Mechanisms Of Long-term Plasticity Research Articles

New biophysical rate-based modeling of long-term plasticity in mean-field neuronal population models

The formation of customized neural networks as the basis of brain functions such as receptive field selectivity, learning or memory depends heavily on the long-term plasticity of synaptic connections. However, the current mean-field population models commonly used to simulate large-scale neural network dynamics lack explicit links to the underlying cellular mechanisms of long-term plasticity. In this study, we developed a new mean-field population model, the plastic density-based neural mass model (pdNMM), by incorporating a newly developed rate-based plasticity model based on the calcium control hypothesis into an existing density-based neural mass model. Derivation of the plasticity model was carried out using population density methods. Our results showed that the synaptic plasticity represented by the resulting rate-based plasticity model exhibited Bienenstock-Cooper-Munro-like learning rules. Furthermore, we demonstrated that the pdNMM accurately reproduced previous experimental observations of long-term plasticity, including characteristics of Hebbian plasticity such as longevity, associativity and input specificity, on hippocampal slices, and the formation of receptive field selectivity in the visual cortex. In conclusion, the pdNMM is a novel approach that can confer long-term plasticity to conventional mean-field neuronal population models.

CB1 cannabinoid receptor-mediated plasticity of GABAergic synapses in the mouse insular cortex

The insular cortex plays pivotal roles in taste learning. As cellular mechanisms of taste learning, long-term potentiation (LTP) at glutamatergic synapses is well studied. However, little is known about long-term changes of synaptic efficacy at GABAergic synapses in the insular cortex. Here, we examined the synaptic mechanisms of long-term plasticity at GABAergic synapses in layer V pyramidal neurons of the mouse insular cortex. In response to a prolonged high-frequency stimulation (HFS), GABAergic synapses displayed endocannabinod (eCB)-mediated long-term depression (LTDGABA). When cannabinoid 1 receptors (CB1Rs) were blocked by a CB1R antagonist, the same stimuli caused LTP at GABAergic synapses (LTPGABA) which was mediated by production of nitric oxide (NO) via activation of NMDA receptors. Intriguingly, NO signaling was necessary for the induction of LTDGABA. In the presence of leptin which blocks CB1 signaling, the prolonged HFS caused LTPGABA which was mediated by NO signaling. These results indicate that long-term plasticity at GABAergic synapses in the insular cortex can be modulated by combined effects of eCB and NO signaling. These forms of GABAergic synaptic plasticity in the insular cortex may be crucial synaptic mechanisms in taste learning.

Neonatal Proinflammatory Stress and Deficit of Induction of Long-Term Potentiation in the Hippocampus in Rats: Gender Differences

The characteristics of long-term potentiation (LTP) in field CA1 of living hippocampal slices from rats aged 17–33 days were studied after neonatal administration of bacterial lipopolysaccharide (LPS) to evaluate the long-term effects of neuroinflammation in early ontogeny on synaptic plasticity. A deficit of LTP was seen in field CA1 of the hippocampus of rats surviving neonatal proinflammatory stress (NPS), the most sensitive being the mechanism of induction. In rats of the NPS group, synaptic potentiation was absent or weak in 61% of experiments, though when normal induction occurred, LTP persisted throughout the observation period. Correlation analysis showed that impairments to the development of the mechanisms of long-term plasticity in response to NPS are probably associated with weakening of the afferent influx to hippocampal neurons. Initially low-amplitude responses in field CA1 to application of single stimuli to Schaffer collaterals dominated in the hippocampus of female rats of the NPS group but were significantly rarer in males. This is probably why the early phase of potentiation in living hippocampal slices from females was virtually absent, though slow potentiation gradually developed. The hippocampus of males of the NPS group showed a statistically significant link between the effects of suppression of LTP and synaptic depression of initial potentials; signs of convulsive activity appeared when potentiation was successful. In contrast to males and the results obtained from all control groups, the hippocampus of female rats showed facilitation of the development of post-tetanic potentiation. It is suggested that this effect may be relevant to protective functions, which are evidently less effectively utilized in the hippocampus of males.

