The effect of rTMS over the cerebellum in normal human volunteers on peg-board movement performance.

Low frequency rTMS over the paramedian part of the right cerebellum was used to test the effects of TMS-induced disruption of the cerebellum on performance of the 10-hole pegboard task. A test group (n = 14) showed significantly increased movement times lasting about 3 min after the 5-min 1 Hz rTMS train, compared to a control group who received no rTMS (n = 14), tested in a parallel group design. The increase was greatest for the hand ipsilateral to the stimulation, but the difference between the two hands was not statistically significant. These results suggest that the rTMS affects cerebellar excitability and cause a short-lasting bilateral change in sensory-motor performance.

Functional coupling of motor units is modulated during walking in human subjects.

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies

Interaction between peripheral afferent activity and presynaptic inhibition of ia afferents in the cat.

It has been demonstrated in man that the H-reflex is more depressed by presynaptic inhibition than the stretch reflex. Here we investigated this finding further in the alpha-chloralose-anesthetized cat. Soleus monosynaptic reflexes were evoked by electrical stimulation of the tibial nerve or by stretch of the triceps surae muscle. Conditioning stimulation of the posterior biceps and semitendinosus nerve (PBSt) produced a significantly stronger depression of the electrically than the mechanically evoked reflexes. The depression of the reflexes has been shown to be caused by presynaptic inhibition of triceps surae Ia afferents. We investigated the hypothesis that repetitive activation of peripheral afferents may reduce their sensitivity to presynaptic inhibition. In triceps surae motoneurones, we measured the effect of presynaptic inhibition on excitatory postsynaptic potentials (EPSPs) produced by repetitive activation of the peripheral afferents or by fast and slow muscle stretch. EPSPs evoked by single electrical stimulation of the tibial nerve or by fast muscle stretch were significantly depressed by PBSt stimulation. However, the last EPSP in a series of EPSPs evoked by a train of electrical stimuli (5-6 shocks, 150-200 Hz) was significantly less depressed by the conditioning stimulation than the first EPSP. In addition, the last part of the long-lasting EPSPs evoked by a slow muscle stretch was also less depressed than the first part. A single EPSP evoked by stimulation of the medial gastrocnemius nerve was less depressed when preceded by a train of stimuli applied to the same nerve than when the same train of stimuli was applied to a synergistic nerve. The decreased sensitivity of the test EPSP to presynaptic inhibition was maximal when it was evoked within 20 ms after the train of EPSPs. It was not observed at intervals longer than 30 ms. These findings suggest that afferent activity may decrease the efficiency of presynaptic inhibition. We propose that the described interaction between afferent nerve activity and presynaptic inhibition may partly explain why electrically and mechanically evoked reflexes are differently sensitive to presynaptic inhibition.

Coupling of antagonistic ankle muscles during co-contraction in humans.

In 35 healthy human subjects coupling of EMGs recorded from the tibialis anterior (TA) and soleus (Sol) muscles during voluntary co-contraction was analysed in the time and frequency domains. Two patterns were observed in different subjects or in the same subject on different occasions. One pattern consisted of central peaks in the cumulant density function of the two signals, which was often accompanied by coherence in the 15-35 Hz frequency band. The other pattern consisted of a central trough in the cumulant density function, which was mostly accompanied by coherence around 10 Hz. When this was the case oscillations were usually observed in the cumulant density function with time lags of 100 ms. Both patterns could be observed in the same subject, but usually not at the same time. Coherence around 10 Hz associated with a central trough in the cumulant density function was less common during weak than during strong co-contraction. The central peak with coherence in the 15-35 Hz frequency band in contrast tended to be most common during weak contraction. There was a tendency for the 10-Hz coherence with central trough to occur when the contractions had been maintained for some time. Both patterns could be observed when sensory feedback in large diameter afferents was blocked by ischaemia. When a central peak with coherence in the 15-35 Hz frequency band was observed for paired TA and Sol EMG recordings (10 out of 19 subjects), a coupling in the same frequency band was also observed between the EMG activities from the two muscles and the EEG activity recorded from the leg area of the motor cortex. When the central trough and the coherence around 10 Hz was observed for the EMG recordings (8 out of 19 subjects), no significant coherence was observed between EEG and EMG in 7 of the 8 subjects. In the last subject coherence around 10 Hz was observed. It is suggested that these findings signify the existence of two different central input systems to antagonistic ankle motoneurones: one input activates one muscle while depressing the antagonist and the other coactivates antagonistic motoneurones. The data suggest that at least the latter input depends on motor cortical activity.

rTMS over the cerebellum can increase corticospinal excitability through a spinal mechanism involving activation of peripheral nerve fibres.

OBJECTIVES: Single-pulse transcranial magnetic stimulation (TMS) over the cerebellum affects corticospinal excitability by a cerebellar and a peripheral mechanism. We have investigated whether any of the long-lasting effects of repetitive TMS (rTMS) over cerebellum can also be attributed to peripheral effects. METHODS: Five hundred conditioning stimuli at 1 Hz were given over either the right cerebellum using a double-cone coil, or over the right posterior neck using a figure-8-coil. Corticospinal excitability was assessed by measuring the amplitude of motor evoked potentials (MEPs) evoked in the right and left hand and forearm muscles. Hoffman reflexes (H-reflex) were also obtained in the right flexor carpi radialis muscle. RESULTS: rTMS over either the right cerebellum or the right posterior neck significantly facilitated MEPs in hand and forearm muscles in the right but not in the left arm (n=8) for up to 30 min after the end of the train. rTMS (1 Hz) of the right neck area increased the amplitude of the H-reflex (n=5). CONCLUSIONS: Much of the persisting effects of rTMS over the cerebellum on corticospinal excitability appear to be mediated through stimulation of peripheral rather than central structures. Moreover, the results show that rTMS over peripheral areas can cause long-lasting changes in spinal reflexes.