Spinal associative stimulation: a non-invasive stimulation paradigm to modulate spinal excitability

Interaction of transcutaneous spinal stimulation and transcranial magnetic stimulation in human leg muscles

Transcutaneous spinal stimulation is a noninvasive method that can activate dorsal and/or ventral roots depending on the location and intensity of stimulation. Reflex root-evoked potentials (REPs) were studied in muscles that traditionally evoke large (soleus) and small H-reflexes (tibialis anterior), as well as muscles where H-reflexes are difficult to study (hamstrings). This study characterizes the interaction of the REP and the motor-evoked potential (MEP). Transcranial magnetic stimulation (TMS) delivered 11-25 ms before spinal stimulation resulted in more than linear summation of the two responses. Because of overlap, the modulation was quantified after subtracting the contribution of the conditioning MEP or REP. At rest, the mean-rectified soleus response was facilitated by up to ~250 μV (21-times the MEP or 161% of the REP). The increases were more reliable during a voluntary contraction (up to ~300 μV, 517% of the MEP or 181% of the REP). At the 13-ms interval, the mean-rectified response in the pre-contracted hamstrings was increased by 227% of the MEP or 300% of the REP. In some subjects, TMS could also eliminate the post-activation depression produced using two spinal stimuli, confirming that the interaction can extend to presynaptic spinal neurons. The spatiotemporal facilitation in tibialis anterior was not significant. However, the large MEP was facilitated when the spinal stimulus preceded TMS by 100-150 ms, presumably because of rebound excitation. These strong interactions may be important for inducing motor plasticity and improved training procedures for recovery after neurological damage.

Modulation of human corticospinal excitability by paired associative stimulation

Paired Associative Stimulation (PAS) has come to prominence as a potential therapeutic intervention for the treatment of brain injury/disease, and as an experimental method with which to investigate Hebbian principles of neural plasticity in humans. Prototypically, a single electrical stimulus is directed to a peripheral nerve in advance of transcranial magnetic stimulation (TMS) delivered to the contralateral primary motor cortex (M1). Repeated pairing of the stimuli (i.e., association) over an extended period may increase or decrease the excitability of corticospinal projections from M1, in manner that depends on the interstimulus interval (ISI). It has been suggested that these effects represent a form of associative long-term potentiation (LTP) and depression (LTD) that bears resemblance to spike-timing dependent plasticity (STDP) as it has been elaborated in animal models. With a large body of empirical evidence having emerged since the cardinal features of PAS were first described, and in light of the variations from the original protocols that have been implemented, it is opportune to consider whether the phenomenology of PAS remains consistent with the characteristic features that were initially disclosed. This assessment necessarily has bearing upon interpretation of the effects of PAS in relation to the specific cellular pathways that are putatively engaged, including those that adhere to the rules of STDP. The balance of evidence suggests that the mechanisms that contribute to the LTP- and LTD-type responses to PAS differ depending on the precise nature of the induction protocol that is used. In addition to emphasizing the requirement for additional explanatory models, in the present analysis we highlight the key features of the PAS phenomenology that require interpretation.

Restoration of sensorimotor functions after spinal cord injury

The purpose of this review is to discuss the achievements and perspectives regarding rehabilitation of sensorimotor functions after spinal cord injury. In the first part we discuss clinical approaches based on neuroplasticity, a term referring to all adaptive and maladaptive changes within the sensorimotor systems triggered by a spinal cord injury. Neuroplasticity can be facilitated through the training of movements with assistance as needed, and/or by electrical stimulation techniques. The success of such training in individuals with incomplete spinal cord injury critically depends on the presence of physiological proprioceptive input to the spinal cord leading to meaningful muscle activations during movement performances. The addition of rehabilitation technology, such as robotic devices allows for longer training times and provision of feedback information regarding changes in movement performance. Nevertheless, the improvement of function by such approaches for rehabilitation is limited. In the second part, we discuss preclinical approaches to restore function by compensating for the loss of descending input to spinal networks following complete spinal cord injury. This can be achieved with stimulation of spinal networks or approaches to restore their descending input. Electrical and pharmacological stimulation of spinal neural networks is still in an experimental stage; and despite promising repair studies in animal models, translations to humans up to now have not been convincing. It is likely that combinations of techniques targeting the promotion of axonal regeneration and meaningful plasticity are necessary to advance the restoration of function. In the future, refinement of animal studies may contribute to greater translational success.

