Download Modulation of visceral function by selective stimulation of the left

717
Exp Physiol 89.6 pp 717–725
Experimental Physiology
Modulation of visceral function by selective
stimulation of the left vagus nerve in dogs
J. Rozman1 and M. Bunc2
1
2
ITIS d. o. o. Ljubljana, Center for Implantable Technology and Sensors, Lepi pot 11, Republic of Slovenia
University of Ljubljana, School of Medicine, Institute of Pathophysiology, Zaloška 4, 1000 Ljubljana, Republic of Slovenia
The superficial regions of the left vagus nerves of a dog were selectively stimulated with
39-electrode spiral cuffs having 13 circumferential groups of three electrodes (GTE) to
modulate the function of the innervated internal organs and glands. Under general
anaesthesia, the cuffs were chronically implanted around the nerve in the neck in two adult
Beagle dogs and remained viable for 16 months. The regions were stimulated with biphasic,
rectangular current pulses (2 mA, 200 µs, 20 Hz) delivered to the group of GTE lying close
to the region innervating the specific internal organs or glands. The results showed that
specific electrode configurations had actions on the heart (GTE 9), lungs (GTE 4) and pressure
in the urinary bladder (GTE 1). It was also shown that GTE no. 10 significantly modified the
endocrine function of the pancreas. The results of this study clearly demonstrate that internal
organs and glands can be selectively stimulated via the selective stimulation of innervating
superficial regions of the autonomous peripheral nerve.
(Received 18 May 2004; accepted after revision 27 August 2004; first published online 13 September 2004)
Corresponding author J. Rozman: ITIS d.o.o. Ljubljana, Center for Implantable Technology and Sensors, Lepi pot 11,
1000 Ljubljana, Republic of Slovenia. Email: [email protected]
The left vagus nerve is an important route of information
into the CNS (Berthoud & Neuhuber, 2000). One of its
main functions is to monitor and control the activity of
the internal organs and glands such as the heart, lungs,
stomach, bladder and pancreas. Accordingly, there is a
revival of interest in the influence of the vagal nerve fibres
on these organs (Dixon et al. 1980; Roy et al. 1984; Woods
& Porte, 1987; Berthoud et al. 1990; Schemann & Grundy,
1992; Levy, 1997).
The principal functions of the heart are regulated
by the sympathetic and parasympathetic divisions of
the autonomic nervous system (Levy et al. 1993). The
sympathetic nerves to the heart are facilitatory, whereas
the parasympathetic (vagus) nerves are inhibitory (Mace
& Levy, 1983; Levy, 1984). The vagus nerve can exert
beat-by-beat control of cardiac function, while the onset
and decay of the sympathetic effects are much more
gradual (Levy et al. 1993).
Yang & Levy (1992) have determined the influence of
differences in the time of initiation of sympathetic and
vagal stimulation on the cardiac autonomic interactions
in anaesthetized dogs. Sympathetic stimulation alone
increased the heart rate, vagal stimulation alone decreased
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the heart rate, and combined stimulation also decreased
the heart rate (the vagal effects predominated). The actual
pace set by the sino-atrial (SA) node, however, depends on
the net effect of these antagonistic influences. It was shown
by Dexter et al. (1992) that stimulation at precise times
in successive cardiac cycles can elicit sinus arrhythmias.
Moreover, Matheny & Shaar (1997) have suggested the
technique of vagus nerve stimulation as a means of
temporarily slowing down or arresting the heart during
surgery. To increase the trial cycle length, Carlson et al.
(1992) selectively stimulated parasympathetic nerve fibres
that innervate the SA node.
The activity of the autonomic innervation of the heart
is coordinated by the cardiac control centre in the medulla
oblongata which is, in turn, affected by higher brain
areas and by sensory feedback from baroreceptors in the
aorta and carotid arteries. Sensory nerve activity from the
baroreceptors ascends via the vagus and glassopharyngeal
nerves to the medulla. The left vagus nerve innervates
the atrioventricular (AV) node. There can, however, be
significant overlap in the anatomical distribution. Recently
the central nervous system mediation of the baroreflex
and the chronotropic responsiveness of the heart to vagal
DOI: 10.1113/expphysiol.2004.027953
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J. Rozman and M. Bunc
efferent activity were independently assessed by recording
the responses to electrical stimulation of the left vagus
nerve (Ma et al. 2002). Reduced heart rate variability and
baroreflex sensitivity are markers of cardiac vagal control
and are used to assess activity in the autonomic nervous
system (Levy & Martin, 1981; Huikuri et al. 1999). Both are
powerful and independent indicators of adverse prognosis
in patients with cardiac failure or myocardial infarction
(Xenopoulos et al. 1996).