Presynaptic increase in IP3 receptor type 1 concentration in the early phase of hippocampal synaptic plasticity

The inositol 1,4,5-trisphosphate receptor (IP3R) subtype IP3R1 is highly enriched in the brain, including hippocampal neurons. It plays an important function in regulating intracellular calcium concentrations. Residing on the smooth endoplasmic reticulum (sER), the IP3R1 mobilizes calcium into the cytosol upon binding the intracellular signaling molecule IP3, whose concentration is increased by stimulating certain metabotropic glutamate receptors. Increased calcium may mediate synaptic changes occurring during long-term plasticity, which includes molecular mechanisms underlying memory encoding. The exact synaptic localization of IP3R1 in the central nervous system (CNS) remains unclear. We hypothesized that IP3R1, in addition to its known expression in soma and dendritic shafts of hippocampal CA1 pyramidal neurons, also may be present in postsynaptic spines. Moreover, we hypothesized that IP3R1 may be present in presynaptic terminals as well, given the importance of calcium in regulating presynaptic neurotransmitter exocytosis. To test these two hypotheses, we used IP3R1 immunocytochemistry at the light and electron microscopical levels in the CA1 area of the hippocampus. Furthermore, we hypothesized that induction of long-term potentiation (LTP) would be accompanied by an increase in synaptic IP3R1 concentrations, thereby facilitating synaptic mechanisms of long term plasticity. To investigate this, we used quantitative immunogold electron microscopy to determine possible changes in IP3R1 concentration in sub-synaptic compartments before and five minutes after high frequency tetanizations. Firstly, our data confirm localization of IP3R1 in both presynaptic terminals and postsynaptic spines. Secondly, the concentration of IP3R1 after tetanization was significantly increased in the presynaptic compartment, suggesting a presynaptic role of IP3R1 in early phases of synaptic plasticity. It is therefore possible that IP3R1 is involved in modulating neurotransmitter release by regulating calcium homeostasis presynaptically.

Long-Term Plasticity of Neurotransmitter Release: Emerging Mechanisms and Contributions to Brain Function and Disease.

Long-lasting changes of brain function in response to experience rely on diverse forms of activity-dependent synaptic plasticity. Chief among them are long-term potentiation and long-term depression of neurotransmitter release, which are widely expressed by excitatory and inhibitory synapses throughout the central nervous system and can dynamically regulate information flow in neural circuits. This review article explores recent advances in presynaptic long-term plasticity mechanisms and contributions to circuit function. Growing evidence indicates that presynaptic plasticity may involve structural changes, presynaptic protein synthesis, and transsynaptic signaling. Presynaptic long-term plasticity can alter the short-term dynamics of neurotransmitter release, thereby contributing to circuit computations such as novelty detection, modifications of the excitatory/inhibitory balance, and sensory adaptation. In addition, presynaptic long-term plasticity underlies forms of learning and its dysregulation participates in several neuropsychiatric conditions, including schizophrenia, autism, intellectual disabilities, neurodegenerative diseases, and drug abuse.

Mechanisms of Long-Term Plasticity of Hippocampal GABAergic Synapses.

Long-term potentiation and depression of synaptic transmission have been considered as cellular mechanisms of memory in studies conducted in recent decades. These studies were predominantly focused on mechanisms underlying plasticity at excitatory synapses. Nevertheless, normal central nervous system functioning requires maintenance of a balance between inhibition and excitation, suggesting existence of similar modulation of glutamatergic and GABAergic synapses. Here we review the involvement of G-protein-coupled receptors in the generation of long-term changes in synaptic transmission of inhibitory synapses. We considered the role of endocannabinoid and glutamate systems, GABAB and opioid receptors in the induction of long-term potentiation and long-term depression in inhibitory synapses. The pre- and postsynaptic effects of activation of these receptors are also discussed.

Attention-related changes in short-term cortical plasticity help to explain fatigue in multiple sclerosis

Background: In multiple sclerosis (MS), pathophysiology of fatigue is only partially known. Objective: The aim of this study was to investigate whether the attention-induced modulation on short- and long-term cortical plasticity mechanisms in primary motor area (M1) is abnormal in patients with MS-related fatigue. Methods: All participants underwent 5-Hz repetitive transcranial magnetic stimulation (rTMS), reflecting short-term plasticity, and paired associative stimulation (PAS), reflecting long-term plasticity, and were asked to focus their attention on the hand contralateral to the M1 stimulated. A group of age-matched healthy subjects acted as control. Results: In patients with MS, 5-Hz rTMS and PAS failed to induce the normal increase in motor-evoked potential (MEP). During the attention-demanding condition, 5-Hz rTMS- and PAS-induced responses differed in patients with MS with and without fatigue. Whereas in patients with fatigue neither technique induced the attention-induced MEP increase, in patients without fatigue they both increased the MEP response, although they did so less efficiently than in healthy subjects. Attention-induced changes in short-term cortical plasticity inversely correlated with fatigue severity. Conclusion: Short-term and long-term plasticity mechanisms are abnormal in MS possibly owing to widespread changes in ion-channel expression. Fatigue in MS reflects disrupted cortical attentional networks related to movement control.