BDNF Val66Met polymorphism alters spinal DC stimulation-induced plasticity in humans

The brain-derived neurotrophic factor gene (BDNF) is one of many genes thought to influence neuronal survival, synaptic plasticity, and neurogenesis. A common single nucleotide polymorphism (SNP) of the BDNF gene due to valine-to-methionine substitution at codon 66 (BDNF Val66Met) in the normal population has been associated with complex neuronal phenotype, including differences in brain morphology, episodic memory, or cortical plasticity following brain stimulation and is believed to influence synaptic changes following motor learning task. However, the effect of this polymorphism on spinal plasticity remains largely unknown. Here, we used anodal transcutaneous spinal direct current stimulation (tsDCS), a novel noninvasive technique that induces plasticity of spinal neuronal circuits in healthy subjects. To investigate whether the susceptibility of tsDCS probes of spinal plasticity is significantly influenced by BDNF polymorphism, we collected stimulus-response curves of the soleus (Sol) H reflex before, during, at current offset, and 15 min after anodal tsDCS delivered at Th11 (2.5 mA, 15 min, 0.071 mA/cm2, and 64 mC/cm2) in 17 healthy, Met allele carriers and 17 Val homozygotes who were matched for age and sex. Anodal tsDCS induced a progressive leftward shift of recruitment curve of the H reflex during the stimulation that persisted for at least 15 min after current offset in Val/Val individuals. In contrast, this shift was not observed in Met allele carriers. Our findings demonstrate for the first time that the BDNF Val66Met genotype impacts spinal plasticity in humans, as assessed by tsDCS, and may be one factor influencing the natural response of the spinal cord to injury or disease.

Assessing brain plasticity across the lifespan with transcranial magnetic stimulation: why, how, and what is the ultimate goal?

Sustaining brain and cognitive function across the lifespan must be one of the main biomedical goals of the twenty-first century. We need to aim to prevent neuropsychiatric diseases and, thus, to identify and remediate brain and cognitive dysfunction before clinical symptoms manifest and disability develops. The brain undergoes a complex array of changes from developmental years into old age, putatively the underpinnings of changes in cognition and behavior throughout life. A functionally “normal” brain is a changing brain, a brain whose capacity and mechanisms of change are shifting appropriately from one time-point to another in a given individual's life. Therefore, assessing the mechanisms of brain plasticity across the lifespan is critical to gain insight into an individual's brain health. Indexing brain plasticity in humans is possible with transcranial magnetic stimulation (TMS), which, in combination with neuroimaging, provides a powerful tool for exploring local cortical and brain network plasticity. Here, we review investigations to date, summarize findings, and discuss some of the challenges that need to be solved to enhance the use of TMS measures of brain plasticity across all ages. Ultimately, TMS measures of plasticity can become the foundation for a brain health index (BHI) to enable objective correlates of an individual's brain health over time, assessment across diseases and disorders, and reliable evaluation of indicators of efficacy of future preventive and therapeutic interventions.

The physiological basis of neurorehabilitation--locomotor training after spinal cord injury

Advances in our understanding of the physiological basis of locomotion enable us to optimize the neurorehabilitation of patients with lesions to the central nervous system, such as stroke or spinal cord injury (SCI). It is generally accepted, based on work in animal models, that spinal neuronal machinery can produce a stepping-like output. In both incomplete and complete SCI subjects spinal locomotor circuitries can be activated by functional training which provides appropriate afferent feedback. In motor complete SCI subjects, however, motor functions caudal to the spinal cord lesion are no longer used resulting in neuronal dysfunction. In contrast, in subjects with an incomplete SCI such training paradigms can lead to improved locomotor ability. Appropriate functional training involves the facilitation and assistance of stepping-like movements with the subjects’ legs and body weight support as far as is required. In severely affected subjects standardized assisted locomotor training is provided by body weight supported treadmill training with leg movements either manually assisted or moved by a driven gait orthosis. Load- and hip-joint related afferent input is of crucial importance during locomotor training as it leads to appropriate leg muscle activation and thus increases the efficacy of the rehabilitative training. Successful recovery of locomotion after SCI relies on the ability of spinal locomotor circuitries to utilize specific multisensory information to generate a locomotor pattern. It seems that a critical combination of sensory cues is required to generate and improve locomotor patterns after SCI. In addition to functional locomotor training there are numbers of other promising experimental approaches, such as tonic epidural electrical or magnetic stimulation of the spinal cord, which both promote locomotor permissive states that lead to a coordinated locomotor output. Therefore, a combination of functional training and activation of spinal locomotor circuitries, for example by epidural/flexor reflex electrical stimulation or drug application (e.g. noradrenergic agonists), might constitute an effective strategy to promote neuroplasticity after SCI in the future.

Motor recovery after spinal cord injury enhanced by strengthening corticospinal synaptic transmission

The corticospinal tract is an important target for motor recovery after spinal cord injury (SCI) in animals and humans. Voluntary motor output depends on the efficacy of synapses between corticospinal axons and spinal motoneurons, which can be modulated by the precise timing of neuronal spikes. Using noninvasive techniques, we developed tailored protocols for precise timing of the arrival of descending and peripheral volleys at corticospinal-motoneuronal synapses of an intrinsic finger muscle in humans with chronic incomplete SCI. We found that arrival of presynaptic volleys prior to motoneuron discharge enhanced corticospinal transmission and hand voluntary motor output. The reverse order of volley arrival and sham stimulation did not affect or decreased voluntary motor output and electrophysiological outcomes. These findings are the first demonstration that spike timing-dependent plasticity of residual corticospinal-motoneuronal synapses provides a mechanism to improve motor function after SCI. Modulation of residual corticospinal-motoneuronal synapses may present a novel therapeutic target for enhancing voluntary motor output in motor disorders affecting the corticospinal tract.

