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ORIGINAL RESEARCH
Evidence for centrally induced cholinergic vasodilatation in
skeletal muscle during voluntary one-legged cycling and
motor imagery in humans
Kei Ishii
1
, Kanji Matsukawa
1
, Nan Liang
1
, Kana Endo
1
, Mitsuhiro Idesako
1
, Hironobu Hamada
2
,
Kazumi Ueno
3
& Tsuyoshi Kataoka
3
1 Department of Integrative Physiology, Graduate School of Biomedical and Health Sciences, Hiroshima University, Hiroshima, Japan
2 Department of Physical Analysis and Therapeutic Sciences, Graduate School of Biomedical and Health Sciences, Hiroshima University,
Hiroshima, Japan
3 Department of Health Care for Adults, Graduate School of Biomedical and Health Sciences, Hiroshima University, Hiroshima, Japan
Keywords
Central command, exercise hyperemia, near-
infrared spectroscopy (NIRS), sympathetic
cholinergic nerve.
Correspondence
Kanji Matsukawa, Department of Integrative
Physiology, Graduate School of Biomedical
and Health Sciences, Hiroshima University,
1-2-3 Kasumi, Minami-ku, Hiroshima 734-
8551, Japan.
Tel: +81 82 257 5435
Fax: +81 82 257 5344
E-mail: matsuk@hiroshima-u.ac.jp
Funding Information
This study was supported by Grants-in-Aid
for Scientific Research (B) and for Exploratory
Research from the Japan Society for the
Promotion of Science (JSPS). K. I. was
supported as a JSPS research fellow.
Received: 22 July 2013; Revised: 22 August
2013; Accepted: 23 August 2013
doi: 10.1002/phy2.92
Physiol Rep, 1 (4), 2013, e00092, doi:
10.1002/phy2.92
Abstract
We have recently reported that central command contributes to increased
blood flow in both noncontracting and contracting vastus lateralis (VL) mus-
cles at the early period of voluntary one-legged cycling. The purpose of this
study was to examine whether sympathetic cholinergic vasodilatation mediates
the increases in blood flows of both muscles during one-legged exercise.
Following intravenous administration of atropine (10 lg/kg), eight subjects
performed voluntary 1-min one-legged cycling (at 35% of maximal voluntary
effort) and mental imagery of the exercise. The relative concentrations of oxy-
genated- and deoxygenated-hemoglobin (Oxy- and Deoxy-Hb) in the bilateral
VL were measured as an index of muscle tissue blood flow with near-infrared
spectroscopy (NIRS). The Oxy-Hb in both noncontracting and contracting VL
increased at the early period of one-legged cycling, whereas the Deoxy-Hb did
not alter at that period. Atropine blunted (P<0.05) the Oxy-Hb responses of
both VL muscles but did not affect the Deoxy-Hb responses. The time course
and magnitude of the atropine-sensitive component in the Oxy-Hb response
were quite similar between the noncontracting and contracting VL muscles.
With no changes in the Deoxy-Hb and hemodynamics, imagery of one-legged
cycling induced the bilateral increases in the Oxy-Hb, which were completely
abolished by atropine. In contrast, imagery of a circle (with no relation to
exercise) did not alter the NIRS signals, irrespective of the presence or absence
of atropine. It is concluded that central command evokes cholinergic vasodila-
tation equally in bilateral VL muscles during voluntary one-legged cycling and
motor imagery.
Introduction
The presence of neurally mediated vasodilator mechanisms
for blood vessels in skeletal muscle is a controversial issue.
Blood flow to noncontracting muscle, however, is predom-
inantly regulated by the sympathetic nervous system,
because muscle contraction is absent and no metabolites
are released in the muscle. It is known that blood flow
and vascular conductance of nonexercising limb increase
rapidly at the early period of one armed or legged exercise,
suggesting neurally mediated vasodilatation in skeletal
muscle (Eklund et al. 1974; Duprez et al. 1989; Taylor
et al. 1989; Fisher and White 2003; Yoshizawa et al. 2008).
Unfortunately, the responses in muscle sympathetic
nerve activity (MSNA) of the contralateral resting limb
during one-legged exercise varied among studies (increased
[Herr et al. 1999], decreased [Saito and Mano 1991], and
unchanged [Ray et al. 1993]). With measurements of limb
ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
This is an open access article under the terms of the Creative Commons Attribution License,
which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
2013 | Vol. 1 | Iss. 4 | e00092
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Physiological Reports ISSN 2051-817X
blood flow and MSNA, Fisher et al. (2005) reported that
vasodilatation in the contralateral leg occurred at the start
of calf static exercise with no accompanying changes in
MSNA to the leg. The inconsistent results for the MSNA
response lead to a possibility that muscle sympathetic
nerve may contain not only sympathetic adrenergic vaso-
constrictor fibers but also other types of sympathetic fibers
that are hardly recorded (Wallin and Sundl€
of 1982; Halli-
will et al. 1997). Indeed, single unit recording of sympa-
thetic postganglionic fibers revealed that some fibers
dissected from nerve bundles entering the cat gastrocne-
mius or tibialis anterior muscle are not spontaneously
active and are not activated by arterial baroreceptor stimu-
lation, whereas they are activated by stimulation of the
hypothalamic defense area (Horeysek et al. 1976; Lopes
and Palmer 1977; Dean and Coote 1986). Because the same
hypothalamic stimulation elicited cholinergic vasodilata-
tion in skeletal muscle (Eliasson et al. 1951; Uvn€
as 1954;
Abrahams et al. 1960, 1964; Bolme et al. 1967; Matsukawa
et al. 1993, 1997; Komine et al. 2003), the normally silent
sympathetic postganglionic fibers are presumably choliner-
gic and may contribute to the vasodilatation. Vascular
innervation of sympathetic cholinergic fibers to skeletal
muscle has been confirmed in some animal species (such
as cat and dog; Bolme and Fuxe 1970; Lundberg et al.
1979) but absent in other species (such as rat, mice, and
monkey; Bolme and Fuxe 1970; Guidry and Landis 2000).
