Repetitive intermittent hypoxia induces respiratory and somatic motor recovery after chronic cervical spinal injury

Prolonged intermittent hypoxia differentially regulates phrenic motor neuron serotonin receptor expression in rats following chronic cervical spinal cord injury

Low-dose (< 2 h/day), acute intermittent hypoxia (AIH) elicits multiple forms of serotonin-dependent phrenic motor plasticity and is emerging as a promising therapeutic strategy to restore respiratory and non-respiratory motor function after spinal cord injury (SCI). In contrast, high-dose (> 8 h/day), chronic intermittent hypoxia (CIH) undermines some forms of serotonin-dependent phrenic motor plasticity and elicits pathology. CIH is a hallmark of sleep disordered breathing, which is highly prevalent in individuals with cervical SCI. Interestingly, AIH and CIH preconditioning differentially impact phrenic motor plasticity. Although mechanisms of AIH-induced plasticity in the phrenic motor system are well-described in naïve rats, we know little concerning how these mechanisms are affected by chronic SCI or intermittent hypoxia preconditioning. Thus, in a rat model of chronic, incomplete cervical SCI (lateral spinal hemisection at C2 (C2Hx), we assessed serotonin type 2A, 2B and 7 receptor expression in and near phrenic motor neurons and compared: 1) intact vs. chronically injured rats; and 2) the impact of preconditioning with varied "doses" of intermittent hypoxia (IH). While there were no effects of chronic injury or intermittent hypoxia alone, CIH affected multiple receptors in rats with chronic C2Hx. Specifically, CIH preconditioning (8 h/day; 28 days) increased serotonin 2A and 7 receptor expression exclusively in rats with chronic C2Hx. Understanding the complex, context-specific interactions between chronic SCI and CIH and how this ultimately impacts phrenic motor plasticity is important as we leverage AIH-induced motor plasticity to restore breathing and other non-respiratory motor functions in people with chronic SCI.

Intermittent Hypoxia-Induced Enhancements in Corticospinal Excitability Predict Gains in Motor Learning and Metabolic Efficiency

Acute intermittent hypoxia (AIH) enhances human motor function after incomplete spinal cord injury. Although the underlying mechanisms in humans are unknown, emerging evidence indicates that AIH facilitates corticospinal excitability to the upper limb. However, the functional relevance of this plasticity remains unexplored, and it is unclear whether similar plasticity can be induced for lower limb motor areas. We recently demonstrated that AIH improves motor learning and metabolic efficiency during split-belt walking. Thus, we hypothesized that AIH increases lower limb excitability and that these enhancements would predict the magnitude of motor learning and the corresponding reductions in net metabolic power. We assessed tibialis anterior (TA) excitability using transcranial magnetic stimulation and quantified changes in spatiotemporal asymmetries and net metabolic power in response to split-belt speed perturbations. We show that AIH enhances TA excitability, and that the magnitude of this facilitation positively correlates with greater spatiotemporal adaptation. Notably, we demonstrate a novel association between increased excitability and reduced net metabolic power during motor learning and savings. Together, our results suggest that AIH-induced gains in excitability predict both the magnitude of motor learning and the associated metabolic efficiency. Determining indices of AIH-induced improvements in motor performance is critical for optimizing its therapeutic reach.

A single sequence of intermittent hypoxia does not alter stretch reflex excitability in able-bodied individuals

Spasticity attributable to exaggerated stretch reflex pathways, particularly affecting the ankle plantar flexors, often impairs overground walking in persons with incomplete spinal cord injury. Compelling evidence from rodent models underscores how exposure to acute intermittent hypoxia (AIH) can provide a unique medium to induce spinal plasticity in key inhibitory pathways mediating stretch reflex excitability and potentially affect spasticity. In this study, we quantify the effects of a single exposure to AIH on the stretch reflex in able-bodied individuals. We hypothesized that a single sequence of AIH will increase the stretch reflex excitability of the soleus muscle during ramp-and-hold angular perturbations applied to the ankle joint while participants perform passive and volitionally matched contractions. Our results revealed that a single AIH exposure did not significantly change the stretch reflex excitability during both passive and active matching conditions. Furthermore, we found that able-bodied individuals increased their stretch reflex response from passive to active matching conditions after both sham and AIH exposures. Together, these findings suggest that a single AIH exposure might not engage inhibitory pathways sufficiently to alter stretch reflex responses in able-bodied persons. However, the generalizability of our present findings requires further examination during repetitive exposures to AIH along with potential reflex modulation during functional movements, such as overground walking.

Acute Intermittent Hypoxia Did Not Alter Pain Sensitivity or Pain Intensity Ratings for Individuals with Chronic Low Back Pain: A Pilot Study

Aims and Objective

The purpose of this pilot study was to explore whether AIH produces changes in pain sensitivity or in reports of self-reported pain intensity for individuals with low back pain.

Methods

In a quasi-experimental, cross-over design we compared participants (n = 9) exposed to normal room air and hypoxia using a commercially available gas blender. The treatment period consisted of 5 consecutive days of randomly assigned to AIH or room air. For the participants initially randomized to AIH there was cross-over to receive 5 more consecutive days of room air. Therefore, this design allowed for between group and within subject assessment of AIH effects. Pain sensitivity was assessed with quantitative sensory testing (QST) for posterior superior iliac spine pressure threshold, plantar thermal threshold, and peak pain ratings. Self-reported pain intensity for low back pain was assessed via the Brief Pain Inventory.

Results

There were no between group differences for AIH and room air in pain sensitivity or self-reported pain intensity. In the within subject analyses larger effect sizes favoring AIH were detected for plantar measures of pain sensitivity but not for self-reported pain intensity.

Conclusion

This study, while presenting null findings, describes an initial step in determining whether AIH can be used to increase pain relief. Based on this pilot study we offer guidance for future research including study design, AIH dosage, participant selection, and using AIH in combination with non-pharmacologic treatments.

Enhanced motor learning and motor savings after acute intermittent hypoxia are associated with a reduction in metabolic cost

Breathing mild bouts of low oxygen air (i.e. acute intermittent hypoxia, AIH) has been shown to improve locomotor function in humans after a spinal cord injury. How AIH-induced gains in motor performance are achieved remains unclear. We examined the hypothesis that AIH augments motor learning and motor retention during a locomotor adaptation task. We further hypothesized that gains in motor learning and retention will be associated with reductions in net metabolic power, consistent with the acquisition of energetically favourable mechanics. Thirty healthy individuals were randomly allocated into either a control group or an AIH group. We utilized a split-belt treadmill to characterize adaptations to an unexpected belt speed perturbation of equal magnitude during an initial exposure and a second exposure. Adaptation was characterized by changes in spatiotemporal step asymmetry, anterior-posterior force asymmetry, and net metabolic power. While both groups adapted by reducing spatial asymmetry, only the AIH group achieved significant reductions in double support time asymmetry and propulsive force asymmetry during both the initial and the second exposures to the belt speed perturbation. Net metabolic power was also significantly lower in the AIH group, with significant reductions from the initial perturbation exposure to the second. These results provide the first evidence that AIH mediates improvements in both motor learning and retention. Further, our results suggest that reductions in net metabolic power continue to be optimized upon subsequent learning and are driven by more energetically favourable temporal coordination strategies. Our observation that AIH facilitates motor learning and retention can be leveraged to design rehabilitation interventions that promote functional recovery. KEY POINTS: Brief exposures to low oxygen air, known as acute intermittent hypoxia (AIH), improves locomotor function in humans after a spinal cord injury, but it remains unclear how gains in motor performance are achieved. In this study, we tested the hypothesis that AIH induces enhancements in motor learning and retention by quantifying changes in interlimb coordination, anterior-posterior force symmetry and metabolic cost during a locomotor adaptation task. We show the first evidence that AIH improves both motor learning and savings of newly learned temporal interlimb coordination strategies and force asymmetry compared to untreated individuals. We further demonstrate that AIH elicits greater reductions in metabolic cost during motor learning that continues to be optimized upon subsequent learning. Our findings suggest that AIH-induced gains in locomotor performance are facilitated by enhancements in motor learning and retention of more energetically favourable coordination strategies.