Neural interfaces for the brain and spinal cord--restoring motor function

Regaining motor function is of high priority to patients with spinal cord injury (SCI). A variety of electronic devices that interface with the brain or spinal cord, which have applications in neural prosthetics and neurorehabilitation, are in development. Owing to our advancing understanding of activity-dependent synaptic plasticity, new technologies to monitor, decode and manipulate neural activity are being translated to patient populations, and have demonstrated clinical efficacy. Brain-machine interfaces that decode motor intentions from cortical signals are enabling patient-driven control of assistive devices such as computers and robotic prostheses, whereas electrical stimulation of the spinal cord and muscles can aid in retraining of motor circuits and improve residual capabilities in patients with SCI. Next-generation interfaces that combine recording and stimulating capabilities in so-called closed-loop devices will further extend the potential for neuroelectronic augmentation of injured motor circuits. Emerging evidence suggests that integration of closed-loop interfaces into intentional motor behaviours has therapeutic benefits that outlast the use of these devices as prostheses. In this Review, we summarize this evidence and propose that several known plasticity mechanisms, operating in a complementary manner, might underlie the therapeutic effects that are achieved by closing the loop between electronic devices and the nervous system.

Measuring and manipulating brain connectivity with resting state functional connectivity magnetic resonance imaging (fcMRI) and transcranial magnetic stimulation (TMS)

Both resting state functional magnetic resonance imaging (fcMRI) and transcranial magnetic stimulation (TMS) are increasingly popular techniques that can be used to non-invasively measure brain connectivity in human subjects. TMS shows additional promise as a method to manipulate brain connectivity. In this review we discuss how these two complimentary tools can be combined to optimally study brain connectivity and manipulate distributed brain networks. Important clinical applications include using resting state fcMRI to guide target selection for TMS and using TMS to modulate pathological network interactions identified with resting state fcMRI. The combination of TMS and resting state fcMRI has the potential to accelerate the translation of both techniques into the clinical realm and promises a new approach to the diagnosis and treatment of neurological and psychiatric diseases that demonstrate network pathology.

Modulation of soleus H reflex by spinal DC stimulation in humans

Transcranial direct current stimulation (tDCS) of the human motor cortex induces changes in excitability within cortical and spinal circuits that occur during and after the stimulation. Recently, transcutaneous spinal direct current stimulation (tsDCS) has been shown to modulate spinal conduction properties, as assessed by somatosensory-evoked potentials, and transynaptic properties of the spinal neurons, as tested by postactivation depression of the H reflex or by the RIII nociceptive component of the flexion reflex in the lower limb. To further explore tsDCS-induced plastic changes in spinal excitability, we examined, in a double-blind crossover randomized study, the stimulus-response curves of the soleus H reflex before, during, at current offset and 15 min after anodal, cathodal, and sham tsDCS delivered at the Th11 level (2.5 mA, 15 min, 0.071 mA/cm2, 0.064 C/cm2) in 17 healthy subjects. Anodal tsDCS induced a progressive leftward shift of the recruitment curve of the soleus H reflex during the stimulation; the effects persisted for at least 15 min after current offset. In contrast, both cathodal and sham tsDCS had no significant effects. This exploratory study provides further evidence for the use of tsDCS as an expedient, noninvasive tool to induce long-lasting plastic changes in spinal circuitry. Increased spinal excitability after anodal tsDCS may have potential for spinal neuromodulation in patients with central nervous system lesions.

Limb segment load inhibits post activation depression of soleus H-reflex in humans

OBJECTIVE

We investigated the effect of various doses of limb segment load on soleus H-reflex amplitude and post activation depression in healthy individuals. We also explored the influence of limb segment load on spinal circuitry in one individual with chronic SCI.

METHODS

Twenty-eight healthy adults and one SCI subject received compressive loads applied to the top of their knee at varied doses of load (10%, 25%, and 50% of the body weight). Soleus H-reflexes were measured before (baseline) and during the loading phase.

RESULTS

There were no significant differences in H-reflex amplitudes during the 50% BW load-on phase as compared to either baseline session or the load-off phase. However, the post activation depression was decreased over 9% (p<0.05) during the load-on phase compared to the load-off phase and scaled according to load (50%>25%>10%). The post activation depression ratio also appears less responsive to varying loads after chronic SCI.

CONCLUSIONS

Limb segment load decreases post-activation depression in humans. These findings suggest that the mechanism associated with post activation depression is modulated by limb segment load, and may be influenced by spinal reorganization after SCI.

SIGNIFICANCE

Future studies will determine if various levels of spasticity modulate the response of limb segment load on post activation depression in those with acute and chronic SCI.