It is not well known to what extent the sympathetic
cholinergic vasodilator system contributes to increasing
blood flow of skeletal muscle in humans, although cen-
trally induced activation of the sympathetic cholinergic
nerve contributes to exercise hyperemia at the onset of
voluntary static exercise in cats (Komine et al. 2008). Cer-
vical sympathectomy and muscarinic blockade had little
influence on increased blood flow to exercising limb after
a brief contraction and during intermittent handgrip
exercise (Corcondilas et al. 1964; Shoemaker et al. 1997;
Brock et al. 1998). Furthermore, histological evidence for
sympathetic cholinergic innervation is lacking in humans
(Bolme and Fuxe 1970). Thus, it is thought that the sym-
pathetic nervous system is not responsible for exercise
hyperemia in contracting muscle, which is induced by a
complicated interplay of locally derived vasoactive sub-
stances and mechanical factors (Shepherd 1983; Sheriff
et al. 1993; R
adegran and Saltin 1998; Saltin et al. 1998;
Wray et al. 2005; Clifford 2007; Joyner and Wilkins 2007;
Kirby et al. 2007). On the other hand, because atropine-
sensitive vasodilatation in a resting limb is observed dur-
ing mental stress (Blair et al. 1959; Dietz et al. 1994) or
contralateral handgrip exercise (Sanders et al. 1989), it
cannot be neglected that neurally mediated vasodilatation
may be masked by the vasodilatation derived metaboli-
cally and mechanically in contacting muscle.
The previous findings were based on limb blood flow
via venous occlusion plethysmography or Doppler ultra-
sound at a relatively low time resolution. Recently, Ishii
et al. (2012) have reexamined a possible contribution of
neurally mediated vasodilatation using an estimate of
muscle tissue blood flow and oxygenation at a higher
time resolution with near-infrared spectroscopy (NIRS).
Both femoral blood flow and tissue blood flow in the
noncontracting vastus lateralis (VL) muscle increased at
the early period of one-legged cycling without accompa-
nying rise in arterial blood pressure (AP) (Ishii et al.
2012). Interestingly, mental imagery of the one-legged
exercise evoked approximately half of the increases in
femoral blood flow and muscular tissue blood flow
observed during the exercise (Ishii et al. 2012). Accord-
ingly, it is likely that descending signal from higher brain
centers in association with cycling imagery is capable of
causing the vasodilatation without any feedback from
contracting muscle. The purpose of this study was (1) to
examine whether the vasodilatation of noncontracting VL
muscle during one-legged exercise is neurally mediated
via sympathetic cholinergic nerve, (2) to examine whether
the vasodilatation during mental imagery of the exercise
is also mediated via sympathetic cholinergic nerve, and
(3) to examine whether such centrally induced vasodilata-
tion occurs not only in the noncontracting but also in the
contracting VL. If the hypotheses are true, central com-
mand may transmit vasodilator signals via sympathetic
cholinergic nerves to bilateral skeletal muscles during
exercise.
Materials and Methods
Subjects
Eight healthy men (age, 22 1 years; height, 170 2 cm;
body weight, 63 4 kg) participated in this study. None
of the subjects suffered from any known cardiovascular
and neuromuscular disease. They did not take any medica-
tions. The experimental procedures and protocols were
performed in accordance with the Declaration of Helsinki
and approved by the Institutional Ethical Committee. The
subjects gave their informed written consent prior to the
experiments. All experiments were performed in thermo-
neutral and soundproof environment.
One-legged cycling exercise
Voluntary one-legged exercise with the right leg was per-
formed for 1 min at 50 rpm in the supine position on a
comfortable reclining seat of a specially designed cycle
ergometer as shown in Figure 1 (Strength Ergo 240
BK-ERG-003; Mitsubishi Electric Engineering, Tokyo,
2013 | Vol. 1 | Iss. 4 | e00092
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ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
Central Command and Cholinergic Vasodilatation K. Ishii et al.
Japan). The subjects arbitrarily started voluntary one-leg-
ged cycling without any cue. The exercise intensity was
set at 35% of the maximal voluntary effort, which was
determined by an incremental one-legged exercise test
conducted on a separate day prior to the main experi-
ments. The right foot was put on a specially designed
shoe affixed at the pedal. The positions of the crank,
pedal, and seat were adjusted so as to allow the subjects
to remain in a comfortable and certain posture. The sub-
jects were instructed to perform one-legged isotonic
cycling with the right leg alone and to maintain the left
leg relaxed throughout the experiments. Torque against
the wheel shaft and pedal displacement of the ergometer
was continuously measured. According to the Borg 6-20
unit scale, the rating of perceived exertion (RPE) was
monitored after each boot of exercise was accomplished.
Motor imagery of voluntary one-legged
exercise
To examine the influence of central command on muscle
blood flow without any feedback from contracting mus-
cle, the subjects were instructed to imagine one-legged
cycling of the right leg (cycling-imagery) for 1 min as
Torque
Pedal displacement
Finometer NIRS
AP
(mmHg)
HR
(beats/min)
Oxy-Hb of
contracng VL
(μMcm)
Oxy-Hb of
non-contracng VL
(μMcm)
Torque
(Nm)
-30 0 30 60 90
0
20
40
60
100
140
-120
0
120
-30 0 30 60 90
0
20
40
60
100
140
0
100
200
0
70
-70
0
70
-70
0
100
200
0
120
-120
One-legged cycling One-legged cycling
(s) (s)
Control
A
BAtropine
Figure 1. (A) The experimental setup. (B) Representative recordings of arterial blood pressure (AP), heart rate (HR), relative concentrations of
oxygenated-hemoglobin (Oxy-Hb) in contracting and noncontracting vastus lateralis (VL) muscles, and developed torque during voluntary
one-legged cycling under control and atropine conditions in a subject. Horizontal dotted lines indicate the baseline levels of the Oxy-Hb. At the
early period of the exercise under the control condition, HR and the Oxy-Hb in both contracting and noncontracting VL increased, while AP
was unchanged. Thereafter, the Oxy-Hb of noncontracting VL remained elevated during the later period of exercise, while the Oxy-Hb of
contracting VL returned near the baseline. Atropine markedly blunted the Oxy-Hb responses in both VL muscles, although it did not affect
torque development.
ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
2013 | Vol. 1 | Iss. 4 | e00092
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K. Ishii et al.Central Command and Cholinergic Vasodilatation
soon as a cue was given. As control, the subjects imagined
a circle (circle-imagery) with no relation to exercise for
1 min. The vividness score of imagery (from 0 [not vivid
at all] to 10 [the most vivid]) was asked after each imag-
ery task (Ishii et al. 2012).
Measurements of muscle blood flow
The relative concentrations of oxygenated- and deoxygen-
ated-hemoglobin (Oxy- and Deoxy-Hb) in the bilateral
VL muscles were measured with NIRS. The basic principle
of NIRS is that near-infrared light from three laser photo-
diodes with different wavelengths penetrates skeletal mus-
cle tissue and some of the light is absorbed by Hb,
myoglobin (Mb), and cytochromes and that others scat-
tered by the tissue are picked up with photodetectors (Fer-
rari et al. 1997; Boushel and Piantadosi 2000; McCully
and Hamaoka 2000). However, it has been indicated that
the Hb in blood vessels of muscle tissue rather than the
other candidates (i.e., Mb and cytochromes) chiefly affects
the signals of NIRS (Seiyama et al. 1988). The muscle oxy-
genation signals of NIRS are dependent on a balance of
oxygen supply and utilization in the tissue. As long as oxy-
gen utilization is constant and at the minimum (e.g., non-
contracting muscle), the signal of Deoxy-Hb is constant
and the signal of Oxy-Hb may reflect muscle tissue blood
flow. On the basis of these rationales, we monitored the
Oxy-Hb as an estimate of tissue blood flow in skeletal
muscle (Ishii et al. 2012). A pair of photoemission and
photodetection probes were placed two-thirds from the
greater trochanter to the top of the patella and attached
4 cm apart on the skin over the left and right VL muscles,
so that near-infrared light intersected the muscle bundles.
The reflected near-infrared light (wavelength: 775, 810,
and 850 nm) through muscle tissue was sampled at a rate
of 6 Hz and converted to optical densities with a near-
infrared spectrometer (NIRO 200; Hamamatsu Photonics,
Hamamatsu, Japan).
Cardiovascular and EMG recordings
The electrocardiogram (ECG) was monitored with a
telemetry system (DynaScope DS-3140; Fukuda Denshi,
Tokyo, Japan). AP was noninvasively and continuously
measured with a Finometer (Finapres Medical Systems
BV, Arnhem, the Netherlands), of which a cuff was
attached to the left middle finger. The AP waveform was
sampled at a frequency of 200 Hz. The beat-to-beat val-
ues of systolic, diastolic, and mean AP (MAP) and heart
rate (HR) were obtained throughout the experiments.
Simultaneously, the beat-to-beat values of cardiac output
(CO), stroke volume (SV), and total peripheral resistance
(TPR) were calculated from the aortic pressure waveform
by using a Modelflow software (BeatScope 1.1; Finapres
Medical Systems BV, Arnhem, the Netherlands).
Electromyogram (EMG) activity of the VL muscle was
bilaterally measured using a pair of silver bar electrodes
attached on the central portion of the muscle belly (Bagn-
oli-2 EMG System; Delsys, Boston, MA). Prior to the
EMG electrode application, skin was cleaned up with
alcohol and preparatory gel. The EMG signals were
amplified (910000) and passed through a bandpass filter
between 20 and 2000 Hz.
Experimental protocols
The NIRS and EMG signals of the bilateral VL muscles
were simultaneously measured in all subjects as well as
the cardiovascular responses and motor performance
(developed torque and pedal displacement of the ergome-
ter). The EMG activity of the noncontracting muscle was
absent in all cases. The NIRS and cardiovascular
responses during experimental interventions were studied
with and without muscarinic blockade on two different
days separated by 5–10 days.
Day 1: without any drug (control condition)
All subjects performed voluntary one-legged cycling with
35% of the maximal voluntary effort, cycling-imagery,
and circle-imagery at intertrial intervals of 5 0.2 min.
Day 2: the effect of muscarinic receptor blockade
(atropine condition)
After the NIRS responses to voluntary one-legged cycling
were obtained, atropine sulfate (10 lg/kg) was intrave-
nously administrated into the right cephalic vein in all
subjects. After a rest period of 7 0.9 min from the
atropine injection, each individual task was conducted at
the sufficient intertrial intervals.
Data analysis
The data of NIRS signals, AP, ECG, and EMG were
stored to a computer at a sampling frequency of 1000 Hz
(MP150; BIOPACK Systems, Santa Barbara, CA) for off-
line analysis. The onset time of voluntary one-legged
cycling was defined as zero according to the onset of
pedal displacement. The changes in Oxy- and Deoxy-Hb,
HR, SV, CO, MAP, and TPR from the baseline levels
were sequentially averaged every 1 sec. The absolute con-
centration of Oxy-Hb could not be obtained, because the
pathlength of the near-infrared light within tissue was not
known in vivo. Instead, the relative changes in Oxy-Hb
were expressed as a percentage against the baseline. To
2013 | Vol. 1 | Iss. 4 | e00092
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ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
Central Command and Cholinergic Vasodilatation K. Ishii et al.
determine the baseline level of the Oxy-Hb, the zero level
of the Oxy-Hb was defined as the minimum value of
Oxy-Hb obtained during inflation of a pneumatic cuff,
wrapped around the upper thigh, with a pressure of 200–
250 mmHg. The baseline value of the Oxy-Hb signal
against the minimum value was taken as 100%. To cancel
a phasic change in the NIRS signals due to movement
artifact, the NIRS signals of the contracting VL were
recalculated by performing a moving average over neigh-
boring 1000 points.
