Effects of afferent stimulation rate on inhibitory spinal pathways in hemiplegic spastic patients.

OBJECTIVE: It has recently been demonstrated in the cat and in healthy subjects that the effects of repetitive afferent fibre stimulation depends on the target spinal neurones. The purpose of this series of experiments was therefore to determine whether central nervous system lesions modify the behaviour of the inhibitory spinal pathways in response to repetitive activation of afferent fibres. METHODS: The H-reflex technique was used to study the effect of increasing the conditioning stimulus rate from 0.16 to 1 Hz on disynaptic inhibition and on presynaptic Ia inhibition on the affected side of 36 hemiplegic patients. RESULTS: The major finding was that, similar to results previously obtained in healthy subjects, increasing the conditioning stimulus rate in hemiplegic patients leads to an increase in the synaptic efficiency of inhibitory spinal circuits. Furthermore, a significant correlation was found between the severity of flexor carpi radialis muscle spasticity and the amount of disynaptic inhibition. CONCLUSIONS: The reinforcement of inhibitory spinal networks induced by repetitive stimulation of afferent fibres is preserved in spastic patients, whereas the mechanisms underlying this phenomena might be altered. SIGNIFICANCE: The results of these experiments open up a number of possibilities for novel spasticity therapies based on non-invasive techniques.

Effects of anodal tDCS on lumbar propriospinal system in healthy subjects.

OBJECTIVE: It has recently been shown that transcranial direct current stimulation (tDCS) (1) can modify lumbar spinal network excitability and (2) decreases cervical propriospinal system excitability. Thus the purpose of this series of experiments was to determine if anodal tDCS applied over the leg motor cortex area induces changes in lumbar propriospinal system excitability. To that end, the effects of anodal tDCS and sham tDCS on group I and group II propriospinal facilitation of quadriceps motoneurones were studied in healthy subjects. METHODS: Common peroneal nerve group I and group II quadriceps H-reflex facilitation was assessed in 15 healthy subjects in two randomised conditions: anodal tDCS condition and sham tDCS condition. Recordings were performed before, during and after the end of the cortical stimulation. RESULTS: Compared to sham, anodal tDCS decreases significantly CPN-induced group I and II quadriceps H-reflex facilitation during and also after the end of the cortical stimulation. CONCLUSIONS: Anodal tDCS induces (1) modulation of lumbar propriospinal system excitability (2) post-effects on spinal network. SIGNIFICANCE: These results open a new vista to regulate propriospinal lumbar system excitability in patients and suggest that anodal tDCS would be interesting for neuro-rehabilitation of patients with central nervous system lesions.

Effects of anodal transcranial direct current stimulation over the leg motor area on lumbar spinal network excitability in healthy subjects.

In recent years, two techniques have become available for the non-invasive stimulation of human motor cortex: transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). The effects of TMS and tDCS when applied over motor cortex should be considered with regard not only to cortical circuits but also to spinal motor circuits. The different modes of action and specificity of TMS and tDCS suggest that their effects on spinal network excitability may be different from that in the cortex. Until now, the effects of tDCS on lumbar spinal network excitability have never been studied. In this series of experiments, on healthy subjects, we studied the effects of anodal tDCS over the lower limb motor cortex on (i) reciprocal Ia inhibition projecting from the tibialis anterior muscle (TA) to the soleus (SOL), (ii) presynaptic inhibition of SOL Ia terminals, (iii) homonymous SOL recurrent inhibition, and (iv) SOL H-reflex recruitment curves. The results show that anodal tDCS decreases reciprocal Ia inhibition, increases recurrent inhibition and induces no modification of presynaptic inhibition of SOL Ia terminals and of SOL-H reflex recruitment curves. Our results indicate therefore that the effects of tDCS are the opposite of those previously described for TMS on spinal network excitability. They also indicate that anodal tDCS induces effects on spinal network excitability similar to those observed during co-contraction suggesting that anodal tDCS activates descending corticospinal projections mainly involved in co-contractions.

Enhanced spinal excitation from ankle flexors to knee extensors during walking in stroke patients.

