Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.</p

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole-bodyrelaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well-established paired motor unit (MU) technique. High-density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20-s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexiontorque and soleus activity were recorded during 33-s Achilles tendon vibration and simultaneous 20-Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n=15, 5 females) and WBR (n=15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post-vibration torque were reduced by WBR (n=19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n=20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs onmotoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non-pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low-magnitude involuntary torque combined with the negative effects of the interventions

Effects of reciprocal inhibition and whole‐body relaxation on persistent inward currents estimated by two different methods

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole-body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well-established paired motor unit (MU) technique. High-density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20-s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33-s Achilles tendon vibration and simultaneous 20-Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post-vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non-pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low-magnitude involuntary torque combined with the negative effects of the interventions.</p