signaling may play a role in disease. We examined contribution of transmembrane signaling molecule ephrinB2 to ALS pathogenesis, in particular its role in driving motor neuron damage by spinal cord astrocytes. In symptomatic SOD1-G93A mice (a well-established ALS model), ephrinB2 expression was dramatically increased in ventral horn astrocytes. Reducing ephrinB2 in the cervical spinal cord ventral horn via viral-mediated shRNA delivery reduced motor neuron loss and preserved respiratory function by maintaining phrenic motor neuron innervation of diaphragm. EphrinB2 expression was also elevated in human ALS spinal cord. These findings implicate ephrinB2 upregulation as both a transcellular signaling mechanism in mutant SOD1-associated ALS and a promising therapeutic target.

Spontaneous Cervical Epidural Hematoma Presenting with Respiratory Failure: A Case Report and Literature Review

CASE

A 62-year-old woman who had an unremarkable medical history presented with sudden headache and neck pain. After the presentation, complete quadriplegia and respiratory arrest developed, and the patient was urgently intubated. Magnetic resonance imaging revealed an extensive epidural hematoma (EH), and emergency hematoma evacuation was performed. At the 1-year follow-up visit, the patient had no motor deficits.

CONCLUSION

We reported a case of spontaneous cervical EH presenting with respiratory failure that was successfully treated with surgical management. Literature review has shown that the surgical outcome is very poor; nevertheless, prompt surgical decompression of the spinal cord can minimize neurological sequelae.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

Impact of cervical spinal cord injury on the relationship between the metabolism and ventilation in rats

Cervical spinal cord injury typically results in respiratory impairments. Clinical and animal studies have demonstrated that respiratory function can spontaneously and partially recover over time after injury. However, it remains unclear whether respiratory recovery is associated with alterations in metabolism. The present study was designed to comprehensively examine ventilation and metabolism in a rat model of spinal cord injury. Adult male rats received sham (i.e., laminectomy) or unilateral mid-cervical contusion injury (height of impact rod: 6.25 or 12.5 mm). Breathing patterns and whole body metabolism (O₂ consumption and CO₂ production) were measured using a whole body plethysmography system conjugated with flow controllers and gas analyzer at the acute (1 day postinjury), subchronic (2 wk postinjury), and chronic (8 wk postinjury) injury stages. The results demonstrated that mid-cervical contusion caused a significant reduction in the tidal volume. Although the tidal volume of contused animals can gradually recover, it remains lower than that of uninjured animals at the chronic injury stage. Although O₂ consumption and CO₂ production were similar between uninjured and contused animals at the acute injury stage, these two metabolic parameters were significantly reduced in contused animals at the subchronic to chronic injury stages. Additionally, the relationships between ventilation, metabolism, and body temperature were altered by cervical spinal cord injury. These results suggest that cervical spinal cord injury causes a complicated reconfiguration of ventilation and metabolism that may enable injured animals to maintain a suitable homeostasis for adapting to the pathophysiological consequences of injury.NEW & NOTEWORTHY Ventilation and metabolism are tightly coupled to maintain appropriate energy expenditure under physiological conditions. Our findings demonstrate that cervical spinal cord injury results in the differential reduction of ventilation and metabolism at the various injury stages and leads to alterations in the relationship between ventilation and metabolism. These results from an animal model provide fundamental knowledge for understanding how cervical spinal cord injury impacts energy homeostasis.

Respiratory Training and Plasticity After Cervical Spinal Cord Injury

While spinal cord injuries (SCIs) result in a vast array of functional deficits, many of which are life threatening, the majority of SCIs are anatomically incomplete. Spared neural pathways contribute to functional and anatomical neuroplasticity that can occur spontaneously, or can be harnessed using rehabilitative, electrophysiological, or pharmacological strategies. With a focus on respiratory networks that are affected by cervical level SCI, the present review summarizes how non-invasive respiratory treatments can be used to harness this neuroplastic potential and enhance long-term recovery. Specific attention is given to "respiratory training" strategies currently used clinically (e.g., strength training) and those being developed through pre-clinical and early clinical testing [e.g., intermittent chemical stimulation via altering inhaled oxygen (hypoxia) or carbon dioxide stimulation]. Consideration is also given to the effect of training on non-respiratory (e.g., locomotor) networks. This review highlights advances in this area of pre-clinical and translational research, with insight into future directions for enhancing plasticity and improving functional outcomes after SCI.