Therefore, by using the method of selective stimulation
of the left vagus nerve, heart rate variability can be altered
towards normal and it is possible to elicit a variety of
chronotropic and inotropic cardiac responses with or
without alterations in systemic arterial pressure (Armour
& Randall, 1985; Seidel et al. 1997; Setty et al. 1998; Sevre
& Rostrup, 2001).
The nerve fibres of the respiratory muscles arise from
the vagus nerve and sympathetic chains. Dixon et al. (1980)
have studied the distribution of the afferent and efferent
nerves to the respiratory muscles in dogs anaesthetized
with chloralose.
The motor neurones that stimulate the respiratory
muscles are controlled by two descending pathways
controlling voluntary and involuntary breathing. The
unconscious rhythmic control of breathing is influenced
by input from the central chemoreceptors in the medulla
and the peripheral chemoreceptors that respond to
changes in the arterial partial pressure of CO2 (PaCO2 ),
pH and the arterial partial pressure of O2 (PaO2 ). The
peripheral chemoreceptors include the aortic bodies,
located around the aortic arch, and the carotid bodies,
located in each common carotid artery at the point where
it branches into the internal and external carotid arteries.
The aortic bodies send sensory information to the medulla
in the vagus nerve (X); the carotid bodies stimulate sensory
fibres in the glossopharingeal nerve (IX). Inspiration and
expiration are produced by contraction and relaxation
of the skeletal muscles in response to activity in the
somatic motor neurones in the spinal cord. Their activity
is controlled, in turn, by descending tracts from neurones
in the respiratory control centres in the medulla and from
neurones in the cerebral cortex. The rhythmicity centre,
which controls automatic breathing and is located in the
medulla, consists of interacting pools of neurones that
fire either during inspiration (I neurones) or expiration
(E neurones). The activity of the I and E neurones varies
in a reciprocal way to produce a rhythmic pattern of
breathing.
Therefore, by using the method of selective stimulation
of the corresponding superficial regions of the left vagus
nerve, the information sent by the aortic bodies could
be modified, and thus involuntary breathing could be
externally controlled (Matran et al. 1991).
The extrinsic motor innervation of the pancreas is
provided by both the sympathetic and the parasympathetic
Exp Physiol 89.6 pp 717–725
nervous system (Ahrén, 2000). A significant proportion
of pancreatic neurones receives excitatory synaptic input
from the vagal preganglionic axons (Miller, 1981).
Stimulation of the vagal preganglionic fibres can directly
affect pancreatic endocrine and exocrine secretion
(Bergman & Miller, 1973; Berthoud & Powley, 1987).
Ahrén & Taborsky (1986) studied the mechanisms of
vagal nerve stimulation of glucagon and insulin secretion
in halothane-anaesthetized dogs and concluded that
vagal nerve stimulation produces a moderate increase
of glucagon secretion and a marked increase of insulin
secretion.
The present study addresses the hypothesis that a certain
superficial region of the peripheral autonomic nerve is
composed mainly of fibres innervating a single internal
organ or gland. Our study was aimed at demonstrating
that stimulation of the autonomic nerve at one single site
with nearby stimulating electrodes within the installed
multielectrode spiral nerve cuff can potentially be used as a
method for external modulation of function of the internal
organs and glands innervated by the corresponding
selectively stimulated superficial region.
Methods
Multi-electrode nerve cuff
A cuff was made by bonding two 0.1 mm thick silicone
sheets together. One sheet, stretched and fixed in that
position, was covered with a layer of adhesive (MED-1511,
NuSil, Carpinteria, CA, USA). A second unstretched
sheet was placed on the adhesive and the composite was
compressed to a thickness of 0.3 mm until the whole curing
process was completed. When released, the composite
curled into a spiral tube as the stretched sheet contracted
to its natural length. The diameter of the cuff was related to
the amount of stretch; the greater the stretch, the smaller
the diameter. Thirty-nine rectangular electrodes with a
width of 0.6 mm and length of 1.5 mm were made of 50 µm
thick platinum ribbon (99.99% purity) and connected to
lead wires (AS 631, Cooner Wire, Chatsworth, CA, USA)
that were mounted on a third silicone sheet with a thickness
of 0.1 mm. They were arranged in three parallel groups
each containing 13 electrodes at a distance of 0.5 mm.