A Research Protocol to Study the Priming Effects of Breathing Low Oxygen on Enhancing Training-Related Gains in Walking Function for Persons With Spinal Cord Injury: The BO2ST Trial

Brief episodes of low oxygen breathing (therapeutic acute intermittent hypoxia; tAIH) may serve as an effective plasticity-promoting primer to enhance the effects of transcutaneous spinal stimulation-enhanced walking therapy (WALKtSTIM) in persons with chronic (>1 year) spinal cord injury (SCI). Pre-clinical studies in rodents with SCI show that tAIH and WALKtSTIM therapies harness complementary mechanisms of plasticity to maximize walking recovery. Here, we present a multi-site clinical trial protocol designed to examine the influence of tAIH + WALKtSTIM on walking recovery in persons with chronic SCI. We hypothesize that daily (eight sessions, 2 weeks) tAIH + WALKtSTIM will elicit faster, more persistent improvements in walking recovery than either treatment alone. To test our hypothesis, we are conducting a placebo-controlled clinical trial on 60 SCI participants who randomly receive one of three interventions: tAIH + WALKtSTIM; Placebo + WALKtSTIM; and tAIH + WALKtSHAM. Participants receive daily tAIH (fifteen 90-sec episodes at 10% O2 with 60-sec intervals at 21% O2) or daily placebo (fifteen 90-sec episodes at 21% O2 with 60-sec intervals at 21% O2) before a 45-min session of WALKtSTIM or WALKtSHAM. Our primary outcome measures assess walking speed (10-Meter Walk Test), endurance (6-Minute Walk Test), and balance (Timed Up and Go Test). For safety, we also measure pain levels, spasticity, sleep behavior, cognition, and rates of systemic hypertension and autonomic dysreflexia. Assessments occur before, during, and after sessions, as well as at 1, 4, and 8 weeks post-intervention. Results from this study extend our understanding of the functional benefits of tAIH priming by investigating its capacity to boost the neuromodulatory effects of transcutaneous spinal stimulation on restoring walking after SCI. Given that there is no known cure for SCI and no single treatment is sufficient to overcome walking deficits, there is a critical need for combinatorial treatments that accelerate and anchor walking gains in persons with lifelong SCI.

Trial Registration

ClinicalTrials.gov, NCT05563103.

Cervical spinal hemisection effects on spinal tissue oxygenation and long-term facilitation of phrenic, renal and splanchnic sympathetic nerve activity

HYPOTHESES

Moderate acute intermittent hypoxia (mAIH) elicits plasticity in both respiratory (phrenic long-term facilitation; pLTF) and sympathetic nerve activity (sympLTF) in rats. Although mAIH produces pLTF in normal rats, inconsistent results are reported after cervical spinal cord injury (cSCI), possibly due to greater spinal tissue hypoxia below the injury site. There are no reports concerning cSCI effects on sympLTF. Since mAIH is being explored as a therapeutic modality to restore respiratory and non-respiratory movements in humans with chronic SCI, both effects are important. To understand cSCI effects on mAIH-induced pLTF and sympLTF, partial or complete C2 spinal hemisections (C2Hx) were performed and, 2 weeks later, we assessed: 1) ipsilateral cervical spinal tissue oxygen tension; 2) ipsilateral & contralateral pLTF; and 3) ipsilateral sympLTF in splanchnic and renal sympathetic nerves.

METHODS

Male Sprague-Dawley rats were studied intact, or after partial (single slice) or complete C2Hx (slice with ∼1 mm aspiration). Two weeks post-C2Hx, rats were anesthetized and prepared for recordings of bilateral phrenic nerve activity and spinal tissue oxygen pressure (PtO2). Splanchnic and renal sympathetic nerve activity was recorded in intact and complete C2Hx rats.

RESULTS

Spinal PtO2 near phrenic motor neurons was decreased after C2Hx, an effect most prominent with complete vs. partial injuries; baseline PtO2 was positively correlated with mean arterial pressure. Complete C2Hx impaired ipsilateral but not contralateral pLTF; with partial C2Hx, ipsilateral pLTF was unaffected. In intact rats, mAIH elicited splanchnic and renal sympLTF. Complete C2Hx had minimal impact on baseline ipsilateral splanchnic or renal sympathetic nerve activity and renal, but not splanchnic, sympLTF remained intact.

CONCLUSION

Greater tissue hypoxia likely impairs pLTF and splanchnic sympLTF post-C2Hx, although renal sympLTF remains intact. Increased sympathetic nerve activity post-mAIH may have therapeutic benefits in individuals living with chronic SCI since anticipated elevations in systemic blood pressure may mitigate hypotension characteristic of people living with SCI.

Acute intermittent hypoxia enhances strength, and modulates spatial distribution of muscle activation in persons with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for facilitating neural plasticity in individuals with chronic incomplete spinal cord injury (iSCI). A single sequence of AIH enhances hand grip strength and ankle plantarflexion torque, but underlying mechanisms are not yet clear. We sought to examine how AIH-induced changes in magnitude and spatial distribution of the electromyogram (EMG) of the biceps and triceps brachii contributes to improved strength. Seven individuals with iSCI visited the laboratory on two occasions, and received either AIH or Sham AIH intervention in a randomized order. AIH consisted of 15 brief (∼60s) periods of low oxygen (fraction of inspired O2 = 0.09) alternating with 60s of normoxia, whereas Sham AIH consisted of repeated exposures to normoxic air. High-density surface EMG of biceps and triceps brachii was recorded during maximal elbow flexion and extension. We then generated spatial maps which distinguished active muscle regions prior to and 60 min after AIH or Sham AIH. After an AIH sequence, elbow flexion and extension forces increased by 91.7 ± 88.4% and 51.7 ± 57.8% from baseline, respectively, whereas there was no difference after Sham AIH. Changes in strength were associated with an altered spatial distribution of EMG and increased root mean squared EMG amplitude in both biceps and triceps brachii muscles. These data suggest that altered motor unit activation profiles may underlie improved volitional strength after a single dose of AIH and warrant further investigation using single motor unit analysis techniques to further elucidate mechanisms of AIH-induced plasticity.

Novel role for non-invasive neuromodulation techniques in central respiratory dysfunction

Respiration is a crucial steady-state function of human life. Central nervous system injury can damage the central respiratory pattern generator (CRPG) or interrupt its outflow, leading to central respiratory paralysis and dysfunction, which can endanger the patient's life. At present, there is no effective means to reverse this process. Commonly used non-invasive neuromodulation techniques include repetitive transcranial magnetic stimulation (rTMS), transcranial direct current stimulation (tDCS) and so forth, which have been widely applied in nervous system diseases and their various secondary symptoms, but rarely in respiratory function. Clinical and animal studies have confirmed that TMS is also suitable for investigating the excitability and plasticity of ascending corticospinal respiratory pathways. In addition, although rTMS and tDCS differ in their respective mechanisms, both can regulate respiratory networks in healthy individuals and in diseased states. In this review, we provide an overview of the physiology of respiration, the use of TMS to assess the excitability of corticophrenic pathways in healthy individuals and in central respiratory disorders, followed by an overview of the animal and clinical studies of rTMS, tDCS and so forth in regulating respiratory circuits and the possible mechanisms behind them. It was found that the supplementary motor area (SMA) and the phrenic motor neuron (PMN) may be key regulatory areas. Finally, the challenges and future research directions of neuroregulation in respiratory function are proposed. Through understanding how neuromodulation affects the respiratory neural circuit non-invasively, we can further explore the therapeutic potential of this neuromodulation strategy, so as to promote the recovery of respiratory function after central nervous system diseases or injury.

Acute intermittent hypoxia alters disease course and promotes CNS repair including resolution of inflammation and remyelination in the experimental autoimmune encephalomyelitis model of MS

Remyelination and neurodegeneration prevention mitigate disability in Multiple Sclerosis (MS). We have shown acute intermittent hypoxia (AIH) is a novel, non-invasive and effective therapy for peripheral nerve repair, including remyelination. Thus, we posited AIH would improve repair following CNS demyelination and address the paucity of MS repair treatments. AIH's capacity to enhance intrinsic repair, functional recovery and alter disease course in the experimental autoimmune encephalomyelitis (EAE) model of MS was assessed. EAE was induced by MOG35-55 immunization in C57BL/6 female mice. EAE mice received either AIH (10 cycles-5 min 11% oxygen alternating with 5 min 21% oxygen) or Normoxia (control; 21% oxygen for same duration) once daily for 7d beginning at near peak EAE disease score of 2.5. Mice were followed post-treatment for an additional 7d before assessing histopathology or 14d to examine maintenance of AIH effects. Alterations in histopathological correlates of multiple repair indices were analyzed quantitatively in focally demyelinated ventral lumbar spinal cord areas to assess AIH impacts. AIH begun at near peak disease significantly improved daily clinical scores/functional recovery and associated histopathology relative to Normoxia controls and the former were maintained for at least 14d post-treatment. AIH enhanced correlates of myelination, axon protection and oligodendrocyte precursor cell recruitment to demyelinated areas. AIH also effected a dramatic reduction in inflammation, while polarizing remaining macrophages/microglia toward a pro-repair state. Collectively, this supports a role for AIH as a novel non-invasive therapy to enhance CNS repair and alter disease course following demyelination and holds promise as a neuroregenerative MS strategy.

