Effects of electric and magnetic transcranial stimulation on long latency reflexes

Abstract

The interaction of transcranial electric and magnetic brain stimulation with electrically elicited short- and long latency reflexes (LLR) of hand and forearm flexor muscles has been investigated in normal subjects. In the first paradigm, the motor potential evoked in thenar muscles by transcranial stimulation was conditioned by median nerve stimulation at various conditioning-test intervals. At short intervals (electric: 5-12.5 ms, magnetic: 0-7.5 ms) facilitation occurred that corresponded to the H-reflex and at longer intervals (electric: 25-40 ms, magnetic: 22.5-35 ms) there was a facilitation corresponding to the LLR. Electric and magnetic stimulation resulted in a similar degree of facilitation. A second paradigm investigated the facilitation of the forearm flexor H-reflex by a cutaneo-muscular LLR elicited by radial superficial nerve stimulation and transcranial stimulation used separately or together. When electric and magnetic brain stimulation were compared, magnetic brain stimulation was followed by significant extrafacilitation but electric stimulation was not. This result favours an interaction between the afferent volley eliciting the LLR and transcranial magnetic stimulation most likely at supraspinal level.

The effect of task instruction on the excitability of spinal and supraspinal reflex pathways projecting to the biceps muscle

“…Previous studies using TMS have shown that corticospinal excitability increases following an imposed joint displacement at a time consistent with the passage of the afferent volley through the motor cortex (Day et al 1991;Deuschl et al 1991;Palmer and Ashby 1992;Petersen et al 1998;Lewis et al 2004). This was also evident in our results.…”

“…Previous studies have shown that the excitability of corticospinal pathways are increased during the time period approximately corresponding to the passage of the afferent volley through the motor cortex (Day et al 1991;Deuschl et al 1991;Palmer and Ashby 1992;Petersen et al 1998;Lewis et al 2004). To date, no study has used TMS to investigate the effects of task instruction on corticospinal excitability following imposed perturbations of joint posture.…”

Behaviour of short and long latency reflexes in fatigued human muscles.

“…It has been suggested that the H reflex and LLR evoked by the electrical stimulation of the median nerve at the wrist have the same origin and are both transmitted by Ia fibres, but that the LLR is routed transcortically (Deuschl et al 1985(Deuschl et al , 1991Mariorenzi, Zarola, Caramia, Paradiso & Rossini, 1991). If this is right, the different behaviour of the LLR compared to the H reflex in the sustained contraction could be explained by a stronger descending central drive induced by the electrical stimulation which elicits the reflex responses as a result from a higher central excitability at supraspinal levels during muscle fatigue.…”

Inhibitory action of forearm flexor muscle afferents on corticospinal outputs to antagonist muscles in humans

“…Several investigators have used magnetic TCS to show that the cerebral cortex is involved in long-latency reflexes. Most experiments have focused on hand muscles (Troni et al 1988;Mariorenzi et al 1991;Deuschl et al 1991), where the evidence for transcortical reflexes is strongest (Thilmann et al 1991;Rothwell et al 1991a). Deletis et al (1992) noted that stimulation of the median nerve at the wrist (at a higher intensity than that used here) could inhibit MEPs evoked by electrical TCS in forearm flexors.…”