The plasticity of inhibitory synapses as a factor of long-term modifications

We review the current data on the functions of GABAergic inhibition in the mechanisms of long-term plasticity that underlie learning and memory. We present the main principles of the organization of the inhibitory inputs and molecular composition of synapses. So-called “ionic plasticity” is a specific feature of the inhibitory synapses. The range of ionic plasticity that is associated with the depolarizing effect of GABA is of particular interest. In adult animals, this effect is observed after high-frequency stimulation of synaptic inputs. The main attention was focused on the reaction of disinhibition in the hippocampus that is induced by high-frequency stimulation of the afferent inputs that are used for LTP induction. We argue that the disinhibition effect is crucially important for more effective consolidation of long-term modifications, including structural plasticity.

The vertical lobe of cephalopods: an attractive brain structure for understanding the evolution of advanced learning and memory systems.

In this review we show that the cephalopod vertical lobe (VL) provides a good system for assessing the level of evolutionary convergence of the function and organization of neuronal circuitry for mediating learning and memory in animals with complex behavior. The pioneering work of JZ Young described the morphological convergence of the VL with the mammalian hippocampus, cerebellum and the insect mushroom body. Studies in octopus and cuttlefish VL networks suggest evolutionary convergence into a universal organization of connectivity as a divergence-convergence ('fan-out fan-in') network with activity-dependent long-term plasticity mechanisms. Yet, these studies also show that the properties of the neurons, neurotransmitters, neuromodulators and mechanisms of long-term potentiation (LTP) induction and maintenance are highly variable among different species. This suggests that complex networks may have evolved independently multiple times and that even though memory and learning networks share similar organization and cellular processes, there are many molecular ways of constructing them.

Neuron as a reward-modulated combinatorial switch and a model of learning behavior

This paper proposes a neuronal circuitry layout and synaptic plasticity principles that allow the (pyramidal) neuron to act as a “combinatorial switch”. Namely, the neuron learns to be more prone to generate spikes given those combinations of firing input neurons for which a previous spiking of the neuron had been followed by a positive global reward signal. The reward signal may be mediated by certain modulatory hormones or neurotransmitters, e.g., the dopamine. More generally, a trial-and-error learning paradigm is suggested in which a global reward signal triggers long-term enhancement or weakening of a neuron’s spiking response to the preceding neuronal input firing pattern. Thus, rewards provide a feedback pathway that informs neurons whether their spiking was beneficial or detrimental for a particular input combination. The neuron’s ability to discern specific combinations of firing input neurons is achieved through a random or predetermined spatial distribution of input synapses on dendrites that creates synaptic clusters that represent various permutations of input neurons. The corresponding dendritic segments, or the enclosed individual spines, are capable of being particularly excited, due to local sigmoidal thresholding involving voltage-gated channel conductances, if the segment’s excitatory and absence of inhibitory inputs are temporally coincident. Such nonlinear excitation corresponds to a particular firing combination of input neurons, and it is posited that the excitation strength encodes the combinatorial memory and is regulated by long-term plasticity mechanisms. It is also suggested that the spine calcium influx that may result from the spatiotemporal synaptic input coincidence may cause the spine head actin filaments to undergo mechanical (muscle-like) contraction, with the ensuing cytoskeletal deformation transmitted to the axon initial segment where it may modulate the global neuron firing threshold. The tasks of pattern classification and generalization are discussed within the presented framework.

Synaptic plasticity in cephalopods; more than just learning and memory?