The distance between the spiral groups was 6 mm. Each
electrode marked with the same number within each of
the three parallel spiral groups had the same position.
Accordingly, 13 groups of three electrodes (GTEs) in the
same line in a longitudinal direction were formed. All
electrodes of the central and two outer groups were then
connected to the corresponding lead wires. The silicone
sheet with the arranged electrodes was then bonded on
the inner side of the mechanically opened cuff. The length
of the cuff was optimized so that the surface of the nerve
covered by the spiral cuff would be as small as possible to
prevent damage associated with a reduced blood supply
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Exp Physiol 89.6 pp 717–725
Left vagus nerve modulates visceral function
and excessive mechanical trauma of the nerve. Therefore,
the cuff, with an inner diameter of 2.5 mm, was trimmed
to a length of 18 mm as shown in the inset in the top right
corner of Fig. 1.
Finally, all lead wires were connected to a special
common connector to be implanted within the lateral
subcutaneous tissue of the neck for the time between the
experimental sessions. To connect the common connector
to the outputs of the stimulator a special cable was
developed. At one end of the cable to be connected to
the common connector was a switching module designed
to fit its pins. The switching module permitted a certain
GTE to be connected to the stimulator individually or in
combination with other GTEs. In the switching module
a selective ‘quasi-bipolar’ stimulating configuration was
produced. Namely, the two outer electrodes of a certain
GTE to be connected as anodes to one end of a stimulator
were short-circuited, while the corresponding central
electrode was to be connected as a cathode to the
other end. Furthermore, the common connector was
designed to permit simple and reliable multiple use.
This is very important because the common connector
was designed to be reimplanted several times between
individual experiments without any damage.
Surgical implantation
The experiment was performed on two Beagle dogs. Under
fully aseptic conditions gas-sterilized (ethylene oxide)
cuffs were implanted according to the following protocol
approved by the ethics committee at the Veterinary
Administration of the Republic of Slovenia, Ministry
of Agriculture, Forestry and Food. The animals were
premedicated with medetomidine, 40 µg kg−1 i.m.
(Domitor, Orion Corp., Espoo, Finland) and
methadone, 0.2 mg kg−1 s.c. (Heptanon, Pliva, Zagreb,
Croatia). Induction was performed with propofol, 1.0–
2.0 mg kg−1 i.v. (Diprivan, Zeneca Pharmaceuticals Ltd,
Macclesfield, UK). General anaesthesia was maintained
with isoflurane, 0.8–1.5 vol.% (Forane, Abbott
Laboratories Inc., Abbott Park, IL, USA) in 100%
O2 . When necessary, during surgery analgesia was
sustained with ketamine, 0.5–2.0 mg kg−1 i.v. (Ketamine,
Veyx-Pharma GmbH,
Schwarzenborn,
Germany).
Antibiotics (cefazolin, 20 mg kg−1 i.v.; Cefamezin, Krka,
Novo Mesto, Republic of Slovenia) were administered
perioperatively. The room temperature was kept between
23.4 and 24.4◦ C and the temperature of the skin of the
neck was also continuously monitored. According to our
model, the cuff was installed around the left vagus nerve in
Figure 1. Radiograph of the implanted 39-electrode spiral cuff, showing its position on the left vagus
nerve of a dog
Inset in the top right corner shows the 39-electrode spiral cuff (1) with the subcutaneous common connector
(2) and the switch module (3). Inset in the lower left corner shows a model of the reconstructed cross-sectional
geometry of the left vagus nerve, within the spiral cuff, with the indicated GTEs no. 9, no. 4, no. 1 and no. 10,
close to the superficial regions innervating the heart (6), lung (5), bladder (4) and pancreas (7).
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the neck as shown in Fig. 1. The leads of the implanted cuff
were routed and fixated to the corresponding common
connector under the skin at the lateral side of the neck.
Finally, the incision was closed and the animal allowed
to recover. Analgesia during the early recovery period
was provided by methadone, 0.3–0.5 mg kg−1 s.c. three
times daily. Tramadol, 8.0 mg kg−1 s.c. three times daily
(Tramal, Grünenthal GmbH, Stolberg, Germany) was
administered for a further 2 days.