We have confirmed the reproducibility of the NIRS
response to one-legged exercise in the absence of atro-
pine, because the NIRS responses obtained on different
days were not statistically different. Therefore, the cardio-
vascular and NIRS data in a given trial without atropine
were pooled and averaged as the control data. In addi-
tion, when four of the eight subjects performed two bouts
of voluntary one-legged cycling following atropine, the
data were almost identical and were not statistically dif-
ferent between the bouts. To identify the atropine-sensi-
tive component of the Oxy-Hb response to an
experimental intervention (termed as DOxy-Hb
atr
), the
Oxy-Hb response with atropine was subtracted from the
Oxy-Hb control response in individual subjects. DOxy-
Hb
atr
was also sequentially averaged every 1 sec. The ini-
tial peak (at 10–12 sec from the exercise onset) and later
changes (during 30–60 sec of the exercise) of the Oxy-Hb
and the DOxy-Hb
atr
were averaged among the subjects. In
imagery protocols, the responses of all variables were
obtained as an average over a time period from 30 to
45 sec after the imagery onset as previously reported
(Ishii et al. 2012).
Statistical analysis
The baseline and peak values of hemodynamics, devel-
oped torque, and RPE were compared between the con-
trol and atropine conditions by a paired t-test or a
Wilcoxon signed rank test. The effects of atropine on the
time course data of cardiovascular and NIRS responses
during exercise were analyzed by a two-way analysis of
variance (ANOVA) (factors: drug and time) with repeated
measures. The time course data of DOxy-Hb
atr
were ana-
lyzed by a one-way ANOVA with repeated measures.
When a significant Fvalue in the main effect of time was
present, a Bonferroni post hoc test was performed to
detect a significant difference in mean values from the
baseline control at a given time. The magnitudes of the
Oxy-Hb and DOxy-Hb
atr
responses during one-legged
exercise or mental imagery were compared by a paired t-
test between the control and atropine conditions or
between the contracting (right) and noncontracting (left)
VL. As compared to the baseline levels, the peak changes
in the hemodynamics and NIRS data were statistically
analyzed by a paired t-test. The vividness score between
the control and atropine conditions was compared by a
Wilcoxon signed rank test. A level of statistical signifi-
cance was defined at P<0.05 in all cases. All parameters
are expressed as means SE.
Results
The effects of atropine on the baseline hemodynamics
and motor performance are summarized in Table 1. Atro-
pine increased the baseline HR and CO, whereas the base-
Table 1. The effects of atropine on the baseline and peak changes in hemodynamics, developed torque, and the rating of perceived exertion
during one-legged exercise in eight subjects.
Control Atropine
Before During Change Before During Change
HR (beats/min) 62 2 104 2
1
42 1853
2
118 3
1,2
33 1
2
SV (mL) 87 4994
1
13 1785
2
89 5
1
11 3
CO (L/min) 5.4 0.3 10 0.4
1
4.6 0.2 6.6 0.4
2
9.8 0.4
1
3.3 0.3
2
MAP (mmHg) 93 2 115 3
1
22 2994
2
113 5
1
14 2
2
TPR (mmHg/L/min) 18 0.8 11 0.4
1
6.9 0.6 15 0.9
2
10 0.6
1
5.0 0.6
2
Developed torque (Nm) –12 1––12 1–
RPE (Borg scale) –13 0.5 ––13 0.4 –
Values are means SE. All baseline hemodynamic values were significantly different (P<0.05) between the control and atropine conditions.
Also, the peak values of all hemodynamic variables during the one-legged exercise were significantly (P<0.05) different from the baseline in
both conditions. As compared to the control condition, the peak changes in the hemodynamics, except SV were significantly (P<0.05) blunted
by atropine. The average of periodic torque output during the one-legged exercise was defined developed torque. The developed torque and
rating of perceived exertion (RPE) during one-legged exercise were not significantly (P>0.05) different between the control and atropine condi-
tions. HR, heart rate; SV, stroke volume; CO, cardiac output; MAP, mean arterial blood pressure; TPR, total peripheral resistance.
1
Significant difference (P<0.05) from the baseline value.
2
Significant difference (P<0.05) between the control and atropine conditions.
ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
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K. Ishii et al.Central Command and Cholinergic Vasodilatation
line SV was decreased. The increase in CO was counter-
acted by a decrease in TPR, resulting in a slight increase
in MAP. The same amounts of developed torque and
RPE were observed during one-legged exercise following
atropine (Table 1 and Fig. 1).
The effects of atropine on the
cardiovascular responses to voluntary one-
legged cycling
Atropine attenuated the peak increases in HR and AP
during one-legged exercise in a subject as exemplified in
Figure 1. The effects of atropine on the cardiovascular
responses are summarized in Table 1 and Figure 2. In the
absence of atropine, HR and CO increased and TPR
decreased throughout the exercise, while SV elevated
slightly during the later period of exercise. A counterbal-
ance between the increase in CO and the decrease in TPR
resulted in a rise in MAP immediately after the exercise
onset and during the later period of exercise. Atropine
blunted (P<0.05) the increases in HR and CO, but not
SV, during the exercise (Table 1 and Fig. 2). The decrease
in TPR during the exercise was also blunted by atropine.
Atropine did not alter the initial MAP response but sig-
nificantly (P<0.05) diminished the later increase in
MAP.
The effects of atropine on the NIRS
responses to voluntary one-legged cycling
Figure 1 shows a typical example of the Oxy-Hb
responses in the contracting and noncontracting VL mus-
cles to voluntary one-legged exercise under the control
and atropine conditions in an identical subject. In the
control, both the Oxy-Hb of the noncontracting and con-
tracting VL muscles increased rapidly during the early
period (0–12 sec from the exercise onset) of one-legged
cycling. Thereafter the increase in the Oxy-Hb of the non-
contracting VL was sustained during the exercise, while
the Oxy-Hb of the contracting VL returned near the base-
line in progress of oxygen utilization with muscular activ-
ity. Atropine markedly reduced or abolished the increases
in the Oxy-Hb of bilateral VL muscles during one-legged
exercise (Fig. 1).