Therapeutic Strategies Targeting Respiratory Recovery after Spinal Cord Injury: From Preclinical Development to Clinical Translation

High spinal cord injuries (SCIs) lead to permanent functional deficits, including respiratory dysfunction. Patients living with such conditions often rely on ventilatory assistance to survive, and even those that can be weaned continue to suffer life-threatening impairments. There is currently no treatment for SCI that is capable of providing complete recovery of diaphragm activity and respiratory function. The diaphragm is the main inspiratory muscle, and its activity is controlled by phrenic motoneurons (phMNs) located in the cervical (C3-C5) spinal cord. Preserving and/or restoring phMN activity following a high SCI is essential for achieving voluntary control of breathing. In this review, we will highlight (1) the current knowledge of inflammatory and spontaneous pro-regenerative processes occurring after SCI, (2) key therapeutics developed to date, and (3) how these can be harnessed to drive respiratory recovery following SCIs. These therapeutic approaches are typically first developed and tested in relevant preclinical models, with some of them having been translated into clinical studies. A better understanding of inflammatory and pro-regenerative processes, as well as how they can be therapeutically manipulated, will be the key to achieving optimal functional recovery following SCIs.

Hypoxia extends lifespan and neurological function in a mouse model of aging

There is widespread interest in identifying interventions that extend healthy lifespan. Chronic continuous hypoxia delays the onset of replicative senescence in cultured cells and extends lifespan in yeast, nematodes, and fruit flies. Here, we asked whether chronic continuous hypoxia is beneficial in mammalian aging. We utilized the Ercc1 Δ/- mouse model of accelerated aging given that these mice are born developmentally normal but exhibit anatomic, physiological, and biochemical features of aging across multiple organs. Importantly, they exhibit a shortened lifespan that is extended by dietary restriction, the most potent aging intervention across many organisms. We report that chronic continuous 11% oxygen commenced at 4 weeks of age extends lifespan by 50% and delays the onset of neurological debility in Ercc1 Δ/- mice. Chronic continuous hypoxia did not impact food intake and did not significantly affect markers of DNA damage or senescence, suggesting that hypoxia did not simply alleviate the proximal effects of the Ercc1 mutation, but rather acted downstream via unknown mechanisms. To the best of our knowledge, this is the first study to demonstrate that "oxygen restriction" can extend lifespan in a mammalian model of aging.

Neurotrophins in the Neuropathophysiology, Course, and Complications of Obstructive Sleep Apnea-A Narrative Review

Obstructive sleep apnea (OSA) is a disorder characterized by chronic intermittent hypoxia and sleep fragmentation due to recurring airway collapse during sleep. It is highly prevalent in modern societies, and due to its pleiotropic influence on the organism and numerous sequelae, it burdens patients and physicians. Neurotrophins (NTs), proteins that modulate the functioning and development of the central nervous system, such as brain-derived neurotrophic factor (BDNF), have been associated with OSA, primarily due to their probable involvement in offsetting the decline in cognitive functions which accompanies OSA. However, NTs influence multiple aspects of biological functioning, such as immunity. Thus, extensive evaluation of their role in OSA might enlighten the mechanism behind some of its elusive features, such as the increased risk of developing an immune-mediated disease or the association of OSA with cardiovascular diseases. In this review, we examine the interactions between NTs and OSA and discuss their contribution to OSA pathophysiology, complications, as well as comorbidities.

Initiating daily acute intermittent hypoxia (dAIH) therapy at 1-week after contusion spinal cord injury (SCI) improves lower urinary tract function in rat

Spinal cord injury (SCI) above the level of the lumbosacral spinal cord produces lower urinary tract (LUT) dysfunction, resulting in impairment of urine storage and elimination (voiding). While spontaneous functional recovery occurs due to remodeling of spinal reflex micturition pathways, it is incomplete, indicating that additional strategies to further augment neural plasticity following SCI are essential. To this end, acute intermittent hypoxia (AIH) exposure has been proposed as a therapeutic strategy for improving recovery of respiratory and other somatic motor function following SCI; however, the impact of AIH as a therapeutic intervention to improve LUT dysfunction remains to be determined. Therefore, we examined the effects of daily AIH (dAIH) on both spontaneous micturition patterns and reflex micturition event (rME) behaviors in adult female Sprague-Dawley rats with mid-thoracic moderate contusion SCI. For these experiments, dAIH gas exposures (five alternating 3 min 12% O₂ and 21% O₂ episodes) were delivered for 7 consecutive days beginning at 1-week after SCI, with awake micturition patterns being evaluated weekly for 2-3 sessions before and for 4 weeks after SCI and rME behaviors elicited by continuous infusion of saline into the bladder being evaluated under urethane anesthesia at 4-weeks after SCI; daily normoxia (dNx; 21% O₂ episodes) served as a control. At 1-week post-SCI, both an areflexic phenotype (i.e., no effective voiding events) and a functional voiding phenotype (i.e., infrequent voiding events with large volumes) were observed in spontaneous micturition patterns (as expected), and subsequent dAIH, but not dNx, treatment led to recovery of spontaneous void frequency pattern to pre-SCI levels; both dAIH- and dNx-treated rats exhibited slightly increased void volumes. At 4-weeks post-SCI, rME behaviors showed increased effectiveness in voiding in dAIH-treated (compared to dNx-treated) rats that included an increase in both bladder contraction pressure (delta BP; P = 0.014) and dynamic voiding efficiency (P = 0.018). Based on the voiding and non-voiding bladder contraction behaviors (VC and NVC, respectively) observed in the BP records, bladder dysfunction severity was classified into mild, moderate, and severe phenotypes, and while rats in both treatment groups included each severity phenotype, the primary phenotype observed in dAIH-treated rats was mild and that in dNx-treated rats was moderate (P = 0.044). Taken together, these findings suggest that 7-day dAIH treatment produces beneficial improvements in LUT function that include recovery of micturition pattern, more efficient voiding, and decreased NVCs, and extend support to the use of dAIH therapy to treat SCI-induced LUT dysfunction.

Effects of C2 hemisection on respiratory and cardiovascular functions in rats

High cervical spinal cord injuries induce permanent neuromotor and autonomic deficits. These injuries impact both central respiratory and cardiovascular functions through modulation of the sympathetic nervous system. So far, cardiovascular studies have focused on models of complete contusion or transection at the lower cervical and thoracic levels and diaphragm activity evaluations using invasive methods. The present study aimed to evaluate the impact of C2 hemisection on different parameters representing vital functions (i.e., respiratory function, cardiovascular, and renal filtration parameters) at the moment of injury and 7 days post-injury in rats. No ventilatory parameters evaluated by plethysmography were impacted during quiet breathing after 7 days post-injury, whereas permanent diaphragm hemiplegia was observed by ultrasound and confirmed by diaphragmatic electromyography in anesthetized rats. Interestingly, the mean arterial pressure was reduced immediately after C2 hemisection, with complete compensation at 7 days post-injury. Renal filtration was unaffected at 7 days post-injury; however, remnant systolic dysfunction characterized by a reduced left ventricular ejection fraction persisted at 7 days post-injury. Taken together, these results demonstrated that following C2 hemisection, diaphragm activity and systolic function are impacted up to 7 days post-injury, whereas the respiratory and cardiovascular systems display vast adaptation to maintain ventilatory parameters and blood pressure homeostasis, with the latter likely sustained by the remaining descending sympathetic inputs spared by the initial injury. A better broad characterization of the physiopathology of high cervical spinal cord injuries covering a longer time period post-injury could be beneficial for understanding evaluations of putative therapeutics to further increase cardiorespiratory recovery.

Four weeks of repetitive acute hypoxic preconditioning did not alleviate allergen-induced airway dysfunction in rats

Clinical case series suggest beneficial effects of low-dose intermittent hypoxia in asthma. We tested cardiopulmonary effects of repetitive acute hypoxic preconditioning (RAHP) during allergic inflammation. Brown Norway rats were sensitized to house dust mites (HDM) and exposed to 4-week RAHP or normoxia (SHAM), concurrent with weekly HDM or saline (SAL) challenges. We assessed methacholine responses and lung HIF-1α expression at endpoint, and weekly blood pressure (BP). RAHP relative to SHAM: 1) in HDM-challenged rats, showed no protection against HDM-induced airway dysfunction and did not significantly impact BP (week 4 mean BP difference = 10.51 mmHg, p = 0.09) or HIF-1α expression; 2) in SAL-challenged rats, attenuated airway responses to methacholine, reduced BP (week 4 mean BP average difference = -8.72 mmHg, p = 0.04) and amplified HIF-1α expression (p = 0.0086). Four weeks of RAHP did not mitigate the allergen-induced lower airway dysfunction and may detrimentally affect BP. However, it elicited beneficial cardiopulmonary responses in SAL-challenged rats, concurrent with increased HIF-1α expression.

