A 15-minute session of direct current stimulation does not produce lasting changes in axonal excitability

Modulation of neuromuscular transmission using transcutaneous direct currents: An exploratory study

OBJECTIVE

To assess the in vivo long-lasting effects on neuromuscular transmission using transcutaneous stimulation with anodal and cathodal direct currents applied over the end-plate region (epDCS).

METHODS

An active DCS electrode was placed over the end-plate region of both abductor pollicis brevis and first dorsal interosseous muscles, with a reference electrode located on the forearm. Cathodal or anodal currents were applied (2.5mA during 15min). Repetitive nerve stimulation of the median and ulnar nerves at the wrist was performed before and after DCS: protocol A - 500 stimuli at 3Hz; protocol B - 30 stimuli at 30Hz. For both muscles, we measured changes in amplitude and area between the first and 4th compound muscle action potential (CMAP) and between the first and 500th CMAP (protocol A); and the change in amplitude and area between the first and 30th CMAP (protocol B).

RESULTS

Anodal current did not change any measurement. Using cathodal epDCS and median nerve testing, there was a larger increase in CMAP amplitude (p=0.046) and a smaller decrease in area (p=0.008) between the first and 30th response (protocol B). Using cathodal epDCS and ulnar nerve testing, there was a possible significant smaller amplitude decrease of the CMAP measured, between the first and fourth response (protocol A).

CONCLUSIONS

Cathodal transcutaneous direct currents over the end-plate may modulate end-plate function by increasing the release of quanta of acetylcholine (Ach) and/or the number of Ach receptors available. Future studies should address this topic.

Modulation of sensory nerve fiber excitability by transcutaneous cathodal direct current stimulation

OBJECTIVE

To assess the lasting effects on sensory nerve membrane excitability of transcutaneous peripheral nerve stimulation with cathodal direct currents (pDCS).

METHODS

We performed pDCS in 10 healthy subjects with the active electrode placed over the distal right forearm and the reference electrode on the back of the right hand. We used 5×5cm rubber electrodes and the current applied was 2.5mA during 15min. Three pDCS sessions were performed on the same day: first, a baseline stimulation was performed, followed by a sham stimulation and lastly a cathodal stimulation. Median sensory nerve excitability measurements were performed at baseline and immediately after each pDCS session using the TRONDNF nerve excitability protocol of the QTRAC program (measurement on the second finger).

RESULTS

The protocol was completed and well tolerated in all subjects. RRP (relative refractory period) and refractoriness at 2.5ms were significantly different across the three study conditions, with a significant increase of RRP immediately following cathodal stimulation compared with baseline assessment (mean 4.2 versus 5.3, P=0.002). Other measurements were not modulated by the intervention. Sham-stimulation did not change axonal excitability.

CONCLUSIONS

Cathodal pDCS stimulation increased RRP of sensory fibers, but no other consistent long-lasting effect was observed. This finding might suggest a reduction of sensory fiber excitability induced by cathodal pDCS.