The effects of atropine on the time course data of the
Oxy-Hb and Deoxy-Hb average responses of the VL mus-
cles are summarized in Figure 3. In the absence of atro-
pine, the Oxy-Hb of the noncontracting VL increased
during one-legged exercise with no accompanying changes
in Deoxy-Hb (Fig. 3). In the contracting VL, the Oxy-Hb
tended to increase during the early period of exercise (fol-
lowing a transient drop probably due to movement arti-
fact), while the Deoxy-Hb was unchanged at that period
(Fig. 3). As exercise proceeded, the Oxy-Hb decreased
and the Deoxy-Hb increased until the end of exercise.
Atropine significantly (P<0.05) modified the time
courses of the Oxy-Hb not only in the noncontracting
but also in the contacting VL muscle during one-legged
exercise, whereas atropine did not significantly affect the
time course of the Deoxy-Hb change in either VL muscle
(Fig. 3). The effects of atropine on the initial peak (at
10–12 sec from the exercise onset) and later changes in
the Oxy-Hb (during 30–60 sec from the exercise onset)
Control
Atropine
(s)
*
*
*
*
N.S.
One–legged cycling
Changes in MAP
(mmHg)
Changes in HR
(beats/min)
Changes in SV
(mL)
Changes in CO
(L/min)
Changes in TPR
(mmHg/L/min)
–15
0
15
30
45
–20
0
20
40
0
2
4
6
–15
0
15
30
–30 0 30 60 90
–12
–8
–4
0
4
Figure 2. The time courses of the cardiovascular responses during
voluntary one-legged cycling in the control (○) and atropine (●)
conditions in eight subjects. Atropine blunted the increases in HR
and cardiac output (CO) and the decrease in total peripheral
resistance (TPR) during the exercise, while it did not affect the
increase in stroke volume (SV). The initial mean AP (MAP) response
was not altered by atropine, but the later increase in MAP was
blunted (P<0.05). *Significant difference (P<0.05) between the
control and atropine conditions. NS, not significant (P>0.05).
2013 | Vol. 1 | Iss. 4 | e00092
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ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
Central Command and Cholinergic Vasodilatation K. Ishii et al.
are shown in Figure 4. Atropine significantly blunted
(P<0.05) the initial peak changes of the Oxy-Hb in
bilateral VL muscles. Also, the bilateral Oxy-Hb during
the later period was more decreased (P<0.05) by atro-
pine (Fig. 4).
The DOxy-Hb
atr
(calculated from a difference between
the Oxy-Hb responses in the control and atropine condi-
tions) was considered as the atropine-sensitive component
of the Oxy-Hb response. The time courses of the DOxy-
Hb
atr
during one-legged cycling are summarized in Fig-
ure 5. The DOxy-Hb
atr
of the noncontracting VL muscle
began to increase (P<0.05) by 4.7 1.1% at 11 sec
from the exercise onset and remained increased until the
end of exercise (Fig. 5A). The time course and magnitude
of the DOxy-Hb
atr
in the contracting VL resembled those
of the Oxy-Hb in the noncontracting VL (Fig. 5A).
Indeed, the magnitudes of DOxy-Hb
atr
at the initial peak
and during the later period were similar between the non-
contracting and contracting VL in Figure 5B (the initial
peak, 5.1 1.2% vs. 4.4 0.8%; the later change,
7.0 1.9% vs. 8.4 3.1%, respectively). Thus the atro-
pine-sensitive increase in tissue blood flow occurred
equally in both VL muscles.
The effects of atropine on the NIRS
responses to motor imagery
To determine the influence of central command on mus-
cle blood flow without any feedback from contracting
muscle, the Oxy-Hb responses of the bilateral VL muscles
to mental imagery of one-legged cycling were examined
as shown in Figure 6. Cycling-imagery increased the
–30 0 30 60 90
–100
0
100
200
300
–30 0 30 60 90
–10
–5
0
5
10
–30 0 30 60 90
–100
0
100
200
300
Non–contracng VL Contracng VL
Changes in
Oxy–Hb (%)
Changes in
Deoxy–Hb (μMcm)
One–legged cycling
One–legged cycling
*
(S)
(S)
N.S.
N.S.
*
(S)
(S)
–30 0 30 60 90
–30
–20
–10
0
10
–30 0 30 60 90
–300
–200
–100
0
100
–30 0 30 60 90
–100
–50
0
50
Changes in
Oxy–Hb (μMcm)
*
(S)
*
(S)
Control
Atropine
Figure 3. The time courses of the relative changes in Oxy-Hb and Deoxy-Hb of noncontracting and contracting VL muscles during voluntary
one-legged cycling in eight subjects. The parameters are expressed as means. The relative percent changes in Oxy-Hb were determined by
identifying the zero level with muscle ischemia. In the control condition (○), the Oxy-Hb in noncontracting VL increased during the exercise
while the Deoxy-Hb was unchanged. The Oxy-Hb in contracting VL tended to increase at the early period of exercise (following a transient drop
(↑) due to movement artifact) and subsequently decreased. The Deoxy-Hb remained unchanged at the early period of exercise and then
increased as exercise proceeded. Atropine (●) significantly (P<0.05) decreased the Oxy-Hb responses of both noncontracting and contracting
VL during the exercise, whereas it did not affect the Deoxy-Hb responses in both VL. *Significant difference (P<0.05) between the control and
atropine conditions. NS, not significant (P>0.05).
ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
2013 | Vol. 1 | Iss. 4 | e00092
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K. Ishii et al.Central Command and Cholinergic Vasodilatation
Oxy-Hb of not only left VL but also right VL with no
substantial changes in AP and HR (Fig. 6). The effects of
atropine on the Oxy-Hb responses are summarized in
Figure 7. Atropine abolished (P<0.05) the centrally
induced increases in the Oxy-Hb of both muscles. The
magnitude of the DOxy-Hb
atr
(i.e., the atropine-sensitive
component of the Oxy-Hb response) was equal in both
muscles. The vividness score of cycling-imagery was also
similar between the presence and absence of atropine
(Fig. 7). Cycling-imagery did not change the hemody-
namic variables and the Deoxy-Hb of both muscles from
the baseline levels in either condition.