Natural and targeted circuit reorganization after spinal cord injury

A spinal cord injury disrupts communication between the brain and the circuits in the spinal cord that regulate neurological functions. The consequences are permanent paralysis, loss of sensation and debilitating dysautonomia. However, the majority of circuits located above and below the injury remain anatomically intact, and these circuits can reorganize naturally to improve function. In addition, various neuromodulation therapies have tapped into these processes to further augment recovery. Emerging research is illuminating the requirements to reconstitute damaged circuits. Here, we summarize these natural and targeted reorganizations of circuits after a spinal cord injury. We also advocate for new concepts of reorganizing circuits informed by multi-omic single-cell atlases of recovery from injury. These atlases will uncover the molecular logic that governs the selection of 'recovery-organizing' neuronal subpopulations, and are poised to herald a new era in spinal cord medicine.

Intermittent Hypoxia Differentially Regulates Adenosine Receptors in Phrenic Motor Neurons with Spinal Cord Injury

Cervical spinal cord injury (cSCI) impairs neural drive to the respiratory muscles, causing life- threatening complications such as respiratory insufficiency and diminished airway protection. Repetitive "low dose" acute intermittent hypoxia (AIH) is a promising strategy to restore motor function in people with chronic SCI. Conversely, "high dose" chronic intermittent hypoxia (CIH; ∼8 h/night), such as experienced during sleep apnea, causes pathology. Sleep apnea, spinal ischemia, hypoxia and neuroinflammation associated with cSCI increase extracellular adenosine concentrations and activate spinal adenosine receptors which in turn constrains the functional benefits of therapeutic AIH. Adenosine 1 and 2A receptors (A₁, A_2A) compete to determine net cAMP signaling and likely the tAIH efficacy with chronic cSCI. Since cSCI and intermittent hypoxia may regulate adenosine receptor expression in phrenic motor neurons, we tested the hypotheses that: 1) daily AIH (28 days) downregulates A_2A and upregulates A₁ receptor expression; 2) CIH (28 days) upregulates A_2A and downregulates A₁ receptor expression; and 3) cSCI alters the impact of CIH on adenosine receptor expression. Daily AIH had no effect on either adenosine receptor in intact or injured rats. However, CIH exerted complex effects depending on injury status. Whereas CIH increased A₁ receptor expression in intact (not injured) rats, it increased A_2A receptor expression in spinally injured (not intact) rats. The differential impact of CIH reinforces the concept that the injured spinal cord behaves in distinct ways from intact spinal cords, and that these differences should be considered in the design of experiments and/or new treatments for chronic cSCI.

Caffeine Enhances Intermittent Hypoxia-Induced Gains in Walking Function for People with Chronic Spinal Cord Injury

Incomplete spinal cord injury (iSCI) often results in lifelong walking impairments that limit functional independence. Thus, treatments that trigger enduring improvement in walking after iSCI are in high demand. Breathing brief episodes of low oxygen (i.e., acute intermittent hypoxia, AIH) enhances breathing and walking function in rodents and humans with chronic iSCI. Pre-clinical studies found that AIH also causes the accumulation of extracellular adenosine that undermines AIH-induced functional plasticity. Pharmacologically blocking adenosine A2a receptors (A2aR) prior to AIH resulted in a dramatic improvement in motor facilitation in rodents with iSCI; however, a similar beneficial effect in humans is unclear. Thus, we conducted a double-blind, placebo-controlled, crossover randomized study to test the hypothesis that a non-selective A2aR antagonist (i.e., caffeine) enhances AIH-induced effects on walking function in people with chronic (≥1yr) iSCI. We enrolled 12 participants to receive daily (5 days) caffeine or placebo (4 mg/kg) 30 min before breathing 15, 1.5-min low oxygen (AIH; F_IO₂ = 0.10) or SHAM (F_IO₂ = 0.21) episodes with 1-min intervals. We quantified walking function as the change in the 10-meter walk test (speed) and 6-min walk test (endurance) relative to baseline, on Day 5 post-intervention, and on follow-up Days 12 and 19. Participants walked faster (Day 19; p < 0.001) and farther (Day 19; p = 0.012) after caffeine+AIH and the boost in speed persisted more than after placebo+AIH or caffeine+SHAM (Day 19; p < 0.05). These results support our hypothesis that a caffeine pre-treatment to AIH training shows promise as a strategy to augment walking speed in persons with chronic iSCI.

Oropharyngeal Dysphagia in Acute Cervical Spinal Cord Injury: A Literature Review

Dysphagia (swallowing impairment) is a frequent complication of cervical spinal cord injury (cSCI). Recently published national guidance in the UK on rehabilitation after traumatic injury confirmed that people with cSCI are at risk for dysphagia and require early evaluation while remaining nil by mouth [National Institute for Health and Care Excellence. Rehabilitation after traumatic injury (NG211), 2022, https://www.nice.org.uk/guidance/ng21]. While the pathogenesis and pathophysiology of dysphagia in cSCI remains unclear, numerous risk factors have been identified in the literature. This review aims to summarize the literature on the risk factors, presentation, assessment, and management of dysphagia in patients with cSCI. A bespoke approach to dysphagia management, that accounts for the multiple system impairment in cSCI, is presented; the overarching aim of which is to support effective management of dysphagia in patients with cSCI to prevent adverse clinical consequences.

Ultrasound Analysis of Respiration-Related Muscles in Rats

The purpose of this study was to evaluate the effectiveness of ultrasound techniques in the analysis of respiratory-related muscles in rats. Respiratory parameters, including diaphragm end-expiratory thickness, mean rectus abdominis (RA) thickness, and RA area, were measured by ultrasound and compared with histological findings. Spearman’s correlation and Logistic regression analysis were used to detect the differences in the correlation between ultrasound results and histological examinations, and Student’s t test was used to compare the differences between ultrasound results and histological examination data. The results showed that there was no significant difference between the end-expiratory thickness of the diaphragm, the average thickness of RA, and the area of RA in the right RA and histological values under ultrasound detection (p &gt; 0.05), but there was a significant positive correlation between ultrasound, and histological values (p &lt; 0.05).); in addition, tidal volume was significantly positively correlated with total RA area, rapid shallow breathing index (RSBI) was significantly negatively correlated with total RA area, and mean diaphragm TF was significantly positively correlated with tidal volume. In conclusion, ultrasound imaging has a high degree of accuracy and reproducibility and can be used to assess the structure and function of the rat diaphragm and RA.

Respiratory plasticity following spinal cord injury: perspectives from mouse to man

The study of respiratory plasticity in animal models spans decades. At the bench, researchers use an array of techniques aimed at harnessing the power of plasticity within the central nervous system to restore respiration following spinal cord injury. This field of research is highly clinically relevant. People living with cervical spinal cord injury at or above the level of the phrenic motoneuron pool at spinal levels C3-C5 typically have significant impairments in breathing which may require assisted ventilation. Those who are ventilator dependent are at an increased risk of ventilator-associated co-morbidities and have a drastically reduced life expectancy. Pre-clinical research examining respiratory plasticity in animal models has laid the groundwork for clinical trials. Despite how widely researched this injury is in animal models, relatively few treatments have broken through the preclinical barrier. The three goals of this present review are to define plasticity as it pertains to respiratory function post-spinal cord injury, discuss plasticity models of spinal cord injury used in research, and explore the shift from preclinical to clinical research. By investigating current targets of respiratory plasticity research, we hope to illuminate preclinical work that can influence future clinical investigations and the advancement of treatments for spinal cord injury.

The Effects of Therapeutic Intermittent Hypoxia Implementation on Complete Blood Count Parameters: An Experimental Animal Model

Objective: Intermittent hypoxia (IH) implementation is a method performed by intermittently decreasing oxygen concentration in inhaled air at specific rate. This method varies between studies in terms of its application. This study aims to examine the changes in Complete Blood Count (CBC) parameters caused by IH implementation at therapeutic dose ranges with a single model. Methods: Ten Sprague Dawley type adult male rats were divided into two groups. In the study group, FiO2 level of inhaled air, was reduced to 10% in hypoxic cycle. 5 minutes normoxia-hypoxia cycle was used in each 30 minutes experiment period for study group. Control group remained in normoxic air for 30 minutes. 1 cc of blood was taken from mandibular vein from all rats at the end of 6th day. CBC analyzes were performed and differences between two groups were investigated. Results: Significant differences were detected in some CBC parameters between the two groups. It was determined that significant increase in MONO (p

Dose-dependent phosphorylation of endogenous Tau by intermittent hypoxia in rat brain

Intermittent hypoxia, or intermittent low oxygen interspersed with normal oxygen levels, has differential effects that depend on the "dose" of hypoxic episodes (duration, severity, number per day, and number of days). Whereas "low dose" daily acute intermittent hypoxia (dAIH) elicits neuroprotection and neuroplasticity, "high dose" chronic intermittent hypoxia (CIH) similar to that experienced during sleep apnea elicits neuropathology. Sleep apnea is comorbid in >50% of patients with Alzheimer's disease-a progressive, neurodegenerative disease associated with brain amyloid and chronic Tau dysregulation (pathology). Although patients with sleep apnea present with higher Tau levels, it is unknown if sleep apnea through attendant CIH contributes to onset of Tau pathology. We hypothesized CIH characteristic of moderate sleep apnea would increase dysregulation of phosphorylated Tau (phospho-Tau) species in Sprague-Dawley rat hippocampus and prefrontal cortex. Conversely, we hypothesized that dAIH, a promising neurotherapeutic, has minimal impact on Tau phosphorylation. We report a dose-dependent intermittent hypoxia effect, with region-specific increases in 1) phospho-Tau species associated with human Tauopathies in the soluble form and 2) accumulated phospho-Tau in the insoluble fraction. The latter observation was particularly evident with higher CIH intensities. This important and novel finding is consistent with the idea that sleep apnea and attendant CIH have the potential to accelerate the progression of Alzheimer's disease and/or other Tauopathies.NEW & NOTEWORTHY Sleep apnea is highly prevalent in people with Alzheimer's disease, suggesting the potential to accelerate disease onset and/or progression. These studies demonstrate that intermittent hypoxia (IH) induces dose-dependent, region-specific Tau phosphorylation, and are the first to indicate that higher IH "doses" elicit both endogenous, (rat) Tau hyperphosphorylation and accumulation in the hippocampus. These findings are essential for development and implementation of new treatment strategies that minimize sleep apnea and its adverse impact on neurodegenerative diseases.

