The effects of transcranial magnetic stimulation on vibratory-induced presynaptic inhibition of the soleus H reflex

Abstract

A single-pulse transcranial magnetic stimulus (TMS) may induce contraction in many muscles of the body at the same time. This is specially the case when using the double-cone coil to obtain the motor evoked potentials in leg muscles. Even if intensity is kept below threshold for the soleus muscle, TMS induces facilitation of the soleus H reflex that is separated into two phases: the first, peaking at 10-20 ms and the second, peaking at 70-90 ms. We investigated the possibility that TMS-induced facilitation of the H reflex was related, at least in part, to the reafferentation volley reaching the alpha motoneuron after synchronized contraction of other muscles in the body. To test this hypothesis, we examined the effects of vibration on the TMS-induced facilitation of the soleus H reflex. As expected, vibration applied over the triceps tendon caused a significant reduction in H reflex amplitude: 42.4 ± 6.4 % of control values. When conditioned by TMS at intervals corresponding to the first phase, the H reflex was facilitated to the same extent in both conditions: with and without vibration. However, at intervals corresponding to the second facilitation phase, there was a significantly reduced facilitation with vibration. These differential effects of vibration on the two phases of the TMS-induced facilitation of the H reflex indicate a different mechanism for each facilitation phase. The first phase could result from direct corticospinal excitatory input, while the second phase might depend on inputs via Ia afferents from heteronymous muscles.

Effects of postural and voluntary muscle contraction on modulation of the soleus H reflex by transcranial magnetic stimulation

“…The post hoc analysis showed a significant facilitation at ISIs 10 and 20 ms, no effect at ISIs 40-60 ms, and again significant facilitation at ISIs 70-100 ms. According to these findings, which were consistent with those of previous reports (Goulart and Valls-Solé 2001;Serranova et al 2008;Benito Penalva et al 2010;Guzmán-López et al 2012), we defined the baseline H reflex modulation pattern as composed by three phases: F1, noF, and F2. For each condition, we calculated the percentage change in the size of the H reflex at each of these three phases (F1 for ISIs 10-20 ms, noF for ISIs 40-60 ms, and F2 for ISIs 70-100 ms).…”

“…This is likely what generates for the most part the facilitation of the H reflex seen in F1 at rest. However, the fact that this facilitation phase is not inhibited during vibration raised the possibility of a transient effect of TMS over the presynaptic inhibitory mechanisms, contributing also to F1 (Valls-Solé et al 1994;Iles 1996;Guzmán-López et al 2012). Contrary to F1, F2 is inhibited by vibration, which suggests that this phase is conveyed via inputs that are affected by presynaptic inhibition.…”

“…This may reveal inhibitory effects over the reafferentation, assumedly responsible for F2 (Guzmán-López et al 2012).…”

“…These effects of muscle contraction at very short intervals may be explained by the action of reciprocal innervation patterns that may prepare the spinal machinery for fast changes in movement direction for functional tasks (Geertsen et al 2010;Taube et al 2011). However, these explanations may not apply to the H reflex modulation by TMS at longer intervals, where other mechanisms of reafferentation mediated throughout Ia afferents may play a role (Serranova et al 2008;Costa et al 2011;Guzmán-López et al 2012;Roy et al 2014).…”

Physiological Basis of Transcranial Magnetic Stimulation

“…Furthermore, the power of this coil and the depth to which its pulses can penetrate is 70 % greater in comparison with these same parameters for a circular coil because of the increased coupling of the magnetic fields of the two loops that are angled inward in the double cone coil. This type of coil has been used in protocols, for example, in which the motor representation of the lower limbs in the motor cortex (Stokić et al [26]; Guzman-Lopez et al [27]), the corticospinal axons at the cervicomedullary junction (Taylor and Gandevia [28]), and the cerebellum (Ugawa et al [29]; Werhahn et al [30]; Pinto and Chen [31]) are stimulated. This coil is typically used when other coil designs fail to evoke responses from target muscles or when an experimenter is attempting to access deeper structures.…”