Single neuron electrophysiology of transcranial magnetic stimulation. I. Passive responses

We employed the same TMS modeling system as [1]. It is comprised of a multicompartment Layer 5 pyramidal neuron from cat visual cortex [3] and incorporates a modeled TMS stimulus. To determine the source of the orientation selectivity of neuronal response, we removed active membrane channels from the model cell and recorded the passive membrane response along a piece-wise continuous axis spanning the length of the neuron.

An orthodromic TMS stimulus evoked a distance-related bimodal depolarization in the basal dendrites (peak: proximal 26 mV, distal 18 mV) that peaked within 0.5 ms of stimulus onset. In the apical dendrites, the stimulus induced a small depolarization (2-3 mV) proximal to the soma. Depolarization converted to hyperpolarization within 250 um of the soma and increased to become larger than 30 mV at the distal dendritic tip (1000 um). The spatial profile of the membrane response was inverted (i.e. depolarization became hyperpolarization and hyperpolarization became depolarization) following an antidromic E-field stimulus.

In response to an orthodromically directed E-field, current driven into the basal dendrites dissipates rapidly into this highly branched structure and thereby prevents action potential generation at the soma. Alternatively, during an antidromic stimulus, the basal dendrites are rapidly hyperpolarized and current driven into the distal apical dendrites rapidly dissipates. However sufficient depolarization is evoked at the soma to initiate cell discharge. This behavior is likely controlled by the primary apical dendrite which exhibits increased resistance for the first 800 um from the soma.