Effects of acute intermittent hypoxia on corticospinal excitability within the primary motor cortex

Purpose

Acute intermittent hypoxia (AIH) is a safe and non-invasive treatment approach that uses brief, repetitive periods of breathing reduced oxygen air alternated with normoxia. While AIH is known to affect spinal circuit excitability, the effects of AIH on cortical excitability remain largely unknown. We investigated the effects of AIH on cortical excitability within the primary motor cortex.

Methods

Eleven healthy, right-handed participants completed two testing sessions: (1) AIH (comprising 3 min in hypoxia [fraction of inspired oxygen ~ 10%] and 2 min in normoxia repeated over five cycles) and (2) normoxia (NOR) (equivalent duration to AIH). Single- and paired-pulse transcranial magnetic stimulations were delivered to the primary motor cortex, before and 0, 25, and 50 min after AIH and normoxia.

Results

The mean nadir in arterial oxygen saturation was lower (p < 0.001) during the cycles of AIH (82.5 ± 4.9%) than NOR (97.8 ± 0.6%). There was no significant difference in corticospinal excitability, intracortical facilitation, or intracortical inhibition between AIH and normoxia conditions at any time point (all p > 0.05). There was no association between arterial oxygen saturation and changes in corticospinal excitability after AIH (r = 0.05, p = 0.87).

Conclusion

Overall, AIH did not modify either corticospinal excitability or excitability of intracortical facilitatory and inhibitory circuits within the primary motor cortex. Future research should explore whether a more severe or individualised AIH dose would induce consistent, measurable changes in corticospinal excitability.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00421-022-04982-8.

The effect of acute intermittent hypoxia on human limb motoneurone output

NEW FINDINGS

What is the central question of this study? Does a single session of repeated bouts of acute intermittent hypoxic breathing enhance the motoneuronal output of the limb muscles of healthy able-bodied participants? What is the main finding and its importance? Compared to breathing room air, there were some increases in motoneuronal output following acute intermittent hypoxia, but the increases were variable across participants, in time after the intervention and depended on which neurophysiological measure was checked.

ABSTRACT

Acute intermittent hypoxia (AIH) induces persistent increases in output from rat phrenic motoneurones. Studies in people with spinal cord injury suggest AIH improves limb performance, perhaps via postsynaptic changes at cortico-motoneuronal synapses. We assessed whether limb motoneurone output in response to reflex and descending synaptic activation is facilitated after one session of AIH in healthy able-bodied volunteers. Fourteen participants completed two experimental days, either AIH or a sham intervention (randomised crossover design). We measured H-reflex recruitment curves and homosynaptic post-activation depression (HPAD) of the H reflex in soleus, and motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) and their recruitment curves, in first dorsal interosseous. All measurements were performed at rest and occurred at baseline, 0, 20, 40, and 60 minutes post-intervention. The intervention was 30 minutes of either normoxia (sham, FiO₂ ≈ 0.21) or AIH (alternate 1-minute hypoxia [FiO₂ ≈ 0.09], 1-minute normoxia). After AIH the H-reflex recruitment curve shifted leftward. Lower stimulation intensities were needed to evoke 5%, 50%, and 99% of the maximal H reflex at 40 and 60 minutes after AIH (P<0.04). The maximal H reflex, recruitment slope and HPAD, were unchanged after AIH. MEPs evoked by constant intensity TMS were larger 40 minutes after AIH (P = 0.027). There was no change in MEP recruitment or the maximal MEP. In conclusion, some measures of the evoked responses from limb motoneurones increased after a single AIH session, but only at discrete time points. It is unclear to what extent these changes alter functional performance. This article is protected by copyright. All rights reserved.

Nerve root magnetic stimulation improves locomotor function following spinal cord injury with electrophysiological improvements and cortical synaptic reconstruction

Following a spinal cord injury, there are usually a number of neural pathways that remain intact in the spinal cord. These residual nerve fibers are important, as they could be used to reconstruct the neural circuits that enable motor function. Our group previously designed a novel magnetic stimulation protocol, targeting the motor cortex and the spinal nerve roots, that led to significant improvements in locomotor function in patients with a chronic incomplete spinal cord injury. Here, we investigated how nerve root magnetic stimulation contributes to improved locomotor function using a rat model of spinal cord injury. Rats underwent surgery to clamp the spinal cord at T10; three days later, the rats were treated with repetitive magnetic stimulation (5 Hz, 25 pulses/train, 20 pulse trains) targeting the nerve roots at the L5–L6 vertebrae. The treatment was repeated five times a week over a period of three weeks. We found that the nerve root magnetic stimulation improved the locomotor function and enhanced nerve conduction in the injured spinal cord. In addition, the nerve root magnetic stimulation promoted the recovery of synaptic ultrastructure in the sensorimotor cortex. Overall, the results suggest that nerve root magnetic stimulation may be an effective, noninvasive method for mobilizing the residual spinal cord pathways to promote the recovery of locomotor function.

Acute intermittent hypoxia and respiratory muscle recruitment in people with amyotrophic lateral sclerosis: A preliminary study

Respiratory failure is the main cause of death in amyotrophic lateral sclerosis (ALS). Since no effective treatments to preserve independent breathing are available, there is a critical need for new therapies to preserve or restore breathing ability. Since acute intermittent hypoxia (AIH) elicits spinal respiratory motor plasticity in rodent ALS models, and may restore breathing ability in people with ALS, we performed a proof-of-principle study to investigate this possibility in ALS patients. Quiet breathing, sniff nasal inspiratory pressure (SNIP) and maximal inspiratory pressure (MIP) were tested in 13 persons with ALS and 10 age-matched controls, before and 60 min post-AIH (15, 1 min episodes of 10% O₂, 2 min normoxic intervals) or sham AIH (continuous normoxia). The root mean square (RMS) of the right and left diaphragm, 2nd parasternal, scalene and sternocleidomastoid muscles were monitored. A vector analysis was used to calculate summated vector magnitude (Mag) and similarity index (SI) of collective EMG activity during quiet breathing, SNIP and MIP maneuvers. AIH facilitated tidal volume and minute ventilation (treatment main effects: p < 0.05), and Mag (ie. collective respiratory muscle activity; p < 0.001) during quiet breathing in ALS and control subjects, but there was no effect on SI during quiet breathing. SNIP SI decreased in both groups post-AIH (p < 0.005), whereas Mag was unchanged (p = 0.09). No differences were observed in SNIP or MIP post AIH in either group. Discomfort was not reported during AIH by any subject, nor were adverse events observed. Thus, AIH may be a safe way to increase collective inspiratory muscle activity during quiet breathing in ALS patients, although a single AIH presentation was not sufficient to significantly increase peak inspiratory pressure generation. These preliminary results provide evidence that AIH may improve breathing function in people with ALS, and that future studies of prolonged, repetitive AIH protocols are warranted.

Therapeutic acute intermittent hypoxia: A translational roadmap for spinal cord injury and neuromuscular disease

We review progress towards greater mechanistic understanding and clinical translation of a strategy to improve respiratory and non-respiratory motor function in people with neuromuscular disorders, therapeutic acute intermittent hypoxia (tAIH). In 2016 and 2020, workshops to create and update a "road map to clinical translation" were held to help guide future research and development of tAIH to restore movement in people living with chronic, incomplete spinal cord injuries. After briefly discussing the pioneering, non-targeted basic research inspiring this novel therapeutic approach, we then summarize workshop recommendations, emphasizing critical knowledge gaps, priorities for future research effort, and steps needed to accelerate progress as we evaluate the potential of tAIH for routine clinical use. Highlighted areas include: 1) greater mechanistic understanding, particularly in non-respiratory motor systems; 2) optimization of tAIH protocols to maximize benefits; 3) identification of combinatorial treatments that amplify plasticity or remove plasticity constraints, including task-specific training; 4) identification of biomarkers for individuals most/least likely to benefit from tAIH; 5) assessment of long-term tAIH safety; and 6) development of a simple, safe and effective device to administer tAIH in clinical and home settings. Finally, we update ongoing clinical trials and recent investigations of tAIH in SCI and other clinical disorders that compromise motor function, including ALS, multiple sclerosis, and stroke.