The outstanding behavioural capacity of cephalopods is underpinned by a highly sophisticated nervous system anatomy and neural mechanisms that often differ significantly from similarly complex systems in vertebrates and insects. Cephalopods exhibit considerable behavioural flexibility and adaptability, and it might be expected that this should be supported by evident cellular and synaptic plasticity. Here, we review what little is known of the cellular mechanisms that underlie plasticity in cephalopods, particularly from the point of view of synaptic function. We conclude that cephalopods utilise short-, medium-, and long-term plasticity mechanisms that are superficially similar to those so far described in vertebrate and insect synapses. These mechanisms, however, often differ significantly from those in other animals at the biophysical level and are deployed not just in the central nervous system, but also to a limited extent in the peripheral nervous system and neuromuscular junctions.

Inhibition of glycine transporter-1 reduces cue-induced nicotine-seeking, but does not promote extinction of conditioned nicotine cue responding in the rat

Pharmacological stimulation of N-methyl-D-aspartate receptors (NMDAr) could enhance the outcome of cue-exposure therapy for smoking cessation. NMDAr stimulation can be achieved by increasing pharmacologically the synaptic levels of glycine, a necessary co-agonist. Here, we evaluate the effects of SSR504734, a selective inhibitor of glycine type I transporter (GlyT1) in an extinction-reinstatement procedure inducing robust and lasting nicotine-seeking behavior in rats. Male Wistar rats were trained to associate discriminative stimuli (S(D)s) with the availability of nicotine (0.03 mg/kg/65 μL/2 second/infusion) or sucrose (45-mg pellet) versus non-reward in two-lever operant cages. Reinforced response was followed by cue signaling 20-second time-out (CSs). Once the training criterion was met, rats underwent extinction of lever presses, in the absence of reinforcers, S(D) s and CSs. Re-exposure to nicotine or sucrose S(D+)/CS(+), but not non-reward S(D-)/CS(-), revived responding at the previously reinforced lever. Acute pre-treatment with SSR504734 (10 mg/kg i.p.) reduced nicotine-seeking but not sucrose-seeking behavior without influencing rats' locomotor activity. Sub-chronic treatment (10 mg/kg i.p. for 5 days) during daily exposure to S(D+)/CS(+) reduced nicotine-seeking; however, this effect was transient, with return to S(D+)/CS(+) responding at 72 hours. Full recovery to S(D+)/CS(+) responding was observed after 1 month suggesting that SSR504734 sub-acute treatment did not engage the long-term plasticity mechanisms probably involved in nicotine-seeking. In conclusion, GlyT1-inhibitors might offer a therapeutic opportunity for acute cue-controlled nicotine-seeking, but the lack of persistent effects of the sub-chronic treatment associated with nicotine cues exposure suggests that short-term administration of GlyT1-inhibitor SSR504734 is not sufficient to promote extinction of nicotine-cue conditioned responding.

The effects of enriched environment on BDNF expression in the mouse cerebellum depending on the length of exposure

Environmental enrichment (EE) has been proposed as a factor that improves neuronal connectivity and brain plasticity. The induction of molecular mechanisms that takes place in the cortex, nucleus accumbens and hippocampus resulting from exposure to EE has been attributed partly to the role of neurotrophins as brain-derived neurotrophic factor (BDNF). Recent data directly implicate this neurotrophin in the modulation of plasticity changes in the cerebellum produced by living under environmental enrichment. In the present study, we aimed to assess the effects of different lengths of exposure to EE on cerebellar BDNF expression and western blotting analysis. On the whole, the present data has shown that BDNF increased under EE. However, changes in expression as a result of extending the duration of EE were only seen in Purkinje neurons. In Purkinje neurons, long-term exposure was required in order to fully express this neurotrophin. These data support BDNF as one of the long-term plasticity mechanisms induced by environment, suggesting that cerebellar plasticity can be stimulated as a response to challenges generated by environment. Our findings could have functional implications for various neurodegenerative disorders such as spinocerebellar ataxias, autism, schizophrenia and certain prion encephalopathies, most of them pathologies which have demonstrated to be characterized by alterations in Purkinje neurons and to show a partial recovery by exposure to EE.