Respiratory axon regeneration in the chronically injured spinal cord

Promoting the combination of robust regeneration of damaged axons and synaptic reconnection of these growing axon populations with appropriate neuronal targets represents a major therapeutic goal following spinal cord injury (SCI). A key impediment to achieving this important aim includes an intrinsic inability of neurons to extend axons in adult CNS, particularly in the context of the chronically-injured spinal cord. We tested whether an inhibitory peptide directed against phosphatase and tensin homolog (PTEN: a central inhibitor of neuron-intrinsic axon growth potential) could restore inspiratory diaphragm function by reconnecting critical respiratory neural circuitry in a rat model of chronic cervical level 2 (C2) hemisection SCI. We found that systemic delivery of PTEN antagonist peptide 4 (PAP4) starting at 8 weeks after C2 hemisection promoted substantial, long-distance regeneration of injured bulbospinal rostral Ventral Respiratory Group (rVRG) axons into and through the lesion and back toward phrenic motor neurons (PhMNs) located in intact caudal C3-C5 spinal cord. Despite this robust rVRG axon regeneration, PAP4 stimulated only minimal recovery of diaphragm function. Furthermore, re-lesion through the hemisection site completely removed PAP4-induced functional improvement, demonstrating that axon regeneration through the lesion was responsible for this partial functional recovery. Interestingly, there was minimal formation of putative excitatory monosynaptic connections between regrowing rVRG axons and PhMN targets, suggesting that (1) limited rVRG-PhMN synaptic reconnectivity was responsible at least in part for the lack of a significant functional effect, (2) chronically-injured spinal cord presents an obstacle to achieving synaptogenesis between regenerating axons and post-synaptic targets, and (3) addressing this challenge is a potentially-powerful strategy to enhance therapeutic efficacy in the chronic SCI setting. In conclusion, our study demonstrates a non-invasive and transient pharmacological approach in chronic SCI to repair the critically-important neural circuitry controlling diaphragmatic respiratory function, but also sheds light on obstacles to circuit plasticity presented by the chronically-injured spinal cord.

Long-Distance Axon Regeneration Promotes Recovery of Diaphragmatic Respiratory Function after Spinal Cord Injury

Compromise in inspiratory breathing following cervical spinal cord injury (SCI) is caused by damage to descending bulbospinal axons originating in the rostral ventral respiratory group (rVRG) and consequent denervation and silencing of phrenic motor neurons (PhMNs) that directly control diaphragm activation. In a rat model of high-cervical hemisection SCI, we performed systemic administration of an antagonist peptide directed against phosphatase and tensin homolog (PTEN), a central inhibitor of neuron-intrinsic axon growth potential. PTEN antagonist peptide (PAP4) robustly restored diaphragm function, as determined with electromyography (EMG) recordings in living SCI animals. PAP4 promoted substantial, long-distance regeneration of injured rVRG axons through the lesion and back toward PhMNs located throughout the C3–C5 spinal cord. These regrowing rVRG axons also formed putative excitatory synaptic connections with PhMNs, demonstrating reconnection of rVRG-PhMN-diaphragm circuitry. Lastly, re-lesion through the hemisection site completely ablated functional recovery induced by PAP4. Collectively, our findings demonstrate that axon regeneration in response to systemic PAP4 administration promoted recovery of diaphragmatic respiratory function after cervical SCI.

