FT Modelling and Simulation (2)

Somatosensory evoked potentials (SEPs) are produced by stimulation of a peripheral nerve and the related conduction of the volley to the somatosensory cortex.Lumbosacral SEPs may contain mixed signal components originating from synaptic potentials and volley conduction along the white mater of the spinal cord.In this study, we present an analytical model to study the frequency components in longitudinal spinal volley conduction.For simulating the body surface potential generated by pulse conduction along a single axon the model contains a source term related to the action potential and a volume conductor term related to pulse propagation along the axon.The body surface potential and its frequency spectrum are obtained by convolution of the source and volume conductor terms.The compound action potential (CAP) propagating along a neural pathway is obtained by the superposition of multiple axonal signals with individual temporal dispersion.Assuming a normal distribution of temporal dispersion, the CAP and its frequency spectrum are obtained.For mimicking the anatomy of a neural tract within the lumbosacral spinal cord we investigated three values of depth (35, 50 and 65 mm).Furthermore, we assigned three different conduction velocities (45, 60 and 75 m/s) to each depth.We assumed that temporal dispersion is reduced for higher conduction velocity and assigned three values for its standard deviation (1.0, 1.25 and 1.5 ms).For all simulation we obtained a welldefined frequency spectrum with a single peak in the frequency range of 115 to 215 Hz.The 50% amplitude bandwidth was always between 42 Hz and 396 Hz.Our simulation suggests, that signal components generated by longitudinal spinal volley conduction are mainly contained in a well-defined frequency band of approximately 40 to 400 Hz, whereas a relatively large bandwidth of 20 Hz to 3000 Hz is an accepted standard to measure signals over the spinal cord.