Is the Frequency in Somatosensory Electrical Stimulation the Key Parameter in Modulating the Corticospinal Excitability of Healthy Volunteers and Stroke Patients with Spasticity?

Motor potential evoked by transcranial magnetic stimulation depends on the placement protocol of recording electrodes: a pilot study

OBJECTIVE

There seems to be no consensus in the literature regarding the protocol of surface electromyography (sEMG) electrode placement for recording motor evoked potentials (MEP) in transcranial magnetic stimulation (TMS) applications. Thus, the aim of this study was to investigate the effect on the MEP amplitude bytwo different protocols for electrode placement.

METHODS

sEMG electrodes were placed on three upper arm muscles (biceps brachii, flexor carpi radialis, and flexor pollicis brevis) of six right-handed subjects following two different protocols (1 and 2), which varied according to the interelectrode distance and location relative to the muscle. TMS pulses were applied to the hotspot of biceps brachii, while sEMGwas recorded from the two protocols and for each muscle simultaneously.

MAIN RESULTS

Greater MEP amplitudes were obtained for Protocol 1 compared to Protocol 2 (P < 0.05).

SIGNIFICANCE

Different electrode placement protocols may result in distinct MEP amplitudes, which should be taken into account when adjusting the intensity on single and repetitive TMS sessions.

Sensory Sub- and Suprathreshold TENS Exhibit No Immediate Effect on Postural Steadiness in Older Adults with No Balance Impairments

Transcutaneous electrical nerve stimulation (TENS) has been reported to attenuate postural sway; however, the results are inconclusive, with some indicating the effect and others not. The study aimed to evaluate the effect of sensory sub- and suprathreshold low-frequency TENS applied through the plantar surface and posterior aspect of shanks on postural sway. In a group of healthy community-dwelling older adults, TENS was delivered with two different current intensities: (1) subsensory which is below conscious perception and (2) suprasensory threshold which is within the range of conscious perception. Frequencies of the TENS stimulation were sweeping from 5 to 180 Hz and were delivered through the plantar surface and posterior shanks of both legs. Postural sway was measured with a force platform in eyes-open and eyes-closed conditions. To evaluate potential fast adaptability to TENS stimuli, the results were evaluated in two time intervals: 30 seconds and 60 seconds. The results indicated that TENS with the chosen frequencies and electrode placement did not affect postural sway in both the sub- and suprathreshold intensities of TENS, in eyes-open and eyes-closed conditions, and in 30-second and 60-second time intervals. In conclusion, given that in this study sub- and suprathreshold TENS applied via the plantar surface of the feet did not attenuate postural sway, it would be easy to conclude that this type of electrical stimuli is ineffective and no further research is required. We must caution against this, given the specificity of the electrode placements. We recommend that future research be performed consisting of individuals with balance impairments and with different positions of electrodes.

Is somatosensory electrical stimulation effective in relieving spasticity? A systematic review

Spasticity is a sensorimotor disorder widely recognized as one of the features that contribute to patients' disability. Transcutaneous electric neural stimulation (TENS/SES) has been adopted in spasticity rehabilitation as an alternative to pharmacological agents. Although previous studies have reported clinical benefits of TENS/SES in relieving spasticity, there is no clarity on how and whether this therapeutic modality affects specific neural circuitries. Thus, this systematic review aimed to verify the efficacy of TENS/SES in the control of spasticity and its consequences in spinal and corticospinal excitability. This study was carried out according to PRISMA recommendations using SCOPUS, PubMed, BVS, Google Scholar and BASE databases screening, which provided 483 references. Six additional records were found from other sources. All these records were submitted to a filtering process following the eligibility criteria, and 44 studies were selected for further analysis. Ten were replicas. Consequently, 34 studies were read in full with the aim of checking their eligibility criterion, which resulted in 10 manuscripts for qualitative synthesis. Even though they evaluated the effects of TENS/SES both at the spinal and/or corticospinal levels, the electrophysiological results seem to be inconsistent, corroborating the lack of agreement between them and with clinical outcomes.

Neuromodulation by combined sensory and motor stimulation in the peripheral nerve: tendon organ afferent activity

OBJECTIVE

Neuromuscular stimulation is a therapeutic approach to treat impairments such as stroke or pain, although the resulting inputs to the nervous system due to the stimulation are not well quantified. Stimulation activates both afferents and efferents, and the resulting neural activity is a mix of the effects of both: the changes in afferent activity due to efferent physiological actions plus the alterations due to afferent stimulation. This study quantitatively describes the resulting Golgi tendon organ Ib afferent activity in response to mixed afferent and efferent stimulation.

APPROACH

Neural and stimulated action potentials interact by means of collision, neural resetting, and refractory block. We simulated the action potential patterns of Ib afferents in the human first dorsal interosseous during constant voluntary, stimulated, and combined contractions, varying both stimulation rate and location, and both with and without simultaneous Ib stimulation.

MAIN RESULTS

Efferent stimulation affects the firing rate versus force relationships of individual tendon organs, but the effects on the population mean are very small. In contrast, afferent stimulation changes the firing rate versus force relationship of individual afferents by increasing the firing rate, decreasing the force resolution, broadening the distribution of action potential rates, and temporally phase locking a portion of the action potentials. Force resolution is retained in the population mean. The effects of afferent stimulation change with the stimulation rate and location, and with receptor firing rates.

SIGNIFICANCE

This is the first quantitative description of the changes in afferent feedback during combined efferent and afferent nerve stimulation. The small effects of efferent stimulation on the population response implies that tendon organs could provide accurate force feedback during stimulated contractions. The effects of afferent stimulation can be altered by choice of stimulus rate and site and are generalizable to other afferents.