Muscles of Breathing: Development, Function, and Patterns of Activation

This review is a comprehensive description of all muscles that assist lung inflation or deflation in any way. The developmental origin, anatomical orientation, mechanical action, innervation, and pattern of activation are described for each respiratory muscle fulfilling this broad definition. In addition, the circumstances in which each muscle is called upon to assist ventilation are discussed. The number of "respiratory" muscles is large, and the coordination of respiratory muscles with "nonrespiratory" muscles and in nonrespiratory activities is complex-commensurate with the diversity of activities that humans pursue, including sleep (8.27). The capacity for speech and adoption of the bipedal posture in human evolution has resulted in patterns of respiratory muscle activation that differ significantly from most other animals. A disproportionate number of respiratory muscles affect the nose, mouth, pharynx, and larynx, reflecting the vital importance of coordinated muscle activity to control upper airway patency during both wakefulness and sleep. The upright posture has freed the hands from locomotor functions, but the evolutionary history and ontogeny of forelimb muscles pervades the patterns of activation and the forces generated by these muscles during breathing. The distinction between respiratory and nonrespiratory muscles is artificial, as many "nonrespiratory" muscles can augment breathing under conditions of high ventilator demand. Understanding the ontogeny, innervation, activation patterns, and functions of respiratory muscles is clinically useful, particularly in sleep medicine. Detailed explorations of how the nervous system controls the multiple muscles required for successful completion of respiratory behaviors will continue to be a fruitful area of investigation. © 2019 American Physiological Society. Compr Physiol 9:1025-1080, 2019.

Astrocyte progenitor transplantation promotes regeneration of bulbospinal respiratory axons, recovery of diaphragm function, and a reduced macrophage response following cervical spinal cord injury

Stem/progenitor cell transplantation delivery of astrocytes is a potentially powerful strategy for spinal cord injury (SCI). Axon extension into SCI lesions that occur spontaneously or in response to experimental manipulations is often observed along endogenous astrocyte "bridges," suggesting that augmenting this response via astrocyte lineage transplantation can enhance axon regrowth. Given the importance of respiratory dysfunction post-SCI, we transplanted glial-restricted precursors (GRPs)-a class of lineage-restricted astrocyte progenitors-into the C2 hemisection model and evaluated effects on diaphragm function and the growth response of descending rostral ventral respiratory group (rVRG) axons that innervate phrenic motor neurons (PhMNs). GRPs survived long term and efficiently differentiated into astrocytes in injured spinal cord. GRPs promoted significant recovery of diaphragm electromyography amplitudes and stimulated robust regeneration of injured rVRG axons. Although rVRG fibers extended across the lesion, no regrowing axons re-entered caudal spinal cord to reinnervate PhMNs, suggesting that this regeneration response-although impressive-was not responsible for recovery. Within ipsilateral C3-5 ventral horn (PhMN location), GRPs induced substantial sprouting of spared fibers originating in contralateral rVRG and 5-HT axons that are important for regulating PhMN excitability; this sprouting was likely involved in functional effects of GRPs. Finally, GRPs reduced the macrophage response (which plays a key role in inducing axon retraction and limiting regrowth) both within the hemisection and at intact caudal spinal cord surrounding PhMNs. These findings demonstrate that astrocyte progenitor transplantation promotes significant plasticity of rVRG-PhMN circuitry and restoration of diaphragm function and suggest that these effects may be in part through immunomodulation.

Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Clinical and Neurophysiological Changes after Targeted Intrathecal Injections of Bone Marrow Stem Cells in a C3 Tetraplegic Subject

High-level quadriplegia is a devastating condition with limited treatment options. Bone marrow derived stem cells (BMSCs) are reported to have immunomodulatory and neurotrophic effects in spinal cord injury (SCI). We report a subject with complete C2 SCI who received three anatomically targeted intrathecal infusions of BMSCs under a single-patient expanded access investigational new drug (IND). She underwent intensive physical therapy and was followed for >2 years. At end-point, her American Spinal Injury Association Impairment Scale (AIS) grade improved from A to B, and she recovered focal pressure touch sensation over several body areas. We conducted serial neurophysiological testing to monitor changes in residual connectivity. Motor, sensory, and autonomic system testing included motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), electromyography (EMG) recordings, F waves, galvanic skin responses, and tilt-table responses. The quality and magnitude of voluntary EMG activations increased over time, but remained below the threshold of clinically obvious movement. Unexpectedly, at 14 months post-injury, deep inspiratory maneuvers triggered respiratory-like EMG bursting in the biceps and several other muscles. This finding means that connections between respiratory neurons and motor neurons were newly established, or unmasked. We also report serial analysis of MRI, International Standards for Neurological Classification of SCI (ISNCSCI), pulmonary function, pain scores, cerebrospinal fluid (CSF) cytokines, and bladder assessment. As a single case, the linkage of the clinical and neurophysiological changes to either natural history or to the BMSC infusions cannot be resolved. Nevertheless, such detailed neurophysiological assessment of high cervical SCI patients is rarely performed. Our findings indicate that electrophysiology studies are sensitive to define both residual connectivity and new plasticity.

Local BDNF Delivery to the Injured Cervical Spinal Cord using an Engineered Hydrogel Enhances Diaphragmatic Respiratory Function

We developed an innovative biomaterial-based approach to repair the critical neural circuitry that controls diaphragm activation by locally delivering brain-derived neurotrophic factor (BDNF) to injured cervical spinal cord. BDNF can be used to restore respiratory function via a number of potential repair mechanisms; however, widespread BDNF biodistribution resulting from delivery methods such as systemic injection or lumbar puncture can lead to inefficient drug delivery and adverse side effects. As a viable alternative, we developed a novel hydrogel-based system loaded with polysaccharide-BDNF particles self-assembled by electrostatic interactions that can be safely implanted in the intrathecal space for achieving local BDNF delivery with controlled dosing and duration. Implantation of BDNF hydrogel after C4/C5 contusion-type spinal cord injury (SCI) in female rats robustly preserved diaphragm function, as assessed by in vivo recordings of compound muscle action potential and electromyography amplitudes. However, BDNF hydrogel did not decrease lesion size or degeneration of cervical motor neuron soma, suggesting that its therapeutic mechanism of action was not neuroprotection within spinal cord. Interestingly, BDNF hydrogel significantly preserved diaphragm innervation by phrenic motor neurons (PhMNs), as assessed by detailed neuromuscular junction morphological analysis and retrograde PhMN labeling from diaphragm using cholera toxin B. Furthermore, BDNF hydrogel enhanced the serotonergic axon innervation of PhMNs that plays an important role in modulating PhMN excitability. Our findings demonstrate that local BDNF hydrogel delivery is a robustly effective and safe strategy to restore diaphragm function after SCI. In addition, we demonstrate novel therapeutic mechanisms by which BDNF can repair respiratory neural circuitry.SIGNIFICANCE STATEMENT Respiratory compromise is a leading cause of morbidity and mortality following traumatic spinal cord injury (SCI). We used an innovative biomaterial-based drug delivery system in the form of a hydrogel that can be safely injected into the intrathecal space for achieving local delivery of brain-derived neurotrophic factor (BDNF) with controlled dosing and duration, while avoiding side effects associated with other delivery methods. In a clinically relevant rat model of cervical contusion-type SCI, BDNF hydrogel robustly and persistently improved diaphragmatic respiratory function by enhancing phrenic motor neuron (PhMN) innervation of the diaphragm neuromuscular junction and by increasing serotonergic innervation of PhMNs in ventral horn of the cervical spinal cord. These exciting findings demonstrate that local BDNF hydrogel delivery is a safe and robustly effective strategy to maintain respiratory function after cervical SCI.