In contrast to cycling-imagery, circle-imagery with no
relation to exercise did not change the Oxy-Hb in both
left and right VL (Fig. 7) as well as the hemodynamics
and the Deoxy-Hb. Following atropine, these variables
also remained unchanged during circle-imagery. The
DOxy-Hb
atr
was not significant from the baseline in both
muscles (Fig. 7). Thus the atropine-sensitive increases in
blood flow in bilateral VL were induced selectively by the
mental imagery in association with exercise.
Discussion
This study has tested the hypothesis that central com-
mand causes cholinergic vasodilatation in noncontracting
muscle during voluntary exercise and that the centrally
induced cholinergic vasodilatation also contributes to
exercise hyperemia. The new findings of this study are
that (1) atropine abolished the increase in Oxy-Hb of the
noncontracting VL at the early period of voluntary one-
legged cycling; (2) atropine also decreased the Oxy-Hb
response in the contracting VL at the early period of the
exercise; (3) the time course and magnitude of the atro-
pine-sensitive component of the Oxy-Hb response were
similar between the noncontracting and contracting VL;
(4) mental imagery of the one-legged exercise caused the
increases in Oxy-Hb of bilateral VL to the same extent,
which were completely abolished by atropine; (5) regard-
less of the presence or absence of atropine, circle-imagery
evoked no changes in Oxy-Hb of the bilateral muscles.
Because the Deoxy-Hb in the noncontracting VL did not
significantly alter throughout one-legged exercise and
mental imagery, the Oxy-Hb response may reflect the
changes in muscular tissue blood flow. Moreover, the
Oxy-Hb response in the contracting VL at the early
period of the exercise may also reflect the changes in the
tissue blood flow, because the Deoxy-Hb in the contract-
ing VL did not significantly alter at that period. Taken
together, it is likely that central command transmits a
cholinergic vasodilator signal equally to bilateral skeletal
muscles at the early period of voluntary exercise and con-
tributes at least partly to the initial exercise hyperemia in
humans.
Cholinergic vasodilatation in noncontracting
muscle during voluntary exercise
Femoral blood flow and vascular conductance of the
nonexercising limb increase without changing an internal
diameter of the artery and perfusion pressure at the
early period of voluntary one-legged exercise (Yoshizawa
et al. 2008; Ishii et al. 2012), indicating that the initial
vasodilatation occurs in downstream resistance vessels.
–10
–5
0
5
10
–30
–20
–10
0
10
Contracng VLNon–contracng VL
Effects on inial peak changes (at 10–12 s) Effects on later changes (during 30–60 s)
Changes in
Oxy–Hb (%)
††
Control
Atropine
†
†
Contracng VLNon–contracng VL
*
*
*
Figure 4. The initial peak (at 10–12 sec) and later changes (during 30–60 sec) in the Oxy-Hb of noncontracting and contracting VL during
voluntary one-legged cycling under the control (□) and atropine (■) conditions in eight subjects. Atropine significantly (P<0.05) decreased the
initial peak change in the Oxy-Hb in not only noncontracting but also contracting VL muscle. The Oxy-Hb values in both muscles during the
later period of exercise were further decreased (P<0.05) by atropine. *Significant difference (P<0.05) from the baseline. †Significant
difference (P<0.05) between the control and atropine conditions.
2013 | Vol. 1 | Iss. 4 | e00092
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ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
Central Command and Cholinergic Vasodilatation K. Ishii et al.
As inadvertent muscular activity was not observed in
noncontracting muscle, neither metabolite nor mechani-
cal deformation of skeletal muscle and/or blood vessels
is relevant with the vasodilatation. Of course, acetylcho-
line spillover from motor nerves could not account for
the vasodilatation in the noncontracting muscle. When
the Oxy-Hb of the noncontracting VL was measured as
an estimate of muscular tissue blood flow, the Oxy-Hb
increase appeared to precede the increase in femoral
blood flow (Ishii et al. 2012), suggesting that the vasodi-
latation occurred first in the noncontracting muscle and
the reduction in vascular resistance increased limb blood
flow. Sanders et al. (1989) demonstrated that intra-
arterial injection of atropine blocked the reduction in
vascular resistance in nonexercising forearm during con-
tralateral static handgrip, suggesting that acetylcholine
may be involved in the vasodilator mechanism. In agree-
ment with the study, we found that atropine abolished
the increase in the Oxy-Hb of noncontracting VL during
one-legged exercise. Acetylcholine may be neurally
released from sympathetic cholinergic terminals and/or
locally released from endothelial cells stimulated by an
increase in blood flow or shear stress (Milner et al.
1990; Martin et al. 1996; Joyner and Dietz 2003). How-
ever, it seems that a local release of acetylcholine from
endothelium if any will be substantially delayed from the
initial increase in muscle blood flow after the exercise
onset because Koller and Kaley (1990) showed that an
increase in red blood cell velocity per se increased the
diameter of arterioles with a time delay of 8 sec in the
rat cremaster muscle. Furthermore, we found that motor
imagery evoked atropine-sensitive vasodilatation in the
bilateral muscles without changing the hemodynamics.
Because the vasodilatation observed during motor imag-
ery is purely induced by a neural mechanism but not a
local mechanism, a sympathetic cholinergic mechanism
is more likely to be responsible for the vasodilatation.
Such neurally mediated mechanism plays a role in caus-
–30 0 30 60 90
–5
0
5
10
15
Non–contracng VL Contracng VL
ΔOxy–Hbatr (%)
One–legged cycling
One–legged cycling
(S) (S)
**
–30 0 30 60 90
–5
0
5
10
15
0
5
10
15
0
5
10
15
ΔOxy–Hbatr (%)
BAtropine–sensive inial peak changes Atropine–sensive later changes
N.S.
N.S.
Contracng VLNon–contracng V L
Contracng VLNon–contrac ng VL
**
*
*
A
Figure 5. (A) The time courses of the DOxy-Hb
atr
of noncontracting and contracting VL muscles during voluntary one-legged cycling in eight
subjects. The DOxy-Hb
atr
of noncontracting VL (taken as the atropine-sensitive component of the Oxy-Hb response) increased (P<0.05) at
11 sec from the exercise onset. The time course and magnitude of the DOxy-Hb
atr
of contracting VL resembled those of the DOxy-Hb
atr
of
noncontracting VL. (B) The atropine-sensitive initial peak (at 10–12 sec) and later changes (during 30–60 sec) in the Oxy-Hb responses of
noncontracting and contracting VL during voluntary one-legged cycling in eight subjects. The increases in DOxy-Hb
atr
at the initial peak and
during the later period of exercise were similar between noncontracting and contracting VL. *Significant difference (P<0.05) from the
baseline. NS, not significant (P>0.05).
ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
2013 | Vol. 1 | Iss. 4 | e00092
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K. Ishii et al.Central Command and Cholinergic Vasodilatation
ing the rapid vasodilatation in noncontracting muscle
during one-legged exercise, presumably due to either
central command or a reflex arising from mechanosensi-
tive afferents in contracting muscle (i.e., muscle
mechanoreflex). To assess which candidate is more likely,
Ishii et al. (2012) examined the response in the Oxy-Hb
during motor-driven passive one-legged cycling. As com-
pared to voluntary one-legged cycling, the increase in
the Oxy-Hb in noncontracting VL during passive cycling
was smaller and developed more slowly (Ishii et al.
2012). Accordingly, it is suggested that muscle mechano-
reflex does not account for the initial vasodilatation in
the noncontracting muscle and central command is more
responsible for the cholinergic vasodilatation. In addi-
tion, we suppose that the vasodilatation by central
command may function not only at the early period of
one-legged exercise but also during the later period of
exercise, because the atropine-sensitive component of the
Oxy-Hb was sustained throughout the exercise.
Centrally induced cholinergic vasodilatation
during motor imagery
Mental imagery of exercise is supposed to simulate cen-
tral control of the cardiovascular adaptation to exercise
with no afferent feedback from contracting skeletal mus-
cle (Decety et al. 1993; Williamson et al. 2002; Ishii et al.
2012). When examining whether central command
involved in motor imagery induces cholinergic vasodilata-
tion in skeletal muscle, we found that imagery of the
one-legged exercise induced the atropine-sensitive
increases in Oxy-Hb of the bilateral VL muscles without
changing the hemodynamics. Thus, it is suggested that
central command is capable of evoking cholinergic vaso-
dilatation in both VL muscles without any feedback from
skeletal muscle. Irrespective of the presence or absence of
atropine, circle-imagery had the same vividness score as
motor imagery but evoked no changes in the Oxy-Hb
and hemodynamics. Taken together, activation of central
“exercise-related” circuits is essential for selectively evok-
ing the cholinergic vasodilatation in skeletal muscle.
Nonetheless, it is not explicit whether central descending
signal is identical between one-legged exercise and its
mental imagery, because motor imagery evoked selectively
cholinergic muscle hyperemia without accompanying
tachycardia. In addition, the time courses of the Oxy-Hb
and cardiovascular responses were different from those
during voluntary one-legged exercise. This is because vol-
untary one-legged cycling started arbitrary without any
cue whereas mental imagery of the exercise was slowly
developed after a cue was given. Despite the limitations,
the mental imagery data are important to identify central
control of muscle circulation by evoking cholinergic vaso-
dilatation.
HR
(beats/min)
AP
(mmHg)
Oxy–Hb of
right VL
(μMcm)
Oxy–Hb of
le VL
(μMcm)
(s) (s)
03060
–50
0
50
40
70
100
40
70
100
03060
0
100
200
0
100
200
0
–50
50
0
–50
50
0
–50
50
Cycling–imagery Cycling–imagery
Control Atropine
Figure 6. Representative data of AP, HR, and Oxy-Hb of the right and left VL muscles during imagery of one-legged cycling under the control
and atropine conditions in a subject. In the control, cycling-imagery increased the Oxy-Hb in bilateral VL muscles without changing HR and AP.
Atropine abolished the increases in Oxy-Hb of both VL muscles.
2013 | Vol. 1 | Iss. 4 | e00092
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ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
Central Command and Cholinergic Vasodilatation K. Ishii et al.
In addition to the cholinergic vasodilator signal,
central command might induce an adrenergic vasocon-
strictor signal. However, because no vasoconstrictor
response appeared following atropine, central command
in relation to the exercise imagery did not produce any
vasoconstrictor signal. On the other hand, imagery of
one-legged exercise is not identical with mental stress
(such as emotional stress, mental arrhythmia, and color
word conflict test), because mental stress usually causes
the pressor and tachycardia response (Blair et al. 1959;
Linde et al. 1989; Dietz et al. 1994; Halliwill et al. 1997;
Carter et al. 2005).
–2
0
2
4
–2
0
2
4
–2
0
2
4
–2
0
2
4
0
2
4
6
8
10
0
2
4
6
8
10
Changes in
Oxy–Hb (%)
Right VLLe VL
†
Right VLLe VL
ΔOxy–Hbatr (%)
Vividness
score of
imagery
AtropineControl AtropineControl
Right VLLe VL Right VLLe VL
Cycling–imagery Circle–imagery
N.S.
N.S.
N.S.
N.S.
N.S. N.S.
†
**
**Control
Atropine
Figure 7. The effects of atropine on the Oxy-Hb response of the bilateral VL muscles and the extent of vividness during cycling- and circle-
imagery in eight subjects. In the control condition (□), cycling-imagery caused the bilateral increases in the Oxy-Hb, which were abolished
(P<0.05) by atropine (■). The DOxy-Hb
atr
of the left and right VL muscle (taken as the atropine-sensitive component of the Oxy-Hb response)
increased to the same extent during cycling-imagery. In contrast, circle-imagery did not change the Oxy-Hb in bilateral VL in either control or
atropine condition. The DOxy-Hb
atr
in both VL muscles during circle-imagery was not significant from the baseline. The vividness score during
cycling- or circle-imagery was similar between the control and atropine conditions. *Significant difference (P<0.05) from the baseline.
†Significant difference (P<0.05) between the control and atropine conditions. NS, not significant (P>0.05).
ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
2013 | Vol. 1 | Iss. 4 | e00092
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K. Ishii et al.Central Command and Cholinergic Vasodilatation
Does centrally induced cholinergic
vasodilatation contribute to exercise
hyperemia?
