References from this review were identified from general reading and personal discussions with investigators in the field of DBS. Only papers published in English were reviewed.
ReviewDeep brain stimulation for Parkinson's disease: disrupting the disruption
Section snippets
How does DBS work?
High-frequency stimulation via electrode contacts in deep brain nuclei appears to have the same effect as a lesion—stimulation blocks neuronal transmission in the stimulated nucleus. Yet, in theory at least, stimulation according to the variables commonly used in human beings would be expected to activate myelinated axons rather than blocking them. Stimulation of the spinal cord, for example, is intended to do just that.7
So what happens when electrical stimulation is delivered through electrode
Stimulatory effects of DBS
In some instances, DBS seems to facilitate rather than inhibit neural systems. Single stimuli delivered to the STN evoke potentials that can be recorded at the scalp.17 The elements involved have a short chronaxie, so again they are likely to be large axons. With paired stimuli or trains, the evoked potentials are increased rather than decreased. This characteristic suggests that the activated neural elements have a short refractory period, and that the increased potentials are postsynaptic. In
Inhibitory effects of DBS
Several studies have examined neuronal activity during stimulation or in the period after the end of stimulation. In rat brain slice preparations, high-frequency electrical stimulation produces extended inactivation of voltage-gated sodium and calcium channels in STN neurons.21 Studies in hippocampal slices have shown that high-frequency stimulation leads to a depolarisation block, possibly related to an extracellular increase in potassium.22 Now there are also some data from studies in human
Downstream effects
Some experiments have also examined the effects of stimulation on downstream neurons, one or more synapses away from the point of stimulation.
What happens to cortical networks after DBS?
Stimulation of deep brain structures produces profound effects in remote brain areas. The downstream effects of DBS on cortical activity have been studied with functional imaging (PET and functional MRI [fMRI]), electroencephalography (EEG), and transcranial magnetic stimulation (TMS). These are complementary methods: functional imaging studies have high spatial resolution, whereas EEG methods have high temporal resolution, and measure neuronal activity rather than the more indirect measure of
Conclusion
Surprisingly little is known about the actions of DBS in patients. The stimulus variables used in DBS in patients are likely to activate large axons. The activation of large axons at high frequency could inhibit a nucleus by the presynaptic release of inhibitory transmitters, or could cause a neural network to dysfunction either by inducing decay at downstream synapses or by disrupting or stabilising the pattern of neuronal firing in the target area. DBS may not work in the same way at each
Search strategy selection and criteria
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