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.

Brief exposure to systemic hypoxia enhances plasticity of the central nervous system in spinal cord injured animals and man

PURPOSE OF REVIEW

We have known for many decades that animals that sustain injuries to the neuraxis, which result in respiratory impairment, are able to develop rapid neural compensation for these injuries. This compensation, which is linked to the systemic hypoxia resulting from damage to the respiratory apparatus, is a potent manifestation of neural plasticity. Hypoxia-induced plasticity is also applicable to somatic neural systems that regulate motor activity in extremity muscles. We report on recent developments in our understanding of the mechanisms underlying this seemingly beneficial action of acute intermittent hypoxia (AIH).

RECENT FINDINGS

AIH improves breathing in animal models of spinal cord injury, and increases strength and endurance in individuals with incomplete spinal injuries. The role of AIH as a therapeutic intervention remains to be confirmed but it has proved to be well tolerated for use in humans with no adverse effects reported to date. The effects of AIH emerge rapidly and persist for several hours raising the possibility that the intervention may serve as a priming mechanism for facilitating rehabilitation and promoting recovery after neurologic injury in man.

SUMMARY

AIH is emerging as a potent and relatively inexpensive modality for inducing neuroplasticity, so it may prove feasible to use AIH in a clinical setting.

Phrenic motor neuron survival below cervical spinal cord hemisection

Cervical spinal cord injury (cSCI) severs bulbospinal projections to respiratory motor neurons, paralyzing respiratory muscles below the injury. C2 spinal hemisection (C2Hx) is a model of cSCI often used to study spontaneous and induced plasticity and breathing recovery post-injury. One key assumption is that C2Hx dennervates motor neurons below the injury, but does not affect their survival. However, a recent study reported substantial bilateral motor neuron death caudal to C2Hx. Since phrenic motor neuron (PMN) death following C2Hx would have profound implications for therapeutic strategies designed to target spared neural circuits, we tested the hypothesis that C2Hx minimally impacts PMN survival. Using improved retrograde tracing methods, we observed no loss of PMNs at 2- or 8-weeks post-C2Hx. We also observed no injury-related differences in ChAT or NeuN immunolabeling within labelled PMNs. Although we found no evidence of PMN loss following C2Hx, we cannot rule out neuronal loss in other motor pools. These findings address an essential prerequisite for studies that utilize C2Hx as a model to explore strategies for inducing plasticity and/or regeneration within the phrenic motor system, as they provide important insights into the viability of phrenic motor neurons as therapeutic targets after high cervical injury.

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.

Development of a Spinal Cord Injury Model Permissive to Study the Cardiovascular Effects of Rehabilitation Approaches Designed to Induce Neuroplasticity

Simple Summary

People living with high-level spinal cord injury experience worse cardiovascular health than the general population. In most spinal cord injuries, there are some remaining functioning pathways leading from the brain through the spinal cord to the organs and muscles, but not enough to sustain normal levels of function. Recently, therapies that aim to increase the strength of connections in these remaining pathways have shown great potential in restoring walking, hand, and breathing function in the spinal cord injured population. In order to test these therapies for their effects on cardiovascular function, we developed a new type of spinal cord injury rat model that spares enough pathways for these therapies to act upon but still produces measurable reductions in heart and blood vessel function that can be targeted with interventions/treatments.

Abstract

As primary medical care for spinal cord injury (SCI) has improved over the last decades there are more individuals living with neurologically incomplete (vs. complete) cervical injuries. For these individuals, a number of promising therapies are being actively researched in pre-clinical settings that seek to strengthen the remaining spinal pathways with a view to improve motor function. To date, few, if any, of these interventions have been tested for their effectiveness to improve autonomic and cardiovascular (CV) function. As a first step to testing such therapies, we aimed to develop a model that has sufficient sparing of descending sympathetic pathways for these interventions to target yet induces robust CV impairment. Twenty-six Wistar rats were assigned to SCI (n = 13) or naïve (n = 13) groups. Animals were injured at the T3 spinal segment with 300 kdyn of force. Fourteen days post-SCI, left ventricular (LV) and arterial catheterization was performed to assess in vivo cardiac and hemodynamic function. Spinal cord lesion characteristics along with sparing in catecholaminergic and serotonergic projections were determined via immunohistochemistry. SCI produced a decrease in mean arterial pressure of 17 ± 3 mmHg (p < 0.001) and left ventricular contractility (end-systolic elastance) of 0.7 ± 0.1 mmHg/µL (p < 0.001). Our novel SCI model produced significant decreases in cardiac and hemodynamic function while preserving 33 ± 9% of white matter at the injury epicenter, which we believe makes it a useful pre-clinical model of SCI to study rehabilitation approaches designed to induce neuroplasticity.

Mechanisms of Stem Cell Therapy in Spinal Cord Injuries

Every year, 0.93 million people worldwide suffer from spinal cord injury (SCI) with irretrievable sequelae. Rehabilitation, currently the only available treatment, does not restore damaged tissues; therefore, the functional recovery of patients remains limited. The pathophysiology of spinal cord injuries is heterogeneous, implying that potential therapeutic targets differ depending on the time of injury onset, the degree of injury, or the spinal level of injury. In recent years, despite a significant number of clinical trials based on various types of stem cells, these aspects of injury have not been effectively considered, resulting in difficult outcomes of trials. In a specialty such as cancerology, precision medicine based on a patient’s characteristics has brought indisputable therapeutic advances. The objective of the present review is to promote the development of precision medicine in the field of SCI. Here, we first describe the multifaceted pathophysiology of SCI, with the temporal changes after injury, the characteristics of the chronic phase, and the subtypes of complete injury. We then detail the appropriate targets and related mechanisms of the different types of stem cell therapy for each pathological condition. Finally, we highlight the great potential of stem cell therapy in cervical SCI.

How Are Adenosine and Adenosine A2A Receptors Involved in the Pathophysiology of Amyotrophic Lateral Sclerosis?

Adenosine is extensively distributed in the central and peripheral nervous systems, where it plays a key role as a neuromodulator. It has long been implicated in the pathogenesis of progressive neurogenerative disorders such as Parkinson’s disease, and there is now growing interest in its role in amyotrophic lateral sclerosis (ALS). The motor neurons affected in ALS are responsive to adenosine receptor function, and there is accumulating evidence for beneficial effects of adenosine A_2A receptor antagonism. In this article, we focus on recent evidence from ALS clinical pathology and animal models that support dynamism of the adenosinergic system (including changes in adenosine levels and receptor changes) in ALS. We review the possible mechanisms of chronic neurodegeneration via the adenosinergic system, potential biomarkers and the acute symptomatic pharmacology, including respiratory motor neuron control, of A_2A receptor antagonism to explore the potential of the A_2A receptor as target for ALS therapy.

Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats

Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Further, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH vs. 2 weeks of dAIH preconditioning on spontaneous and evoked responses recorded in anesthetized, paralyzed (with pancuronium bromide) and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at C2 delivered prior to- and 60 min post-AIH (or an equivalent time in controls). Charge-balanced biphasic pulses (100 µs/phase) of progressively increasing intensity (100 to 700 µA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (~60% from baseline) was observed after a single exposure to moderate AIH (3 x 5 min; 5 min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.

Intermittent hypoxia and respiratory recovery in pre-clinical rodent models of incomplete cervical spinal cord injury

Impaired respiratory function is a common and devastating consequence of cervical spinal cord injury. Accordingly, the development of safe and effective treatments to restore breathing function is critical. Acute intermittent hypoxia has emerged as a promising therapeutic strategy to treat respiratory insufficiency in individuals with spinal cord injury. Since the original report by Bach and Mitchell (1996) concerning long-term facilitation of phrenic motor output elicited by brief, episodic exposure to reduced oxygen, a series of studies in animal models have led to the realization that acute intermittent hypoxia may have tremendous potential for inducing neuroplasticity and functional recovery in the injured spinal cord. Advances in our understanding of the neurobiology of acute intermittent hypoxia have prompted us to begin to explore its effects in human clinical studies. Here, we review the basic neurobiology of the control of breathing and the pathophysiology and respiratory consequences of two common experimental models of incomplete cervical spinal cord injury (i.e., high cervical hemisection and mid-cervical contusion). We then discuss the impact of acute intermittent hypoxia on respiratory motor function in these models: work that has laid the foundation for translation of this promising therapeutic strategy to clinical populations. Lastly, we examine the limitations of these animal models and intermittent hypoxia and discuss how future work in animal models may further advance the translation and therapeutic efficacy of this treatment.