Selective stimulation of the left vagus nerve
Two months after implantation, the first stimulation
session was performed. However, to obtain initial data
about the level of glucagon, insulin and C-peptide
in the blood, samples from the femoral artery were
drawn before any experimental activities. During
the stimulation sessions, the dogs were anaesthetized
according to the aforementioned procedure. After
taking the subcutaneously implanted common connector
belonging to the cuff out of the body, it was thoroughly
cleaned and dried. The common connector was then
placed on the cleaned skin close to the surgical site. Then
the common connector and the surgical site were covered
with self-adhesive sterile surgical foil during the entire
experiment. The connection of the common connector
to the outputs of the stimulator was made simply by
perforating the self-adhesive sterile surgical foil with the
pins of the switch module and inserting them in the
common connector. It was crucial for the stimulations
that the connection permitted reliable mechanical and
galvanical connections. Moreover, the common connector
and the surgical site were insulated from the ambient
atmosphere.
The switches of the switching module were alternately
turned on so as to connect the electrodes in a certain
GTE to the stimulator. Each of the 13 GTEs was then
denoted by a consecutive number, as shown in the
inset in the bottom left corner of Fig. 1, which is a
geometric model of the cuff fitted on the left vagus
nerve. However, considering the dimensions of the
nerve and the cuff it was expected that certain GTEs
remained out of contact with the superficial regions of the
nerve.
The relative positions of GTEs closest to the superficial
regions of the nerve innervating the heart, the lungs
and the bladder were determined experimentally. This
was done by delivering stimulating pulses quasi-bipolarly
to all 13 GTEs within the cuff. The superficial regions
on the circumference of the nerve innervating the
aforementioned organs that were in contact with the
corresponding GTE were selectively stimulated using
rectangular, biphasic, charge-balanced current pulses with
an intensity of 1.3–2.0 mA and a frequency of 20 Hz. The
GTEs that elicited the largest measurable response in one
Exp Physiol 89.6 pp 717–725
of the aforementioned organs were indicated as relevant
to the investigation.
Accordingly, when stimuli were delivered to the GTE
that was in contact with the superficial region of the
vagus nerve innervating the heart, the heart rate began
to fall. Thus the defined superficial region was denoted
as region 6, shown in the inset in the bottom left corner
of Fig. 1. The current amplitude was then adjusted so
that a continuous cessation of heart rate was obtained.
This GTE was then indicated as relevant for selective
stimulation of the heart. To quantify the changes in
heart rate produced by selective stimulation of the
corresponding superficial region of the left vagus nerve the
first, second and third derivative of ECG were recorded.
In our study, this chronotropic response to left vagus
nerve stimulation was evaluated simply by recording the
ECG using stainless-steel hypodermic needles inserted
percutaneously in the animal’s limbs. The ECG signals
were delivered to a custom-designed differential amplifier
and to a DigiPack 1200 high-performance data acquisition
system connected to a PC, featuring a Digidata 1200A
high-performance data acquisition system designed and
manufactured by Axon Instruments and high-speed data
acquisition software. Furthermore, to quantify the changes
in baroreflex sensitivity, which is another marker of cardiac
vagal control, arterial and vein (CVP) blood pressures were
measured using a disposable pressure transducer system
for invasive blood pressure monitoring (model DPT6000; Smiths Medical Deutschland GmbH, Kirchseeon,
Germany; company section: pvb-Critical Care).
Furthermore, when stimuli were delivered to the GTE
that was in contact with the superficial region of the nerve
innervating the respiratory muscles, the breathing rate
was reduced and its character was altered. Thus defined,
the superficial region was denoted as region 5, shown
in the inset in the bottom left of Fig. 1. The current
amplitude was then adjusted so that a continuous cessation
of breathing was obtained. This GTE was then indicated as
relevant for selective stimulation of the lungs. To validate
the changes in the rhythm and character of breathing
produced by selective stimulation of the corresponding
superficial region of the left vagal nerve, variations of
the circumference of the thorax were measured. This was
done using a metal belt instrumented with a customdesigned force transducer, which was mounted around the
chest. The signals obtained from the force transducer were
delivered to a custom-designed bridge amplifier and to the
data acquisition system connected to the PC.
