PubMed · 14486107
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W PFAU. 1962. [Chronaxy and voice classification].. https://pubmed.ncbi.nlm.nih.gov/14486107/
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OBJECTIVE: The neuronal elements mediating the effects of deep brain stimulation (DBS) are unknown. The objective was to determine the strength-duration properties of the neuronal elements that mediate paresthesias evoked by thalamic microstimulation. METHODS: The strength-duration properties of the neuronal elements causing paresthesias were measured using intraoperative microstimulation of the human thalamus. The sample included both concordant (reported in the same region as the mapped sensory receptive fields) and discordant paresthesias (reported in a region different than the mapped sensory receptive fields). RESULTS: There were no significant differences between the chronaxies of concordant and discordant paresthesias. There was no significant correlation between chronaxie and rheobase for concordant paresthesias, but a strong negative correlation existed for discordant paresthesias. CONCLUSIONS: Chronaxies did not distinguish the neuronal elements mediating concordant and discordant paresthesias, but correlations between chronaxie and rheobase suggest that concordant paresthesias were produced by activation of local cells while discordant paresthesias were caused by activation of axons of passage. SIGNIFICANCE: The similarity between the strength-duration properties of paresthesias evoked by thalamic stimulation, tremor reduction evoked by thalamic DBS, and EMG responses to thalamic DBS does not mean that these effects are caused by the same neural elements.
The immediate motor effects of stimulation through electrodes chronically implanted in the globus pallidus internus (GPI) were studied in 9 subjects with Parkinson's disease. Single stimuli (at >>0.4 Hz) produced short latency facilitation of voluntarily activated contralateral muscles in all subjects. The latency and distribution of the facilitation, its probably monosynaptic nature, and the short chronaxie and refractory period of the activated neural elements suggest that the facilitation results from the direct excitation of the fast conducting corticospinal pathway. The facilitation of motoneurons followed high frequency (e.g. 200 Hz) stimulation without decrement and occurred at stimulus intensities well below those required to produce a visible muscle contraction. We conclude that, while there may be other effects, GPI stimulation through electrodes may activate the corticospinal tract, even when the stimuli are below the threshold for a visible muscle contraction, and that continuous stimulation may do so continuously. This may be an unwanted side effect, but possible therapeutic actions are considered. The reproducible short latency facilitation enabled us to estimate current spread from the quadripolar electrodes used for deep brain stimulation. When the current is sufficient to excite large myelinated fibers near one of the quadripolar electrodes, an additional 1-mA current will activate similar fibers at an additional distance of 1.8 mm with bipolar stimulation and at a distance of 5.7 mm with monopolar stimulation.
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