Microstimulation of spinal motoneurons: a model study

A computer model of a cat spinal motoneuron was developed and used to study neuronal excitation by extracellular electric fields. The model included a myelinated axon, initial segment, axon hillock, a three compartment model of the soma, and a tapering dendrite. The extracellular space was modeled as an infinite homogeneous medium, and the electrode by a point current source. Threshold profiles were generated for electrode to neuron distances between 50 μm and 1 mm. The results demonstrate that the model reproduced excitation patterns documented in single cell mapping experiments. The threshold profile along the axon had a distinct sinusoidal shape with peaks at the center of the internodes and valleys at the nodes. The depth of modulation of this sinusoidal pattern was dependent on the distance from the electrode to the neural structure. The relationship between the threshold current and the electrode to fiber distance was non-linear. The slope of the relationship was steeper for the excitation of the cell body compared to excitation of the axon