Protocol-Specific Effects of Intermittent Hypoxia Pre-Conditioning on Phrenic Motor Plasticity in Rats with Chronic Cervical Spinal Cord Injury

"Low-dose" acute intermittent hypoxia (AIH; 3-15 episodes/day) is emerging as a promising therapeutic strategy to improve motor function after incomplete cervical spinal cord injury (cSCI). Conversely, chronic "high-dose" intermittent hypoxia (CIH; > 80-100 episodes/day) elicits multi-system pathology and is a hallmark of sleep apnea, a condition highly prevalent in individuals with cSCI. Whereas daily AIH (dAIH) enhances phrenic motor plasticity in intact rats, it is abolished by CIH. However, there have been no direct comparisons of prolonged dAIH versus CIH on phrenic motor outcomes after chronic cSCI. Thus, phrenic nerve activity and AIH-induced phrenic long-term facilitation (pLTF) were assessed in anesthetized rats. Experimental groups included: 1) intact rats exposed to 28 days of normoxia (Nx28; 21% O2; 8 h/day), and three groups with chronic C2 hemisection (C2Hx) exposed to either: 2) Nx28; 3) dAIH (dAIH28; 10, 5-min episodes of 10.5% O2/day; 5-min intervals); or 4) CIH (IH28-2/2; 2-min episodes; 2-min intervals; 8 h/day). Baseline ipsilateral phrenic nerve activity was reduced in injured versus intact rats but unaffected by dAIH28 or IH28-2/2. There were no group differences in contralateral phrenic activity. pLTF was enhanced bilaterally by dAIH28 versus Nx28 but unaffected by IH28-2/2. Whereas dAIH28 enhanced pLTF after cSCI, it did not improve baseline phrenic output. In contrast, unlike shorter protocols in intact rats, CIH28-2/2 did not abolish pLTF in chronic C2Hx. Mechanisms of differential responses to dAIH versus CIH are not yet known, particularly in the context of cSCI. Further, it remains unclear whether enhanced phrenic motor plasticity can improve breathing after cSCI.

Efficacy and time course of acute intermittent hypoxia effects in the upper extremities of people with cervical spinal cord injury

Spinal cord injuries (SCI) disrupt neural pathways between the brain and spinal cord, causing impairment of motor function and loss of independent mobility. Spontaneous plasticity in spared neural pathways improves function but is often insufficient to restore normal function. One unique approach to augment plasticity in spinal synaptic pathways is acute intermittent hypoxia (AIH), meaning brief exposure to mild bouts of low oxygen, interspersed with normoxia. While the administration of AIH elicits rapid plasticity and enhances volitional somatic motor output in the lower-limbs of people with incomplete SCI, it is not known if AIH-induced neuroplasticity is equally prevalent in spinal motor pathways regulating upper-extremity motor-function. In addition, how long the motor effects are retained following AIH has not yet been established. The goal of this research was to investigate changes in hand strength and upper-limb function elicited by episodic hypoxia, and to establish how long these effects were sustained in persons with incomplete cervical SCI. We conducted a randomized, blinded, placebo-controlled and cross-over design study consisting of a single AIH or sham AIH session in 14 individuals with chronic, incomplete cervical SCI. In a subset of six participants, we also performed a second protocol to determine the cumulative effects of repetitive AIH (i.e., two consecutive days). In both protocols, hand dynamometry and clinical performance tests were performed pre- and post-exposure. We found that a single AIH session enhanced bilateral grip and pinch strength, and that this effect peaked ~3 h post-intervention. The strength change was substantially higher after AIH versus sham AIH. These findings demonstrate the potential of AIH to improve upper-extremity function in persons with chronic SCI, although follow-up studies are needed to investigate optimal dosage and duration of effect.

[Into thin air - Altitude training and hypoxic conditioning: From athlete to patient]

INTRODUCTION

Hypoxic exposure should be considered as a continuum, the effects of which depend on the dose and individual response to hypoxia. Hypoxic conditioning (HC) represents an innovative and promising strategy, ranging from improved human performance to therapeutic applications.

STATE OF THE ART

With the aim of improving sports performance, the effectiveness of hypoxic exposure, whether natural or simulated, is difficult to demonstrate because of the large variability of the protocols used. In therapeutics, the benefits of HC are described in many pathological conditions such as obesity or cardiovascular pathologies. If the HC benefits from a strong preclinical rationale, its application to humans remains limited.

PERSPECTIVES

Advances in training and acclimation will require greater personalization and precise periodization of hypoxic exposures. For patients, the harmonization of HC protocols, the identification of biomarkers and the development and subsequent validation of devices allowing a precise control of the hypoxic stimulus are necessary steps for the development of HC.

CONCLUSIONS

From the athlete to the patient, HC represents an innovative and promising field of research, ranging from the improvement of human performance to the prevention and treatment of certain pathologies.

Novel Influences of Sex and APOE Genotype on Spinal Plasticity and Recovery of Function after Spinal Cord Injury

Spinal cord injuries can abolish both motor and sensory function throughout the body. Spontaneous recovery after injury is limited and can vary substantially between individuals. Despite an abundance of therapeutic approaches that have shown promise in preclinical models, there is currently a lack of effective treatment strategies that have been translated to restore function after spinal cord injury (SCI) in the human population. We hypothesized that sex and genetic background of injured individuals could impact how they respond to treatment strategies, presenting a barrier to translating therapies that are not tailored to the individual. One gene of particular interest is APOE, which has been extensively studied in the brain because of its allele-specific influences on synaptic plasticity, metabolism, inflammation, and neurodegeneration. Despite its prominence as a therapeutic target in brain injury and disease, little is known about how it influences neural plasticity and repair processes in the spinal cord. Using humanized mice, we examined how the ε3 and ε4 alleles of APOE influence the efficacy of therapeutic intermittent hypoxia (IH) in inducing spinally-mediated plasticity after cervical SCI (cSCI). IH is sufficient to enhance plasticity and restore motor function after experimental SCI in genetically similar rodent populations, but its effect in human subjects is more variable (Golder and Mitchell, 2005; Hayes et al., 2014). Our results demonstrate that both sex and APOE genotype determine the extent of respiratory motor plasticity that is elicited by IH, highlighting the importance of considering these clinically relevant variables when translating therapeutic approaches for the SCI community.

Acute intermittent hypoxia enhances regeneration of surgically repaired peripheral nerves in a manner akin to electrical stimulation

The intrinsic repair response of injured peripheral neurons is enhanced by brief electrical stimulation (ES) at time of surgical repair, resulting in improved regeneration in rodents and humans. However, ES is invasive. Acute intermittent hypoxia (AIH) - breathing alternate cycles of regular air and air with ~50% normal oxygen levels (11% O₂), considered mild hypoxia, is an emerging, promising non-invasive therapy that promotes motor function in spinal cord injured rats and humans. AIH can increase neural activity and under moderately severe hypoxic conditions improves repair of peripherally crushed nerves in mice. Thus, we posited an AIH paradigm similar to that used clinically for spinal cord injury, will improve surgically repaired peripheral nerves akin to ES, including an impact on regeneration-associated gene (RAG) expression-a predictor of growth states. Alterations in early RAG expression were examined in adult male Lewis rats that underwent tibial nerve coaptation repair with either 2 days AIH or normoxia control treatment begun on day 2 post-repair, or 1 h ES treatment (20 Hz) at time of repair. Three days post-repair, AIH or ES treatments effected significant and parallel elevated RAG expression relative to normoxia control at the level of injured sensory and motor neuron cell bodies and proximal axon front. These parallel impacts on RAG expression were coupled with significant improvements in later indices of regeneration, namely enhanced myelination and increased numbers of newly myelinated fibers detected 20 mm distal to the tibial nerve repair site or sensory and motor neurons retrogradely labeled 28 mm distal to the repair site, both at 25 days post nerve repair; and improved return of toe spread function 5-10 weeks post-repair. Collectively, AIH mirrors many beneficial effects of ES on peripheral nerve repair outcomes. This highlights its potential for clinical translation as a non-invasive means to effect improved regeneration of injured peripheral nerves.

Daily acute intermittent hypoxia combined with walking practice enhances walking performance but not intralimb motor coordination in persons with chronic incomplete spinal cord injury

Persons living with incomplete spinal cord injuries (SCI) often struggle to regain independent walking due to deficits in walking mechanics. They often dedicate many weeks of gait training before benefits to emerge, with additional training needed for benefits to persist. Recent studies in humans with SCI found that daily bouts of breathing low oxygen (acute intermittent hypoxia, AIH) prior to locomotor training elicited persistent (weeks) improvement in overground walking speed and endurance. AIH-induced improvements in overground walking may result from changes in control strategies that also enhance intralimb coordination; however, this possibility remains untested. Here, we examined the extent to which daily AIH combined with walking practice (AIH + WALK) improved overground walking performance and intralimb motor coordination in persons with chronic, incomplete SCI.

