Information Technology Reference
In-Depth Information
Electrical stimulation of nuclei in the basal ganglia of the brain as a treatment for
Parkinson's disease, also known as
deep brain stimulation
(or DBS), was approved
by the US Food and Drug Administration and became commercially available in
1997 [151]. The apparatus consists of a stimulus generator implanted under the
collar bone and a subcutaneous lead connecting the stimulator to an electrode fixed
at the cranium and reaching the basal ganglia in the center of the human brain.
Following implantation, a wireless link facilitates communication with the implant
for the routine adjustment of the stimulus waveform by medical staff. In this manner,
the treatment can be tuned or optimized over time while avoiding side effects. The
neural signals emanating from the basal ganglia during DBS have been recorded and
analyzed by Dostrovsky et al. [36], Wu et al. [162], Wingeier et al. [158], and Rossi
et al. [130]. Moreover, there have been studies regarding the use of information
contained in the neural activity of the basal ganglia as a control signal or regulator
of the stimulus apparatus [106, 146, 134, 78, 39, 90, 12].
13.2 Nerve Stimulation
The simplest model of electrical nerve stimulation was introduced by Arvanitaki and
uses the passive membrane model with membrane resistance
R
m
and capacitance
C
m
[4, 95]. In this scenario, assuming the stimulus current applied across the cell
membrane is a constant
I
s
, then the change in transmembrane voltage becomes
I
s
R
m
1
e
−
t
/
R
m
C
m
V
m
(
t
)=
−
.
(13.1)
Moreover, given a threshold voltage
V
th
, then the minimum stimulus current
needed for the transmembrane voltage to reach
Δ
Δ
V
th
is found for
t
=
∞
and is called
the
rheobase current
:
I
rh
=
Δ
V
th
R
m
.
(13.2)
Also, another useful measure of stimuli is the time required to reach
Δ
V
th
when
I
s
=
2
I
rh
. This is called
chronaxy
or
chronaxie
[95, 154] and is calculated as
t
c
=
R
m
C
m
ln2
.
(13.3)
As an example, Fig. 13.1 illustrates the decay of the minimum amplitude needed for
stimulating a neuron as pulse width increases [99].
More sophisticated distributed models such as the core conductor model incor-
porate the shape of the neuron axon and conductivity of external media [24, 95].
Moreover, the shape and timing of stimuli are also influential as shown in detailed
studies by Warman, McIntyre, Grill, and others [154, 99, 100, 54]. However, the pas-
sive membrane model with appropriate effective values for
R
m
and
C
m
remains a
useful approximation for many applications [125, 74].
