Proceedings of The Physiological Society

Future Physiology (Leeds, UK) (2017) Proc Physiol Soc 39, PC13

Poster Communications

The reliability of the TMS-conditioned Hoffmann reflex in forearm muscles

A. Capozio1

1. Biological Sciences, University of Leeds, Leeds, United Kingdom.

  • Fig. 1. Raw recordings of Mmax from across 3 test days. Each trace represents the average of 8 trials.

  • Fig. 2. Intra-session and inter-session variability of the TMS motor threshold in a representative subject. Each symbol represents one recording. Red stars represent the average value of the 6 recordings.

A transcranial magnetic stimulation (TMS) pulse can facilitate the occurrence of the monosynaptic reflex (H-reflex) elicited by peripheral nerve stimulation (PNS) if given at the right interstimulus interval (ISI) (1). This phenomenon may represent a mechanism by which spinal cord circuits are modulated by corticospinal descending activity (2). TMS-conditioned H-reflexes show abnormal patterns of activity in the lower limbs of spinal cord injured patients but can be restored by gait training (3). Despite the potential clinical and diagnostic application, the between-session reliability of TMS-conditioned monosynaptic reflex in forearm muscles has never been estimated. Aim of this project is therefore to record physiological parameters including TMS motor thresholds, H-reflex thresholds, maximum motor response (Mmax) and TMS-conditioned H-reflex in a group of healthy volunteers over the course of three sessions. Electromyographic (EMG) activity was recorded from Flexor Carpi Radialis muscle. Mmax was elicited by stimulating the median nerve at the cubital fossa with increasingly strong 1ms electrical pulses, until reaching a plateau in the amplitude of the recorded EMG (4). Following this, the stimulation intensity was adjusted to produce H-reflexes of 10% Mmax amplitude. Magnetic stimulation was delivered to the left motor cortex and the Motor evoked potentials (MEPs) recorded from the FCR muscle. The motor threshold was defined as the minimal stimulation intensity which elicit MEPs with an amplitude of 50 μV in at least 50% of trials (5). Finally, magnetic and electric pulses were given in conjunction at ISI ranging from -10 ms (PNS first) to 10 ms (TMS first). In the piloting phase, we observed a reliable and consistent facilitation of the H-reflex (increase in amplitude relative to baseline H-reflex values) when a TMS pulse was given 10 ms prior to median nerve stimulation. This ISI was therefore chosen in all following sessions. Data from a representative subject (Fig. 1) show how Mmax ranged from 10.21 to 10.31 mV over three sessions. Similarly, TMS motor threshold was stable across sessions, in the range of 0.5-1 mV with an intensity of 62% of maximum stimulator output (Fig. 2). Delivering a magnetic pulse 10 ms prior median nerve stimulation resulted in a facilitation of the H-reflex (pk-pk increase of 0.48 mV). We aim to find high degree of reliability (ICC>0.70) for all the estimated parameters. This study can potentially lead to the implementation of protocols to provide diagnostic information in people with motor disorders and to evaluate the effects of therapeutic intervention on cortical and spinal circuits.

Where applicable, experiments conform with Society ethical requirements