METHODS

We recruited 11 persons with chronic (> 1 year), incomplete SCI to participate in a randomized, double-blind, balanced, crossover study. Participants first received either daily (5 consecutive days) AIH (15, 90-s episodes of 10.0% O₂ with 60s intervals at 20.9% O₂) or SHAM (15, 90s episodes at 20.9% O₂ with 60s intervals at 20.9% O₂) followed by 30-min of overground walking practice. They received the second treatment after a minimum 2-week washout period. We quantified overground walking performance, in terms of speed and endurance, using the 10-Meter Walk Test (10MWT) and 6-Minute Walk Test (6MWT), respectively. We quantified intralimb motor coordination using kinematic variability measures of foot trajectory (i.e., endpoint variability, EV) and of inter-joint coupling between the hip and knee, as well as between the knee and ankle joints (i.e., angular coefficient of correspondence, ACC). We compared the changes in walking performance relative to baseline (BL) between daily AIH + WALK and daily SHAM+WALK on treatment day 5 (T₅), 1-week follow-up (F₁), and 2-weeks follow-up (F₂). We also compared these changes between participants who used bilateral walking aids (N = 5) and those who did not. To assess the effects of daily AIH + WALK on intralimb coordination, we compared potential treatment-induced changes in EV and ACC relative to BL at F₁ and F₂.

RESULTS

Participants improved overground walking performance (speed and endurance) after daily AIH + WALK, but not SHAM+WALK. Following daily AIH + WALK, participants decreased their 10MWT time at T₅ by 28% (95% CI 0.2-10.1 s, p = 0.04), F₁ by 28% (95% CI 1.1-13.5 s, p = 0.01), and F₂ by 27% (95% CI 1.4-13.9 s, p = 0.01) relative to BL. The greatest decreases in the 10MWT occurred in participants who used bilateral walking aids (p < 0.05). We also found daily AIH + WALK resulted in an increase in 6MWT distance at T₅ by 22% (95% CI 13.3-72.6 m, p = 0.001), F₁ by 21% (95% CI 13.1-72.5 m, p = 0.001), and F₂ by 16% (95% CI 2.9-62.2 m, p = 0.02). However, measures of EV and ACC during self-selected walking conditions did not change following daily AIH + WALK (all p-values >0.50).

CONCLUSIONS

Consistent with prior studies, daily AIH + WALK triggered improvements in walking speed and endurance that persisted for weeks after treatment. Greatest improvements in speed occurred in participants who used bilateral walking aids. No change in EV and ACC may suggest that intralimb motor coordination was not a significant gait training priority during daily AIH + WALK.

Prolonged acute intermittent hypoxia improves forelimb reach-to-grasp function in a rat model of chronic cervical spinal cord injury

Repetitive acute intermittent hypoxia (AIH - brief, episodes of low inspired oxygen) elicits spinal motor plasticity, resulting in sustained improvements of respiratory and non-respiratory motor function in both animal models and humans with chronic spinal cord injury (SCI). We previously demonstrated that 7 days of AIH combined with task-specific training improves performance on a skilled locomotor task for at least 3 weeks post-treatment in rats with incomplete SCI. Here we investigated the effect of repetitive AIH administered for 12 wks on a forelimb reach-to-grasp task in a rat model of chronic, incomplete cervical SCI. In a replicated, sham-controlled, randomized and blinded study, male Spraque-Dawley rats were subject to partial hemisection at the 3rd cervical spinal segment, and exposed to daily AIH (10, 5 min episodes of 11% inspired O₂; 5 min intervals of 21% O₂) or sham normoxia (continuous 21% O₂) for 7 days beginning 8 weeks post-injury. Treatments were then reduced to 4 daily treatments per week, and continued for 11 weeks. Performance on 2 pre-conditioned motor tasks, single pellet reaching and horizontal ladder walking, was recorded each week for up to 12 weeks after initiating treatment; performance on spontaneous adhesive removal was also tested. SCI significantly impaired reach-to-grasp task performance 8 weeks post-injury (pre-treatment). Daily AIH improved reaching success by the first week of treatment versus sham controls, and this difference was maintained at 12 weeks (p < 0.0001). Daily AIH did not affect step asymmetry or stride length during ladder walking or adhesive removal time. Thus, prolonged AIH combined with task-specific training improved forelimb reach-to-grasp function in rats with a chronic cervical hemisection, but not off-target motor tasks. This study further supports the idea that daily AIH improves limb function when combined with task-specific training.

Baseline Arterial CO2 Pressure Regulates Acute Intermittent Hypoxia-Induced Phrenic Long-Term Facilitation in Rats

Moderate acute intermittent hypoxia (mAIH) elicits a progressive increase in phrenic motor output lasting hours post-mAIH, a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). mAIH-induced pLTF is initiated by activation of spinally-projecting raphe serotonergic neurons during hypoxia and subsequent serotonin release near phrenic motor neurons. Since raphe serotonergic neurons are also sensitive to pH and CO₂, the prevailing arterial CO₂ pressure (PaCO₂) may modulate their activity (and serotonin release) during hypoxic episodes. Thus, we hypothesized that changes in background PaCO₂ directly influence the magnitude of mAIH-induced pLTF. mAIH-induced pLTF was evaluated in anesthetized, vagotomized, paralyzed and ventilated rats, with end-tidal CO₂ (i.e., a PaCO₂ surrogate) maintained at: (1) ≤39 mmHg (hypocapnia); (2) ∼41 mmHg (normocapnia); or (3) ≥48 mmHg (hypercapnia) throughout experimental protocols. Although baseline phrenic nerve activity tended to be lower in hypocapnia, short-term hypoxic phrenic response, i.e., burst amplitude (Δ = 5.1 ± 1.1 μV) and frequency responses (Δ = 21 ± 4 bpm), was greater than in normocapnic (Δ = 3.6 ± 0.6 μV and 8 ± 4, respectively) or hypercapnic rats (Δ = 2.0 ± 0.6 μV and −2 ± 2, respectively), followed by a progressive increase in phrenic burst amplitude (i.e., pLTF) for at least 60 min post mAIH. pLTF in the hypocapnic group (Δ = 4.9 ± 0.6 μV) was significantly greater than in normocapnic (Δ = 2.8 ± 0.7 μV) or hypercapnic rats (Δ = 1.7 ± 0.4 μV). In contrast, although hypercapnic rats also exhibited significant pLTF, it was attenuated versus hypocapnic rats. When pLTF was expressed as percent change from maximal chemoreflex stimulation, all pairwise comparisons were found to be statistically significant (p < 0.05). We conclude that elevated PaCO₂ undermines mAIH-induced pLTF in anesthetized rats. These findings contrast with well-documented effects of PaCO₂ on ventilatory LTF in awake humans.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

Serotonergic innervation of respiratory motor nuclei after cervical spinal injury: Impact of intermittent hypoxia

Although cervical spinal cord injury (cSCI) disrupts bulbo-spinal serotonergic projections, partial recovery of spinal serotonergic innervation below the injury site is observed after incomplete cSCI. Since serotonin contributes to functional recovery post-injury, treatments to restore or accelerate serotonergic reinnervation are of considerable interest. Intermittent hypoxia (IH) was reported to increase serotonin innervation near respiratory motor neurons in spinal intact rats, and to improve function after cSCI. Here, we tested the hypotheses that spontaneous serotonergic reinnervation of key respiratory (phrenic and intercostal) motor nuclei: 1) is partially restored 12 weeks post C2 hemisection (C2Hx); 2) is enhanced by IH; and 3) results from sprouting of spared crossed-spinal serotonergic projections below the site of injury. Serotonin was assessed via immunofluorescence in male Sprague Dawley rats with and without C2Hx (12 wks post-injury); individual groups were exposed to 28 days of: 1) normoxia; 2) daily acute IH (dAIH28: 10, 5 min 10.5% O2 episodes per day; 5 min normoxic intervals); 3) mild chronic IH (IH28-5/5: 5 min 10.5% O2 episodes; 5 min intervals; 8 h/day); or 4) moderate chronic IH (IH28-2/2: 2 min 10.5% O2 episodes; 2 min intervals; 8 h/day), simulating IH experienced during moderate sleep apnea. After C2Hx, the number of ipsilateral serotonergic structures was decreased in both motor nuclei, regardless of IH protocol. However, serotonergic structures were larger after C2Hx in both motor nuclei, and total serotonin immunolabeling area was increased in the phrenic motor nucleus but reduced in the intercostal motor nucleus. Both chronic IH protocols increased serotonin structure size and total area in the phrenic motor nuclei of uninjured rats, but had no detectable effects after C2Hx. Although the functional implications of fewer but larger serotonergic structures are unclear, we confirm that serotonergic reinnervation is substantial following injury, but IH does not affect the extent of reinnervation.