title: Origins of the somatic N20 and high-frequency oscillations evoked by trigeminal stimulation in the piglets.
abstract: OBJECTIVE
  In humans, the somatic evoked potentials (SEPs) and magnetic fields (SEFs) elicited by peripheral nerve stimulation contain high-frequency oscillations (HFOs) around 600 Hz superimposed on the initial cortical response N20. Responses elicited by snout stimulation in the swine also contain similar HFOs during the rising phase of the porcine N20. This study examined the generators of the N20 and HFOs in the swine.
  
  
  METHODS
  We recorded intracortical SEPs and multi-unit activities in the sulcal area of the primary somatosensory cortex (SI) simultaneously with SEFs. The laminar profiles of the potential and current-source-density (CSD) were analyzed.
  
  
  RESULTS
  The CSD analysis revealed that the N20 was produced by two dipolar generators, both directed toward the cortical surface. After the arrival of the initial thalamocortical volley in layer IV, the sink of the first generator shifted toward shallower layers II-III with a velocity of 0.109+/-0.038 m/s (mean+/-SD). The sink of the second generator moved to layer V. The initial thalamocortical axonal component of the HFO was produced by repolarizing current with the sink in layer IV. The CSD laminar profile of the postsynaptic component was very similar to the profile of intracortical N20. The current sink within each cycle of HFO propagated upward with a velocity of 0.633+/-0.189 m/s, indicating backpropagation.
  
  
  CONCLUSIONS
  We propose that the N20 is generated by two sets of excitatory neurons which also produce the HFOs. Although the loci of synaptic inputs are unknown, these neurons appear to fire initially in the soma and produce backpropagating spikes toward distal apical dendrites.
  
  
  CONCLUSIONS
  These conclusions relate the N20 to the HFO and provide a new explanation of how the current underlying the N20 is invariantly directed toward superficial layers across species.
authors.affiliation: Department of Neurology (MSC1-5620), University of New Mexico School of Medicine, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Ikeda
authors.forename: Yaozhi
authors.initials: Y
authors.lastname: Wang
authors.forename: Yoshio C
authors.initials: YC
authors.lastname: Okada
mesh_terms: D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D001683:Biological Clocks / Q000502:physiology*; D004558:Electric Stimulation / Q000379:methods; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D013552:Swine; D014276:Trigeminal Nerve / Q000502:physiology*