When stimuli were delivered to the GTE that was in
contact with the superficial region of the vagus nerve
innervating the bladder, the pressure in the bladder began
to fall. Thus defined, the superficial region was denoted
as region 4, shown in the inset in the bottom left of
Fig. 1. The current amplitude was then adjusted so that
maximum reduction in pressure was obtained. This GTE
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Exp Physiol 89.6 pp 717–725
Left vagus nerve modulates visceral function
was then indicated as relevant to selective stimulation of
the bladder. To measure the changes of pressure in the
bladder a special catheter inserted through the urethra
and connected to a disposable pressure transducer system
for invasive blood pressure monitoring (model DPT6000; Smiths Medical Deutschland GmbH) was used.
The signal obtained from the pressure transducer was
delivered to a custom-designed bridge amplifier and to
the aforementioned data acquisition system connected to
the PC.
Finally, the relative position of the GTE closest to the
superficial region of the nerve innervating the pancreas
was determined indirectly. This was done by delivering
a train of the aforementioned stimuli for periods of
10 s to each of the 13 GTEs within the cuff so that all
the superficial regions were selectively stimulated. Each
stimulation session was proceeded by a pause of 5 s
during which the existing GTE was disconnected from the
stimulator and the next one was connected. Thus defined,
the superficial region was denoted as region 7, shown in
the inset in the bottom left of Fig. 1. By the end of each
pause, blood samples from the femoral artery were drawn.
They were placed on ice, centrifuged at 4◦ C immediately
after the end of the experiments, and the plasma was
separated and frozen at −20◦ C until radioimmunoassay
(RIA). Plasma glucose was regularly estimated by the
glucose oxidase method. Hematocrit was determined at
regular intervals throughout the experiments. In this way
the GTE that elicited the highest rate of insulin secretion
was determined.
Kits from Linco Research, Inc. (St Charles, MI, USA)
were used for RIA. The first kit was the Canine C-Peptide
radioimmunoassay kit for quantitative determination of
canine C-peptide in serum, plasma and other biological
media. The second kit was the Human Insulin Specific kit
for quantitative determination of insulin in serum, plasma
and other biological media. The third kit was the Glucagon
Radioimmunoassay kit, which uses an antibody specific for
pancreatic glucagon. All of the aforementioned kits are for
research purposes only.
After the last experiment, both animals were killed using
the veterinary drug T61 (Hoechst, Frankfurt, Germany).
The whole study was performed within the time frame
of 3 years. The first year was spent on the development of
the model, development and fabrication of the cuffs, and
for organization of the materials, apparatus and facilities.
Modelling of the electric field generated in the superficial
region of the nerve by a certain GTE was based on the
geometrical model, dissected through the longitudinal axis
of the nerve and selected GTE. To determine the potential
distribution in the superficial regions the Finite Element
Method was used. After the cuffs were implanted, the next
2 months were spent allowing the animals to fully recover
from anaesthesia and tissue healing. In both animals the
cuff remained implanted for a time period of 2 years, but
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during the last 6 months the nerves were not stimulated.
Within the remaining time period of 16 months, four
sessions in each animal were performed. They were
repeated about every 5 months. In this very complex
study we mainly tested selectivity and reproducibility
of stimulation and measured only basic parameters of
modified physiological function in stimulated organs
and glands. Owing to the aforementioned complexity of
the experimental work and the extent of the expected
outcomes of the study a reliable statistical model could not
be developed. Besides, in each animal unique conditions
were established, so only selectivity and intra-individual
reproducibility could be tested. Accordingly, owing to this
lack of a reliable statistical model, we present the results
obtained in the last of four sessions conducted in the
second implanted animal.
Results
The four records of responses elicited in the heart, the lungs
and the bladder by the stimuli delivered for 10 s on GTE
no. 9, which were closest to region 6, innervating the heart,
are presented in Fig. 2. Trace 1, representing breathing
frequency, shows only a minor disturbance in breathing
during the stimulation period. Trace 2, representing the
changes in pressure within the bladder, shows that
vagal stimulation caused no measurable change. Vagal
stimulation did induce a decrease in arterial pressure and
heart rate. In fact, the heart rate response to left vagus
nerve stimulation was nearly abolished in the case when
the stimulation pulses were 2 mA. In particular, trace 4 of
the figure, which represents arterial pressure, shows a rapid
fall during the time period in which current stimuli were
2 mA. Trace 3, representing the ECG, shows a complete
cessation of the ECG recorded within the same period of
time.