Learning complex temporal patterns with resource-dependent spike timing-dependent plasticity

Studies of spike timing-dependent plasticity (STDP) have revealed that long-term changes in the strength of a synapse may be modulated substantially by temporal relationships between multiple presynaptic and postsynaptic spikes. Whereas long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength have been modeled as distinct or separate functional mechanisms, here, we propose a new shared resource model. A functional consequence of our model is fast, stable, and diverse unsupervised learning of temporal multispike patterns with a biologically consistent spiking neural network. Due to interdependencies between LTP and LTD, dendritic delays, and proactive homeostatic aspects of the model, neurons are equipped to learn to decode temporally coded information within spike bursts. Moreover, neurons learn spike timing with few exposures in substantial noise and jitter. Surprisingly, despite having only one parameter, the model also accurately predicts in vitro observations of STDP in more complex multispike trains, as well as rate-dependent effects. We discuss candidate commonalities in natural long-term plasticity mechanisms.

Mechanisms of repetition suppression in models, monkeys, and humans: A case for greater efficiency through enhanced synchronization

Experience with visual objects leads to later improvements in identification speed and accuracy (“repetition priming”), but generally leads to reductions in neural activity in single-cell recording studies in monkeys and fMRI studies in humans (“repetition suppression”). While the cell mechanisms that lead to these activity reductions are unclear, previous studies have implicated relatively local, automatic cortical mechanisms, and slice physiological recordings have identified several candidate short- and long-term plasticity mechanisms. I will show that these plasticity mechanisms when incorporated into a simplified neocortical circuit model are capable of re-producing changes in stimulus selectivity due to repetition as seen in single-cell recording studies in monkey area TE: “scaling” with relatively short-term repetitions and “sharpening” over longer periods of experience. However, these simulations when based on average firing rate fail to provide an account of behavioral priming. In contrast, simulations that retain the spiking property of neurons can potentially account for both repetition suppression and priming by allowing more synchronized and temporally coordinated activity at lower overall rates. I will review the current state of evidence in support of this proposal from monkey single-cell and LFP recordings and human MEG. I will also present new data from intracranial EEG recordings of human epilepsy patients showing that stimulus repetition at both short and long time scales leads to larger amplitude activity fluctuations at low frequencies (<15 Hz). These results indicate that greater neural synchronization accompanies lower overall activity levels following stimulus repetition, constituting a novel efficiency mechanism.

Late phase of long‐term potentiation in the mossy fiber—CA3 hippocampal pathway is critically dependent on metalloproteinases activity

Matrix metalloproteinases (MMPs) are known to play a pivotal role in remodeling of the extracellular matrix and have been implicated in synaptic plasticity, learning and memory. In hippocampus, inhibition of MMPs impairs the maintenance of long term plasticity in Schaeffer collateral-CA1 (Sch/CA1) synapses while its effect on short term plasticity remains a matter of debate. Surprisingly little is known on the role of MMPs in other hippocampal synapses. In this study we have investigated the impact of a broad spectrum MMPs inhibitor, FN-439 on synaptic transmission in mossy fiber-CA3 (MF/CA3) synapses exhibiting profoundly different mechanism of long term potentiation (LTP) as well as robust short-term plasticity, features that clearly distinguish them from the Sch/CA1 synapses. We report, that MMPs blockade before and up to 30 minutes after LTP induction resulted in a severe disruption of the late phase of tetanically induced LTP. However, LTP time course was not changed when FN439 was administered 60 minutes post LTP induction indicating that MMPs activity is required for the consolidation of the synaptic plasticity within a specific time window. The paired-pulse facilitation ratio or post-tetanic potentiation or burst-like pattern of mossy fiber stimulation were not changed in the presence of FN-439 administered for 15 minutes suggesting that temporal pattern of presynaptic transmitter release and, in general, the MF-CA3 fidelity is not significantly affected by MMPs inhibition. We conclude that although the mechanisms of long-term plasticity in MF/CA3 and in Sch/CA1 are profoundly different, MMPs play a crucial role in both pathways in the maintenance of LTP, which is believed to play an important role in learning and memory in the hippocampus.