OBJECTIVES: It is still unclear to what an extent altered reflex activity contributes to gait deficit following stroke. Spinal group I and group II excitations from ankle dorsiflexors to knee extensors were investigated during post-stroke walking. METHODS: Electrical stimulation was applied to the common peroneal nerve (CPN) in the early stance, and the short-latency biphasic excitation in Quadriceps motoneurones was evaluated from the Vastus Lateralis (VL) rectified and averaged (N=50) EMG activity in 14 stroke patients walking at 0.6-1.6 km/h, and 14 control subjects walking at 3.2-4.8 and at 1 km/h. RESULTS: The second peak of the CPN-induced biphasic facilitation in VL EMG activity, which is likely mediated by group II excitatory pathways, was larger on the paretic side of the patients, as compared to their nonparetic side or control subjects, whatever their walking speed. CONCLUSIONS: The spinal, presumed group II, excitation from ankle dorsiflexors to knee extensors is particularly enhanced during post-stroke walking probably due to plastic adaptations in the descending control. SIGNIFICANCE: This adaptation may help to stabilize the knee in early stance when the patients have recover ankle dorsiflexor functions.

The estimation of short intra-cortical inhibition depends on the proportion of spinal motoneurones activated by corticospinal inputs.

OBJECTIVE: The high variability of SICI limits its utility and by extension that of TMS in clinical neurophysiology. Non-linear summation of descending volleys due to heterogeneous motoneurone properties, on which MEP size depends, has not previously been implicated as an issue in SICI evaluation. METHODS: MEP size and SICI were normalised to the test MEP (mV), and as a percentage of M(max) to take account of the proportion of motoneurone pool activated by TMS. Two EMG systems, producing large and small MEPs, were used to determine how the normalisation affected MEPs of different amplitude. RESULTS: M(max) normalisation (i) counteracted the influence of recording conditions on the MEP size, (ii) revealed a significant influence of the test size on SICI (between medium and large MEPs), and of test size on the conditioning intensity (the larger the MEP, the stronger the SICI), and (iii) decreased the variability. CONCLUSIONS: Data normalised to M(max) better reflect the motoneurone recruitment after SICI. To enhance reproducibility, MEP should be normalised to M(max). This adjusts for some of the non-linear properties at the spinal, and possibly, at cortical levels. SIGNIFICANCE: To reduce variability is important because TMS is becoming widely adopted and is being used in patients.

Impact of transcranial direct current stimulation on spinal network excitability in humans.

Transcranial direct current stimulation (tDCS) when applied over the motor cortex, modulates excitability dependent on the current polarity. The impact of this cortical modulation on spinal cord network excitability has rarely been studied. In this series of experiments, performed in healthy subjects, we show that anodal tDCS increases disynaptic inhibition directed from extensor carpi radialis (ECR) to flexor carpi radialis (FCR) with no modification of presynaptic inhibition of FCR Ia terminals and FCR H-reflex recruitment curves. We also show that cathodal tDCS does not modify spinal network excitability. Our results suggest that the increase of disynaptic inhibition observed during anodal tDCS relies on an increase of disynaptic interneuron excitability and that tDCS over the motor cortex in human subjects induces effects on spinal network excitability. Our results highlight the fact that the effects of tDCS should be considered in regard to spinal motor circuits and not only to cortical circuits.

Effects of galvanic mastoid stimulation in seated human subjects.

The vestibular responses evoked by transmastoid galvanic stimulation (GS) in the rectified soleus electromyogram (EMG) in freely standing human subjects disappear when seated. However, a GS-induced facilitation of the soleus monosynaptic (H and tendon jerk) reflex has been described in few experiments in subjects lying prone or seated. This study addresses the issue of whether this reflex facilitation while seated is of vestibulospinal origin. GS-induced responses in the soleus (modulation of the rectified ongoing EMG and of the monosynaptic reflexes) were compared in the same normal subjects while freely standing and sitting with back and head support. The polarity-dependent biphasic responses in the free-standing position were replaced by a non-polarity-dependent twofold facilitation while seated. The effects of GS were hardly detectable in the rectified ongoing voluntary EMG activity, weak for the H reflex, but large and constant for the tendon jerk. They were subject to habituation. Anesthesia of the skin beneath the GS electrodes markedly reduced the reflex facilitation, while a similar, although weaker, facilitation of the tendon jerk was observed when GS was replaced with purely cutaneous stimulation, a tap to the tendon of the sternomastoid muscle, or an auditory click. The stimulation polarity independence of the GS-induced reflex facilitation argues strongly against a vestibular response. However, the vestibular afferent volley, insufficient to produce a vestibular reflex response while seated, could summate with the GS-induced tactile or proprioceptive volley to produce a startle-like response responsible for the reflex facilitation.