1.
Sham transcranial electrical stimulation and its effects on corticospinal excitability: a systematic review and meta-analysis.
Dissanayaka, TD, Zoghi, M, Farrell, M, Egan, GF, Jaberzadeh, S
Reviews in the neurosciences. 2018;(2):223-232
Abstract
Sham stimulation is used in randomized controlled trials (RCTs) to assess the efficacy of active stimulation and placebo effects. It should mimic the characteristics of active stimulation to achieve blinding integrity. The present study was a systematic review and meta-analysis of the published literature to identify the effects of sham transcranial electrical stimulation (tES) - including anodal and cathodal transcranial direct current stimulation (a-tDCS, c-tDCS), transcranial alternating current stimulation (tACS), transcranial random noise stimulation (tRNS) and transcranial pulsed current stimulation (tPCS) - on corticospinal excitability (CSE), compared to baseline in healthy individuals. Electronic databases - PubMed, CINAHL, Scopus, Science Direct and MEDLINE (Ovid) - were searched for RCTs of tES from 1990 to March 2017. Thirty RCTs were identified. Using a random-effects model, meta-analysis of a-tDCS, c-tDCS, tACS, tRNS and tPCS studies showed statistically non-significant pre-post effects of sham interventions on CSE. This review found evidence for statically non-significant effects of sham tES on CSE.
2.
Neuromuscular adaptations to healthy aging.
McNeil, CJ, Rice, CL
Applied physiology, nutrition, and metabolism = Physiologie appliquee, nutrition et metabolisme. 2018;(11):1158-1165
Abstract
Even in the absence of disease or disability, aging is associated with marked physiological adaptations within the neuromuscular system. An ability to perform activities of daily living and maintain independence with advanced age is reliant on the health of the neuromuscular system. Hence, it is critical to elucidate the age-related adaptations that occur within the central nervous system and the associated muscles to design interventions to maintain or improve neuromuscular function in the elderly. This brief review focuses on the neural alterations observed at both spinal and supraspinal levels in healthy humans in their seventh decade and beyond. The topics addressed are motor unit loss and remodelling, neural drive, and responses to transcranial magnetic stimulation of the motor cortex.
3.
Biological and anatomical factors influencing interindividual variability to noninvasive brain stimulation of the primary motor cortex: a systematic review and meta-analysis.
Pellegrini, M, Zoghi, M, Jaberzadeh, S
Reviews in the neurosciences. 2018;(2):199-222
Abstract
Noninvasive brain stimulation (NIBS) modifies corticospinal excitability (CSE) historically in a predictable manner dependent on stimulation parameters. Researchers, however, discuss high degrees of variability between individuals, either responding as expected or not responding as expected. The explanation for this interindividual variability remains unknown with suggested interplay between stimulation parameters and variations in biological, anatomical, and physiological factors. This systematic review and meta-analysis aimed to investigate the effect of variation in inherent factors within an individual (biological and anatomical factors) on CSE in response to NIBS of the primary motor cortex. Twenty-two studies were included investigating genetic variation (n=7), age variation (n=4), gender variation (n=7), and anatomical variation (n=5). The results indicate that variation in brain-derived neurotrophic factor genotypes may have an effect on CSE after NIBS. Variation between younger and older adults also affects CSE after NIBS. Variation between age-matched males and females does not affect CSE after NIBS, but variation across the menstrual cycle does. Variation between skull thickness and brain tissue morphology influences the electric field magnitude that ultimately reaches the primary motor cortex. These findings indicate that biological and anatomical variations may in part account for interindividual variability in CSE in response to NIBS of the primary motor cortex, categorizing individuals as responding as expected (responders) or not responding as expected (nonresponders).
4.
Insights into the bilateral cortical control of human masticatory muscles revealed by transcranial magnetic stimulation.
Nordstrom, MA
Archives of oral biology. 2007;(4):338-42
Abstract
In this brief review I describe details of the functional organisation of the bilateral corticobulbar projections to the trigeminally innervated masticatory muscles, as revealed by transcranial magnetic stimulation of the human brain. The motor cortices of both hemispheres are involved in control of trigeminal motoneurons, however the contralateral hemisphere has the greater excitatory influence. Corticomotoneuronal cells in each hemisphere project to jaw-closer and jaw-opener motoneurons. Less is known about cortically mediated inhibitory effects in the trigeminal motor system, but the available evidence suggests that drive to jaw muscles on each side is affected similarly by intracortical inhibitory processes activated in one hemisphere. Functional studies reveal that the two hemispheres play distinct roles in control of ipsilateral and contralateral muscles, particularly for jaw-closers. Masseter and digastric motor units recruited during low-force contractions do not receive uniform inputs from each hemisphere; the majority of masseter motor units are excited only from the contralateral hemisphere, and while digastric motor units are usually excited from both hemispheres the direct CM cell influence appears to be augmented on the contralateral side by corticobulbar activation of segmental excitatory interneurons. Differences in bilateral cortical control of jaw-closer and jaw-opener muscles may contribute to the more independent control of jaw-closers on each side during functional tasks. Corticobulbar control of the trigeminal muscles during natural tasks such as chewing and speech remains to be investigated with TMS.
5.
Motor cortex stimulation for intractable pain.
Osenbach, RK
Neurosurgical focus. 2006;(6):E7
Abstract
Effective management of neuropathic pain is one of the more challenging endeavors for even the most experienced and skilled pain specialist. Pharmacological therapy is frequently ineffective and/or poorly tolerated, especially in elderly patients. Many if not most surgical procedures have yielded limited success in the treatment of these pain conditions. Motor cortex stimulation (MCS) has emerged as a promising technique for the management of pain in patients with difficult neuropathic and central pain conditions. Although MCS has proven most successful for patients with trigeminal neuropathic/deafferentation pain and central poststroke pain, other conditions are now emerging as potential targets for this therapy. Based on previous as well as ongoing work, it would appear that the future of MCS is indeed bright. Hopefully, as work continues in this area, investigators will be able to develop a better understanding of the mechanisms underlying this modality and be able to further refine the technique of MCS. It is also possible that with the use of noninvasive tools such as transcranial magnetic stimulation, practitioners will be able to predict with accuracy which patients are likely to respond favorably to MCS.
6.
Cortical reorganisation and chronic pain: implications for rehabilitation.
Flor, H
Journal of rehabilitation medicine. 2003;(41 Suppl):66-72
Abstract
Recent neuroscientific evidence has revealed that the adult brain is capable of substantial plastic change in such areas as the primary somatosensory cortex that were formerly thought to be modifiable only during early experience. These findings have implications for our understanding of chronic pain. Functional reorganisation in both the somatosensory and the motor system was observed in neuropathic and musculoskeletal pain. In patients with chronic low back pain and fibromyalgia the amount of reorganisational change increases with chronicity; in phantom limb pain and other neuropathic pain syndromes cortical reorganisation is correlated with the amount of pain. These central alterations may be viewed as pain memories that influence the processing of both painful and nonpainful input to the somatosensory system as well as its effects on the motor system. Cortical plasticity related to chronic pain can be modified by behavioural interventions that provide feedback to the brain areas that were altered by somatosensory pain memories or by pharmacological agents that prevent or reverse maladaptive memory formation.