Presynaptically Expressed Long-Term Potentiation Increases Multivesicular Release at Parallel Fiber Synapses

At a number of synapses, long-term potentiation (LTP) can be expressed by an increase in presynaptic strength, but it is unknown whether presynaptic LTP is expressed solely through an increase in the probability that a single vesicle is released or whether it can increase multivesicular release (MVR). Here, we show that presynaptic LTP decreases inhibition of AMPA receptor EPSCs by a low-affinity antagonist at parallel fiber-molecular layer interneuron (PF-MLI) synapses. This indicates that LTP induction results in larger glutamate concentration transients in the synaptic cleft, a result indicative of MVR, and suggests that MVR can be modified by long-term plasticity. A similar decrease in inhibition was observed when release probability (PR) was increased by forskolin, elevated extracellular Ca2+, and paired-pulse facilitation. Furthermore, we show that MVR may occur under baseline physiological conditions, as inhibition increased when P(R) was lowered by reducing extracellular Ca2+ or by activating presynaptic adenosine receptors. These results suggest that at PF-MLI synapses, MVR occurs under control conditions and is increased when PR is elevated by both short- and long-term plasticity mechanisms.

Comprehensive behavioral phenotyping of calpastatin-knockout mice

BackgroundCalpastatin is an endogenous inhibitor of calpain, intracellular calcium-activated protease. It has been suggested to be involved in molecular mechanisms of long-term plasticity and excitotoxic pathways. However, functions of calpastatin in vivo are still largely unknown. To examine the physiological roles of calpastatin, we subjected calpastatin-knockout mice to a comprehensive behavioral test battery.ResultsCalpastatin-knockout mice showed decreased locomotor activity under stressful environments, and decreased acoustic startle response, but we observed no significant change in hippocampus-dependent memory function.ConclusionThese results suggest that calpastatin is likely to be more closely associated with affective rather than cognitive aspects of brain function.

Contribution of cortical and subcortical electrostimulation in brain glioma surgery: Methodological and functional considerations

The aim of brain glioma surgery is to maximize the quality of resection, while minimizing the risk of sequelae. Due to the frequent location of gliomas in "eloquent areas" and because of major interindividual anatomofunctional variability, the cortical functional organization, effective connectivity and potential for plasticity must be studied for each patient individually. Consequently, in addition to preoperative functional neuroimaging, intraoperative electrostimulation (IES) can be used, under general anesthesia for motor mapping or on awake patient for language and cognitive mapping. This is an easy, accurate, reliable, and safe technique of detection of both cortical and subcortical functionally essential structures. Thus, IES enables: (i) to study the individual cortical functional organization before any resection; (ii) to understand the pathophysiology of areas involved by gliomas; (iii) to map the subcortical structures along the resection, allowing a study of the anatomofunctional connectivity; (iv) to analyze the mechanisms of on-line short-term plasticity, using repeated IES; (v) to tailor the resection according to individual cortico-subcortical functional boundaries, enabling to optimize the benefit:risk ratio of surgery. Moreover, IES can be combined with perioperative functional neuroimaging, before and after surgery, to validate these noninvasive techniques and to better understand the short-term and long-term plasticity mechanisms based on functional cortical reshaping and connectivity changes. Such individual knowledge allows planning multiple-stages surgery. In conclusion, IES enables to increase the impact of surgery on the natural history of gliomas, to preserve the quality of life, and to better understand the dynamic functional anatomy of the brain.

Task-specific changes in motor evoked potentials of lower limb muscles after different training interventions

This study aimed to identify sites and mechanisms of long-term plasticity following lower limb muscle training. Two groups performing either a postural stability maintenance training (SMT) or a ballistic ankle strength training (BST) were compared to a non-training group. The hypothesis was that practicing of a self-initiated voluntary movement would facilitate cortico-spinal projections, while practicing fast automatic adjustments during stabilization of stance would reduce excitatory influence from the primary motor cortex. Training effects were expected to be confined to the practiced task. To test for training specificity, motor evoked potentials (MEP) induced by transcranial magnetic stimulation (TMS) were recorded at rest and during motor tasks that were similar to each training. Intracortical, cortico-spinal, as well as spinal parameters were assessed at rest and during these tasks. The results show high task and training specificity. Training effects were only observable during performance of the trained task. While MEP size was decreased in the SMT group for the trained tasks, MEP recruitment was increased in the BST group in the trained task only. The control group did not show any changes. Background electromyogram levels, M. soleus H-reflex amplitudes and intracortical parameters were unaltered. In summary, it is suggested that the changes of MEP parameters in both training groups, but not in the control group, reflect cortical motor plasticity. While cortico-spinal activation was enhanced in the BST group, SMT may be associated with improved motor control through increased inhibitory trans-cortical effects. Since spinal excitability remained unaltered, changes most likely occur on the supraspinal level.