Similarly, Fig. 3 shows four traces recorded as responses
elicited in the heart, the lungs and the bladder by
stimuli delivered for 10 s to GTE no. 4, which was
closest to region 5, innervating the respiratory muscles.
Trace 1, representing breathing frequency, shows complete
cessation of breathing during the time period in which
the magnitude of the current stimuli was 2 mA. Trace 2,
representing changes in pressure within the bladder, shows
no measurable change. Similarly, trace 3, representing
ECG, does not show any disturbance within the
stimulation period. Trace 4, representing arterial pressure,
does not show any change in pressure.
The four records of responses elicited in the heart,
the lungs and the bladder by stimuli delivered for 10 s
on GTE no. 1, which was closest to region 4, innervating
the bladder, are presented in Fig. 4. Trace 1, representing
breathing frequency, does not show any disturbance in
breathing during the stimulation period. Similarly, as can
J. Rozman and M. Bunc
2
0
34.8
34.6
Trace 2
0
1
Trace 3
Trace 4
0
5
10
15
20
Time (s)
Figure 2. Physiological responses of
heart, lungs, and bladder to stimulation
with the group of 3 electrodes no. 9
Trace 1 shows the physiological
response of the lungs, represented by
breathing frequency. Trace 2 illustrates
internal bladder pressure. Traces 3 and 4
illustrate the response of the heart as
demostrated by ECG and arterial blood
pressure, respectively.
some overlap occurred and some disturbance in the heart
rate was also observed.
Discussion
This work shows for the first time that a multi-electrode
spiral cuff chronically implanted around the left vagus
nerve of a dog can be used to modulate the function of
the internal organs and glands selectively, via selective
stimulation of the corresponding superficial regions of the
nerve.
Normal cardiac rhythm is controlled by pacemaker
activity, which is spontaneously generated by SA nodal
cells. The rhythm is strongly influenced by autonomic
34.6 34.8
0
2
Trace 1
Trace 2
0
1
Trace 3
Trace 4
100
ECG (mV)
Bladder pressure
Breathing (a.u)
(mmHg)
be seen in traces 3 and 4, respectively, vagal stimulation
did not induce any disturbance in heart rate or in arterial
pressure. Trace 2, representing the change in pressure
within the bladder, however, shows a decrease in pressure
from 34.8 mmHg at the beginning of stimulation to
34.74 mmHg at the end of stimulation.
Figure 5 shows the response of the pancreas on
consecutive selective stimulation of the superficial regions
of the nerve using all 13 groups of three electrodes. The
results of RIA of the blood samples drawn from the femoral
artery after stimulation using each of the 13 GTEs showed
that only GTE no. 10, which was close to region 7, caused
a maximum increase in insulin and minor increase in
glucagon secretion. Since GTE no. 10 is close to GTE no. 9,
ABP (mmHg)
Exp Physiol 89.6 pp 717–725
Trace 1
100
ABP (mmHg)
ECG (mV)
Bladder pressure
Breathing (a.u)
(mmHg)
722
0
5
10
15
Time (s)
20
Figure 3. Physiological responses of
heart, lungs, and bladder to stimulation
with the group of 3 electrodes no. 4
Traces as in Fig. 2.
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Left vagus nerve modulates visceral function
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0
2
Trace 1
34.6 34.8
Bladder pressure
Breathing (a.u)
(mmHg)
Exp Physiol 89.6 pp 717–725
Trace 2
Figure 4. Physiological responses of
heart, lungs, and bladder to stimulation
with the group of 3 electrodes no. 1
Traces as in Fig. 2.
1
0
Trace 4
100
ABP (mmHg)
ECG (mV)
Trace 3
0
nerves, with the vagus nerve being dominant over
sympathetic influences at rest. Abnormal cardiac rhythms
may occur when the SA node fails to function normally
or when normal conduction pathways are not followed.
