Abstract
Previous research suggested that anterior-posterior (AP) directed currents induced by TMS in motor cortex (M1) activate interneuron circuits different from those activated by posterior-anterior currents (PA). The present experiments provide evidence that pulse duration also determines the activation of specific interneuron circuits. To use single motor unit (SMU) recordings to confirm the difference in onset latencies of motor-evoked potentials (MEPs) evoked by different current directions and pulse durations: AP30, AP120, PA30 and PA120. To test whether the amplitude of the MEPs is differentially influenced by somatosensory inputs from the hand (short-latency afferent inhibition, SAI), and examine the sensitivity of SAI to changes in cerebellar excitability produced by direct current stimulation (tDCSCb). Surface electromyograms and SMUs were recorded from the first dorsal interosseous muscle. SAI was tested with an electrical stimulus to median or digital nerves ~20-25 ms prior to TMS delivered over the M1 hand area via a controllable pulse parameter TMS (cTMS) device. SAI was also tested during the application of anodal or sham tDCSCb. Because TMS pulse specificity is greatest at low stimulus intensities, most experiments were conducted with weak voluntary contraction to reduce stimulus threshold. AP30 currents recruited the longest latency SMU and surface MEP responses. During contraction SAI was greater for AP30 responses versus all other pulses. Online anodal tDCSCb reduced SAI for the AP30 currents only. AP30 currents activate an interneuron circuit with functional properties different from those activated by other pulse types. Pulse duration and current direction determine what is activated in M1 with TMS.
Highlights
A single Transcranial magnetic stimulation (TMS) pulse over primary motor cortex (M1) activates the axons of excitatory synaptic inputs to corticospinal neurons (CSNs), which initiates descending activity in the corticospinal tract and eventually produces a motor evoked potential (MEP) in contralateral muscles [1]
It is well known that the orientation of the current induced across the central sulcus influences the activation of the Abbreviations: AP, anterior–posterior; controllable pulse parameter TMS (cTMS), controllable pulse parameter transcranial magnetic stimulation device; CSN, corticospinal neuron; MEP, motor evoked potential; posterior–anterior currents (PA), posterior–anterior; tDCSCb, transcranial cerebellar direct current stimulation
Ni et al evaluated the effects of somatosensory inputs from the hand on MEPs evoked by different current directions, and found suppressed more by peripheral nerve inputs (SAI) suppressed I3 waves recruited by PA currents more readily than I3 waves recruited by AP currents [8]
Summary
A single TMS pulse over primary motor cortex (M1) activates the axons of excitatory synaptic inputs to corticospinal neurons (CSNs), which initiates descending activity in the corticospinal tract and eventually produces a motor evoked potential (MEP) in contralateral muscles [1]. Ni et al evaluated the effects of somatosensory inputs from the hand on MEPs (short-latency afferent inhibition, SAI) evoked by different current directions, and found SAI suppressed I3 waves recruited by PA currents more readily than I3 waves recruited by AP currents [8]. They concluded that the late I-waves activated by PA and AP current directions were generated by different excitatory inputs. Pulse duration and current direction determine what is activated in M1 with TMS
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