title: The Frequency-Dependent Neuronal Length Constant in Transcranial Magnetic Stimulation.
abstract: BACKGROUND
  The behavior of the dendritic or axonal membrane voltage due to transcranial magnetic stimulation (TMS) is often modeled with the one-dimensional cable equation. For the cable equation, a length constant λ0 is defined; λ0 describes the axial decay of the membrane voltage in the case of constant applied electric field. In TMS, however, the induced electric field waveform is typically a segment of a sinusoidal wave, with characteristic frequencies of the order of several kHz.
  
  
  OBJECTIVE
  To show that the high frequency content of the stimulation pulse causes deviations in the spatial profile of the membrane voltage as compared to the steady state.
  
  
  METHODS
  We derive the cable equation in complex form utilizing the complex frequency-dependent representation of the membrane conductivity. In addition, we define an effective length constant λeff, which governs the spatial decay of the membrane voltage. We model the behavior of a dendrite in an applied electric field oscillating at 3.9 kHz with the complex cable equation and by solving the traditional cable equation numerically.
  
  
  RESULTS
  The effective length constant decreases as a function of frequency. For a model dendrite or axon, for which λ0 = 1.5 mm, the effective length constant at 3.9 kHz is decreased by a factor 10 to 0.13 mm.
  
  
  CONCLUSIONS
  The frequency dependency of the neuronal length constant has to be taken into account when predicting the spatial behavior of the membrane voltage as a response to TMS.
authors.affiliation: Department of Neuroscience and Biomedical Engineering, Aalto University School of Science Espoo, Finland.
authors.forename: Risto J
authors.initials: RJ
authors.lastname: Ilmoniemi
authors.affiliation: Department of Neuroscience and Biomedical Engineering, Aalto University School of ScienceEspoo, Finland; A.I.R Advanced Analytics for IoT, Comptel CorporationHelsinki, Finland.
authors.forename: Hanna
authors.initials: H
authors.lastname: Mäki
authors.affiliation: Department of Neuroscience and Biomedical Engineering, Aalto University School of Science Espoo, Finland.
authors.forename: Jukka
authors.initials: J
authors.lastname: Saari
authors.affiliation: Faculdade de Ciências, Institute of Biophysics and Biomedical Engineering, Universidade de Lisboa Lisbon, Portugal.
authors.forename: Ricardo
authors.initials: R
authors.lastname: Salvador
authors.affiliation: Faculdade de Ciências, Institute of Biophysics and Biomedical Engineering, Universidade de Lisboa Lisbon, Portugal.
authors.forename: Pedro C
authors.initials: PC
authors.lastname: Miranda
keywords: TMS; cable equation; length constant; membrane potential; transcranial magnetic stimulation