Vagal activation decreases pacemaker rate, which decreases
the slope of repolarization, thereby increasing the time
to reach threshold. Vagal activity can also hyperpolarize
the pacemaker cells during repolarization, which results
in a long time to reach threshold voltage. According to the
results of selective stimulation of the heart, represented by
trace 2 in Fig. 2, showing the ECG, we could assume that
the pacemaker cells were hyperpolarized owing to the
intensity of applied current pulses. This illustrates that one
difficulty observed during stimulation sessions was that the
difference in the current level between threshold excitation
and maximum recruitment was rather low. Consequently,
the heart rate as shown by the R waves remained constant
until it was abruptly terminated at the onset of stimulation
and returned unchanged at the end of stimulation.
The results of selective stimulation of the respiratory
muscles represented by trace 4 in Fig. 3, showing the
rhythm of breathing, shows the complete cessation of
breathing during the stimulation period. However, to
obtain more detailed information on the possibility of
modifying lung function, different values of stimulation
parameters such as current intensity and frequency of
stimuli should be employed.
The results of selective stimulation of the bladder,
represented by trace 3 in Fig. 4, showing pressure within
the bladder, reveals a minor decrease in pressure during
the stimulation period. In this case, however, a degree of
selectivity and the rate of intra-individual reproducibility
are not so great as for heart and lung parameters.
Finally, the results of selective stimulation of the
pancreas, shown in Fig. 5 by the bars representing the
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5
10
15
20
Time (s)
secretion of insulin and glucagon when different GTEs
were used, reveal that a considerable increase in insulin
and minor increase in glucagon secretion could be elicited.
The results obtained in both animals show almost
the same high degree of selectivity and high rate of
intra-individual reproducibility in all stimulation sessions
performed. With respect to selectivity, the problem
of excitational overlap in the regions between the
neighbouring electrodes and dislodgement of the spiral
cuff during any significant movement remained unsolved.
The results of the study could be used in various
animal and especially human basic studies concerning
the neurophysiology of the internal organs and endocrine
glands and their relation to bodily changes and
disease. Ultimately, the method could be used for both
stimulation and recording of electroneurograms (ENGs)
in different combinations when implanted around the
aforementioned nerves.
Figure 5. Physiological response of the pancreas elicited by
stimulation with the group of 3 electrodes no. 10
The physiological responses of the pancreas are represented by
changes in the secretion of insulin and glucagon.
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The methodology as well as the pertaining technological
solutions developed could be used in transfer of the
animal model to the human model of curing diseases of
cardiovascular system and potentially diabetes mellitus.
Namely, it is well known that cardiovascular disease is
the leading cause of death in diabetes mellitus. Moreover,
the methodology, and especially spiral cuffs, could be
successfully used in the transfer of this animal model
to human vagus nerve stimulation (VNS; Murphy &
Patil, 2003). However, the condition for the potentially
successful transfer of the model for selectively modifying
the function of the internal organs and glands in man is
a good functional result demonstrated by these animal
experiments, even over a prolonged period of time.
VNS is already established as a clinically useful
anticonvulsant in drug-resistant patients with epilepsy,
and it may have potential as an antidepressant treatment.
It seems logical to suggest that selective VNS delivered
with different parameters from those commonly used for
epilepsy might produce different CNS effects that would
in turn broaden the clinical indications. Therefore, this
therapy could be significantly improved by the possibility
of selective stimulation of the human left vagus nerve
with the implanted multielectrode spiral cuff (Murphy &
Patil, 2003).
One application already being studied is the use
of VNS in slowing the progress of coronary artery
disease (Zamotrinsky et al. 2001). Another potential
application of VNS is the control of Parkinsonian and
essential tremor. Gastric secretory studies combined
with selective vagal stimulation may be useful in
patients with suspected gastric hypersecretion or with
gastroparesis (Berthoud et al. 2002). Since recent
behavioural and physiological evidence indicates that
the vagus nerves conduct sensory information from the
uterus to the brainstem, conducted information could
also be modified using the selective VNS (Collins et al.
1999).
The long-range goal of our research will be to
understand how the various branches of the autonomic
nervous system regulate the function of the internal
organs and glands. More precisely, in our near future
work, the physiological function of internal organs and
glands modified via the selective stimulation of the
superficial regions in the innervating sympathetic and
parasympathetic nerves will be focused on a certain organ
and studied in detail.
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Acknowledgements
This work was financed by Research Grant no. J2-0542 from the
Ministry of Education, Science and Sport, Ljubljana, Republic of
Slovenia and supported in part by the European Community’s
Human Potential Programme under contract no. HPRN-CT2000-00030 [NeuralPRO].