Differential Response in Novel Stem Cell Niches of the Brain after Cervical Spinal Cord Injury and Traumatic Brain Injury

Populations of neural stem cells (NSCs) reside in a number of defined niches in the adult central nervous system (CNS) where they continually give rise to mature cell types throughout life, including newly born neurons. In addition to the prototypical niches of the subventricular zone (SVZ) and subgranular zone (SGZ) of the hippocampal dentate gyrus, novel stem cell niches that are also neurogenic have recently been identified in multiple midline structures, including circumventricular organs (CVOs) of the brain. These resident NSCs serve as a homeostatic source of new neurons and glial cells under intact physiological conditions. Importantly, they may also have the potential for reparative processes in pathological states such as traumatic spinal cord injury (SCI) and traumatic brain injury (TBI). As the response in these novel CVO stem cell niches has been characterized after stroke but not following SCI or TBI, we quantitatively assessed cell proliferation and the neuronal and glial lineage fate of resident NSCs in three CVO nuclei-area postrema (AP), median eminence (ME), and subfornical organ (SFO) -in rat models of cervical contusion-type SCI and controlled cortical impact (CCI)-induced TBI. Using bromodeoxyuridine (BrdU) labeling of proliferating cells, we find that TBI significantly enhanced proliferation in AP, ME, and SFO, whereas cervical SCI had no effects at early or chronic time-points post-injury. In addition, SCI did not alter NSC differentiation profile into doublecortin-positive neuroblasts, GFAP-expressing astrocytes, or Olig2-labeled cells of the oligodendrocyte lineage within AP, ME, or SFO at both time-points. In contrast, CCI induced a pronounced increase in Sox2- and doublecortin-labeled cells in the AP and Iba1-labeled microglia in the SFO. Lastly, plasma derived from CCI animals significantly increased NSC expansion in an in vitro neurosphere assay, whereas plasma from SCI animals did not exert such an effect, suggesting that signaling factors present in blood may be relevant to stimulating CVO niches after CNS injury and may explain the differential in vivo effects of SCI and TBI on the novel stem cell niches.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

Neuroprotective and Neurorestorative Processes after Spinal Cord Injury: The Case of the Bulbospinal Respiratory Neurons

High cervical spinal cord injuries interrupt the bulbospinal respiratory pathways projecting to the cervical phrenic motoneurons resulting in important respiratory defects. In the case of a lateralized injury that maintains the respiratory drive on the opposite side, a partial recovery of the ipsilateral respiratory function occurs spontaneously over time, as observed in animal models. The rodent respiratory system is therefore a relevant model to investigate the neuroplastic and neuroprotective mechanisms that will trigger such phrenic motoneurons reactivation by supraspinal pathways. Since part of this recovery is dependent on the damaged side of the spinal cord, the present review highlights our current understanding of the anatomical neuroplasticity processes that are developed by the surviving damaged bulbospinal neurons, notably axonal sprouting and rerouting. Such anatomical neuroplasticity relies also on coordinated molecular mechanisms at the level of the axotomized bulbospinal neurons that will promote both neuroprotection and axon growth.

Harnessing the power of cell transplantation to target respiratory dysfunction following spinal cord injury

The therapeutic benefit of cell transplantation has been assessed in a host of central nervous system (CNS) diseases, including disorders of the spinal cord such as traumatic spinal cord injury (SCI). The promise of cell transplantation to preserve and/or restore normal function can be aimed at a variety of therapeutic mechanisms, including replacement of lost or damaged CNS cell types, promotion of axonal regeneration or sprouting, neuroprotection, immune response modulation, and delivery of gene products such as neurotrophic factors, amongst other possibilities. Despite significant work in the field of transplantation in models of SCI, limited attention has been directed at harnessing the therapeutic potential of cell grafting for preserving respiratory function after SCI, despite the critical role pulmonary compromise plays in patient outcome in this devastating disease. Here, we will review the limited number of studies that have demonstrated the therapeutic potential of intraspinal transplantation of a variety of cell types for addressing respiratory dysfunction in SCI.