title: Startle stimuli exert opposite effects on human cortical and spinal motor system excitability in leg muscles.
abstract: Increased excitability of the spinal motor system has been observed after loud and unexpected acoustic stimuli (AS) preceding H-reflexes. The paradigm has been proposed as an electrophysiological marker of reticulospinal tract activity in humans. The brainstem reticular formation also maintains dense anatomical interconnections with the cortical motor system. When a startling AS is delivered, prior to transcranial magnetic stimulation (TMS), the AS produces a suppression of motor evoked potential (MEP) amplitude in hand and arm muscles of healthy subjects. Here we analyzed the conditioning effect of a startling AS on MEP amplitude evoked by TMS to the primary motor leg area. Ten healthy volunteers participated in two experiments that used a conditioning-test paradigm. In the first experiment, a startling AS preceded a suprathreshold transcranial test stimulus. The interstimulus interval (ISI) varied between 20 to 160 ms. When given alone, the test stimulus evoked a MEP amplitude of approximately 0.5 mV in the slightly preinervated soleus muscle (SOL). In the second experiment, the startling AS was used to condition the size of the H-reflex in SOL muscle. Mean MEP amplitude was calculated for each ISI. The conditioning AS suppressed MEP amplitude at ISIs of 30-80 ms. By contrast, H-reflex amplitude was augmented at ISIs of 100-200 ms. In conclusions, acoustic stimulation exerts opposite and ISI-specific effects on the amplitude of MEPs and H-reflex in the SOL muscle, indicating different mechanism of auditory-to-motor interactions at cortical and spinal level of motor system.
authors.affiliation: Clinic of Neurology, Christian-Albrechts-University of Kiel, Germany. tihoilic@gmail.com
authors.forename: T V
authors.initials: TV
authors.lastname: Ilic
authors.forename: M
authors.initials: M
authors.lastname: Pötter-Nerger
authors.forename: I
authors.initials: I
authors.lastname: Holler
authors.forename: H R
authors.initials: HR
authors.lastname: Siebner
authors.forename: N V
authors.initials: NV
authors.lastname: Ilic
authors.forename: G
authors.initials: G
authors.lastname: Deuschl
authors.forename: J
authors.initials: J
authors.lastname: Volkmann
mesh_terms: D000161:Acoustic Stimulation; D000328:Adult; D000704:Analysis of Variance; D002540:Cerebral Cortex / Q000502:physiology*; D004576:Electromyography; D019054:Evoked Potentials, Motor; D005260:Female; D005858:Germany; D006181:H-Reflex; D006801:Humans; D035002:Lower Extremity; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation*; D009433:Neural Inhibition; D013216:Reflex, Startle; D013127:Spinal Nerves / Q000502:physiology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation