Effect of spinal transcutaneous direct current stimulation on somatosensory evoked potentials in humans

Does Trans-Spinal Direct Current Stimulation Alter Phrenic Motoneurons and Respiratory Neuromechanical Outputs in Humans? A Double-Blind, Sham-Controlled, Randomized, Crossover Study

“…tsDCS may have interfered with cortical excitability, as tsDCS applied to the thoracic level modulated the cervicomedullary P30 component of the posterior tibial nerve somatosensory-evoked potential (Cogiamanian et al, 2008) and attenuated the spinal processing of nociceptive inputs (Truini et al, 2011), suggesting that tsDCS influences conduction along ascending (lemniscal/ spinothalamic) tracts. To test this hypothesis, we measured sICI, a common method used to investigate changes in GABA A intracortical synapses in the motor cortex , in the diaphragm (Demoule et al, 2003b;Sharshar et al, 2004).…”

“…Indeed, PMNs can be classified according to their bursting patterns (Kong and Berger, 1986;Lee et al, 2009): early-inspiratory (early-I) PMNs begin to discharge during the initial part of inspiration, whereas late-inspiratory (late-I) PMNs begin bursting after ϳ10 -40% of the inspiratory period has elapsed (Lee and Fuller, 2011). Burst onset differences between PMN subtypes are likely to reflect intrinsic motoneuron properties (Berger, 1979): early-I PMNs are high-resistant small cells, whereas late-I PMNs are low-resistant large cells. Thus, during spontaneous ventilation, in which respiratory-related discharges of PMNs are primarily driven by bulbospinal inputs, early-I PMNs would be recruited before late-I PMNs according to the size principle of motoneuron recruitment established by Henneman et al (1965).…”

“…However, since the origin of CMS-evoked CMAPs in this muscle has been debated [i.e., direct activation of cervical roots as in all other muscles (Chokroverty et al, 1995) or direct activation of the phrenic nerve (Similowski et al, 1997)], we also recorded responses evoked in the BB [i.e., a muscle innervated by part of the same spinal segments (C5-C6) as the diaphragm] and the ADM (i.e., a "control" muscle for which spinal segments C8 -T1 are also activated by the lower part of the TMS coil). Cervical magnetic stimulation (CMS) was delivered by a Magstim Bistim 2 200 (Magstim Company) with a 90-mmdiameter circular coil (Magstim Company).…”

“…Polarity of stimulation refers to the spinal electrode. The stimulus intensity was set at 2.5 mA and was applied over a 15 min period (Cogiamanian et al, 2008;Winkler et al, 2010;Lamy et al, 2012) resulting in a current density of 0.071 mA/cm 2 and a delivered total charge of 64 mC/ cm 2 . The current was ramped up to 2.5 mA over a 10 s period and similarly ramped down at the end of stimulation.…”

“…Recently, there has been a growing interest in application of DC stimulation over the spinal cord [transcutaneous spinal direct current stimulation (tsDCS)] because of its acceptable safety record, low cost, and potential use in outpatients . Indeed, in humans, it has been shown that tsDCS can modulate activity in lemniscal (Cogiamanian et al, 2008), extralemniscal (Cogiamanian et al, 2011;Truini et al, 2011), and corticospinal (Lim and Shin, 2011) tracts, as well as segmental spinal motor circuits (Winkler et al, 2010;Lamy et al, 2012). Although the mechanisms underlying tsDCS aftereffects are poorly understood at the present time, several studies have suggested that (1) glutamatergic neurotransmission is altered after tsDCS (Ahmed and Wieraszko 2012), leading to the assumption that tsDCS aftereffects are likely to occur via activation of neuronal processes terminating in the spinal cord, which are responsible for expression of long-term potentiation in postsynaptic spinal cord motoneurons; (2) DC polarization could affect the intrinsic properties of the motoneurons (Ahmed, 2011); (3) tsDCS could modulate the spontaneous discharges in ventral roots (Fuortes, 1954;Ahmed, 2011); and (4) tsDCS aftereffects differ according to the direction of the current and the position of the electrodes (Ahmed, 2014;Parazzini et al, 2014).…”