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In-Depth Information
13.5 Deep Brain Stimulation
Electrical stimulation of the brain as a treatment for Parkinson's disease was first re-
ported by Benabid et al. [13] in 1987. In particular, during stereotactic neurosurgery
it was observed that stimulating the
ventral intermediate nucleus
(VIM) of the brain
with a sequence of 1-2 V 0.5 ms pulses at 100 Hz blocked symptoms of the disease.
Eventually, the lesioning procedures mentioned previously were replaced by the
implantation of electrodes connected to a pulse generator. Moreover, the physician
could tune the signal generator through a wireless link, thus adjusting the stimulus
parameters.
13.5.1 DBS Mechanism
A primary contributing factor to the inhibitory effect of DBS on the STN and
GPi is likely the release of adenosine by astrocytes as they are electrically stim-
ulated [12]. Also, the same study reports how the inhibition is likely a combination
of adenosine-related and “axonal” effects. That is, there are a number of hypothe-
ses that attempt to explain the inhibitory effect of DBS on the STN and GPi. In
particular, these are: (1) the blocking of action potentials by affecting properties
of ion conductance in the neuron membrane, (2) the preferential stimulation of ax-
ons that terminate at inhibitory synapses rather than neurons themselves, and (3)
the desynchronization of mechanisms occurring in the network as a whole. Out of
these hypotheses, desynchronization seems to be the least refuted and least under-
stood [101].
In practice, the effect of DBS on neural activity can be seen in recordings using
extracellular electrodes that have been taken from patients during surgical implan-
tation of DBS systems, as shown in Fig. 13.4. In particular, the work of Dostrovsky
et al. [36] shows how the activity of pallidal neurons displays a period of quiescence
after each stimulating pulse of DBS. Furthermore, the quiescent period increases
with respect to the DBS pulse amplitude as can be seen in Fig. 13.5. Also, as the
pulses become more dense at higher frequency stimulation, the quiescent periods
seem to overlap, thus causing the inhibitory effect. A more macroscopic view of the
effect of pulse amplitude is provided in Fig. 13.6 [162].
Figure 13.7 shows the neuron activity rate following a stimulus pulse measured
as a percentage of the activity preceding the pulse (baseline activity). As can be seen
in Fig. 13.7, neural activity is nearly 0 after the DBS pulse, but returns to normal
firing after some time (between 50 and 100 ms).
13.5.2 Apparatus
All commercially available DBS systems are currently designed and manufactured
by the Medtronic corporation. By name, the neurostimulators commonly used for
