Fig 1.
In Studies 1 and 2, 20 min of real or sham cervical transcutaneous spinal direct current stimulation (tsDCS) was applied through saline soaked sponge electrodes placed anteriorly below the cervicomental angle, and posteriorly over the spinous process of C7. Real tsDCS involved an initial ramping-up of intensity over 30 s, followed by stimulation at 3 mA for the remainder of the 20 min, whereas sham tsDCS involved an initial ramping-up of intensity over 30 s, 1 min of stimulation at 3 mA, then no stimulation for the remainder of the 20 min. In Study 1 (A), participants sat upright with their right arm relaxed on a pillow on their lap. Transcranial magnetic stimulation (TMS) over the primary motor cortex was used to elicit motor evoked potentials (MEPs) in the biceps brachii, flexor carpi radialis (FCR) and first dorsal interosseous (FDI) muscles. Cervicomedullary stimulation was used to directly activate corticospinal axons at the pyramidal decussation, producing cervicomedullary motor evoked potentials (CMEPs) in biceps and FCR. Electrical stimulation of the brachial plexus was used to elicit maximal compound muscle action potentials (Mmax) in the biceps, FCR and FDI. Three baseline sets of stimuli (5 CMEPs, 1 Mmax and 10 MEPs delivered at 0.1Hz) were delivered prior to real or sham tsDCS, with 5 min between the start of each set. Test sets of stimuli were then delivered at 0, 10, 20 and 30 min after real or sham tsDCS. In Study 2 (B), participants were supine with their right arm out to the side at 45 degrees shoulder abduction, elbow slightly bent, with the palm facing upwards. Electrical median nerve stimulation was used to elicit M waves and H reflexes in the FCR. M-wave and H-reflex recruitment curves were recorded and homosynaptic depression was measured before and after real or sham tsDCS. For baseline measurements, two recruitment curves were recorded, and then one set of stimuli for HD (at intensity for 50% of Hmax). At T1 (0–20 min) and T2 (20–40 min) after real or sham tsDCS, an initial set of stimuli for HD was delivered at the same intensity used for baseline measurements. A recruitment curve was then recorded, and if the stimulus intensity for 50% of Hmax was different to that measured during baseline recordings, a new set of stimuli for HD was delivered at a modified intensity.
Fig 2.
H-reflex recruitment curves, M-wave recruitment curves and H-reflex homosynaptic depression (HD).
(A) H-reflex and M-wave recruitment curves are for a single participant. H-reflex and M-wave peak-to-peak amplitudes were expressed as a percentage of Mmax. For a better alignment of M-wave recruitment curves, stimulus intensity was normalised to motor threshold (MT). H-reflexes were plotted against stimulus intensity (xMT) until just before the curve began to descend. The three parameter sigmoidal function H(s) = Hmax / (1 + em(S50-s)) was fitted to each H-reflex recruitment curve, where H(s) is the size of the H reflex at a given stimulus intensity (s), S50 is the stimulus intensity required to produce an H reflex of 50% of Hmax, and m is the slope parameter of the curve. From sigmoidal functions the following parameters could be calculated: Hmax, slope, Sthresh (stimulus intensity to elicit H reflexes of 5% Hmax), S50 and S99 (stimulus intensity to elicit H reflexes of 99% Hmax). In this individual, there are no obvious differences between baseline H-reflex recruitment curves and those measured at T1 (0–20 min) and T2 (20–40 min) after real or sham tsDCS, apart from a slight decrease in Hmax after both kinds of stimulation. (B) Overlaid traces are muscle responses to median nerve stimulation at intensities for eliciting H reflexes of 50% of Hmax. At each time-point (Baseline, T1: 0-20min, T2: 20–40 min), H reflexes were elicited at either 0.1 Hz (no HD) or 1 Hz (HD). Traces are from a different single participant to that in Fig 2A. In this individual there are no obvious differences in the size of H reflexes before and after real or sham tsDCS, nor are there any apparent differences between real or sham tsDCS in the levels of HD. Calibration: vertical, 1 mV; horizontal, 10 ms.
Table 1.
Mean baseline values for CMEPs, MEPs, H-reflex recruitment curve parameters and HD show no significant differences between days.
Fig 3.
Muscle responses to stimulation at the cervicomedullary junction before and after real or sham tsDCS.
(A) Averaged biceps brachii CMEPs (5–15 traces) for a single participant elicited before (Baseline) and at 0, 10, 20, and 30 min after real or sham tsDCS. No major differences can be observed between responses measured before and after real or sham interventions. Calibration: vertical, 1 mV; horizontal, 10 ms. Biceps (B) and FCR (C) CMEP areas are normalised to Mmax and expressed as a percentage of baseline values. Both group means (n = 12; ◯ = tsDCS; ◇ = sham) and individual data (smaller grey symbols) are represented at 0, 10, 20 and 30 min after real or sham tsDCS. Error bars are 95% CIs. For both biceps and FCR, there are no significant changes in CMEP size over time, and there are no significant differences between real and sham tsDCS.
Fig 4.
Muscle responses to transcranial magnetic stimulation of the primary motor cortex before and after real or sham tsDCS.
(A) Averaged biceps brachii MEPs (10–30 traces) for a single participant elicited before (Baseline) and at 0, 10, 20, and 30 min after real or sham tsDCS. No major differences can be observed between responses measured before and after real or sham interventions. Calibration: vertical, 0.1 mV; horizontal, 10 ms. Biceps (B), FCR (C) and FDI (D) MEP areas are normalised to Mmax and expressed as a percentage of baseline values. Both group means (n = 11; ◯ = tsDCS; ◇ = sham) and individual data (smaller grey symbols) are represented at 0, 10, 20 and 30 min after real or sham tsDCS. Error bars are 95% CIs. For all three muscles, there are no significant changes in MEP size over time, and there are no significant differences between real and sham interventions.
Fig 5.
Sigmoidal functions using group data.
In order to visualise any differences in H-reflex recruitment curves, the group averages (n = 12) of Hmax, m and S50 (taken from equations for each individual’s fitted curves) were used to create three parameter sigmoidal curves using the formula H(s) = Hmax / (1 + em(S50-s)). From these curves a small difference in baseline Hmax between real and sham tsDCS days can be observed, however this difference is not significant (Table 1). Also of note is the decrease in Hmax and slope of the curves at both T1 (0–20 min) and T2 (20–40 min) after tsDCS in comparison to baseline, however these differences occurred similarly on both real and sham tsDCS days, as can be seen in Fig 6A and 6B.
Fig 6.
Variables of the H-reflex recruitment curve and homosynaptic depression (HD) of the H reflex.
Both group means (n = 12; ◯ = tsDCS; ◇ = sham) and individual data (smaller grey symbols) are represented at T1 (0–20 min) and T2 (20–40 min) after real or sham tsDCS. Error bars are 95% CIs. Both Hmax (A) and slope (B) of the H-reflex recruitment curve are decreased in comparison to baseline measurements, however there are no differences between real and sham tsDCS, with similar decreases seen after both kinds of stimulation. For the remaining H-reflex recruitment curve parameters, Sthresh (C), S50 (D) and S99 (E), there are no differences before and after stimulation and there are no differences between real and sham tsDCS. There are similarly no differences in HD of the H reflex, calculated using H1Hz/H0.1Hz (F). Note that values are given as a % of baseline; thus a value greater than 100% indicates less HD and a value less than 100% indicates more HD than baseline.