It has been thought that exercise hyperemia chiefly results
from dilatation of arterial vessels by locally derived vaso-
active substances and mechanical deformation in associa-
tion with muscle contraction (Shepherd 1983; Sheriff
et al. 1993; R
adegran and Saltin 1998; Saltin et al. 1998;
Wray et al. 2005; Clifford 2007; Joyner and Wilkins 2007;
Kirby et al. 2007). On the other hand, the contribution of
a neurally mediated mechanism to exercise hyperemia has
been denied, because cervical sympathectomy and musca-
rinic blockade had no impact on increased blood flow to
an exercising limb (Corcondilas et al. 1964; Shoemaker
et al. 1997; Brock et al. 1998). Nevertheless, as mental
imagery of one-legged exercise evoked the cholinergic
vasodilatation in skeletal muscle in this study (Figs. 6, 7),
it is expected that a centrally induced vasodilator signal
may be transmitted to contracting muscle (Ishii et al.
2012; Matsukawa et al. 2013). Along this line, this study
has shown for the first time that, during the early period
of voluntary one-legged cycling, atropine decreased the
Oxy-Hb response in the contracting VL (Figs. 3, 4). The
atropine-sensitive vasodilatation in contracting muscle
might be masked by metabolic and/or mechanical vasodi-
lator mechanisms in the previous studies. A possibility
that acetylcholine spillover from motor nerves may cause
the atropine-sensitive vasodilatation cannot be denied.
However, it is noteworthy that the atropine-sensitive
component of the Oxy-Hb response in contracting VL
increased with the similar time course and magnitude as
those of the Oxy-Hb in noncontracting VL (Fig. 5). Thus
acetylcholine spillover from motor nerves cannot explain
the increased Oxy-Hb of the contracting muscle, too.
Accordingly, this study provides new evidence that central
command transmits cholinergic vasodilator signal through
the sympathetic nervous system equally to both contract-
ing and noncontracting muscle at the early period of vol-
untary exercise and may contribute, at least in part, to
exercise hyperemia.
Neurally mediated cholinergic vasodilatation in the
contracting muscle observed with NIRS in this study
conflicts with the previous data obtained using ultra-
sound Doppler flowmetry demonstrating that muscarinic
blockade had no impact on increased blood flow to an
exercising limb (Shoemaker et al. 1997; Brock et al.
1998). However, the controversy may be explained by
the following differences between NIRS and Doppler
flowmetry. First, to our knowledge, the most striking
difference between the two flowmetry methods is a dif-
ference in the vascular area detected by each method.
The Doppler flowmetry usually measures blood flow
velocity and internal diameter of a large conduit artery
(e.g., femoral or brachial artery), which supplies blood
flow to the entire vasculature system downstream from
the artery, including muscular, cutaneous, and bone tis-
sues. In contrast, the NIRS data are focused on the vas-
culature involved in a localized region of targeted
skeletal muscle. Second, NIRS has a relatively higher
time resolution than Doppler flowmetry. NIRS provides
the changes in concentration of Oxy-Hb at a sampling
rate of 6 Hz, while Doppler flowmetry provides the
beat-to-beat changes in flow velocity and volume flow of
a conduit artery. Third, it is difficult to continuously
keep good measurement of Doppler blood flow velocity
during exercise, because the Doppler signal is strongly
influenced by a slight change in the insonation angle
against the artery and thereby it appears to be more
intolerant of movement artifact as compared to the
NIRS signal.
Other neurohumoral mechanisms contribute
to vasodilatation in noncontracting muscle
The effects of a- and b-adrenergic blockades on the
increased blood flow in noncontracting skeletal muscle
during one-legged cycling were not examined in this study.
Because b-adrenergic blockade by propranolol attenuated
the peak decrease in vascular resistance of nonexercising
forearm during contralateral static handgrip (Eklund and
Kaijser 1976), b-adrenergic receptors are expected to medi-
ate the vasodilatation. Generally, norepinephrine has a
lower potency on b-adrenergic receptors than a-adrenergic
receptors and usually causes vasoconstriction in skeletal
muscle (Brick et al. 1967; Glick et al. 1967), whereas epi-
nephrine released from the adrenal medulla is a more
potent agonist on b-adrenergic receptors. Thus, epineph-
rine released from the adrenal medulla may contribute to
the increased muscle blood flow during the later period of
exercise (Wakasugi et al. 2010). On the other hand, with-
drawal of sympathetic a-adrenergic vasoconstrictor activity
may contribute to increased blood flow in noncontracting
muscle. However, the possibility is unlikely because phen-
tolamine did not alter the initial vasodilatation in nonexer-
cising limb during exercise (Eklund and Kaijser 1976). On
the contrary, it is pointed out that an increase in sympa-
thetic a-adrenergic vasoconstrictor activity may restrict
blood flow to noncontracting muscle during exercise with
a longer duration and a higher intensity (Eklund and Kaij-
ser 1976; Duprez et al. 1989; Taylor et al. 1989; Yoshizawa
et al. 2008). Seals (1989) reported that MSNA to a resting
leg was unchanged during the initial phase of static hand-
grip at 15–35% of maximal voluntary contraction and
increased during the later period (1.5–2.5 min) of the exer-
cise and that the increase in MSNA was dependent on the
2013 | Vol. 1 | Iss. 4 | e00092
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ª2013 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
the American Physiological Society and The Physiological Society.
Central Command and Cholinergic Vasodilatation K. Ishii et al.
exercise intensity and linearly related to the increase in calf
vascular resistance. Hansen et al. (1994)) showed that
MSNA to both nonexercising and exercising limbs was
unchanged during the first minute of left toe extension and
increased during the second minute of the exercise. This
study found that, following atropine, the Oxy-Hb in the
noncontracting VL decreased gradually during one-legged
exercise (Fig. 3). Taking the previous and present results
into consideration, it is likely that vasoconstriction via a-
adrenergic receptors in noncontracting muscle is masked
by centrally induced cholinergic vasodilatation at the early
period of exercise and thereafter the vasoconstriction may
be evident with time, depending on exercise intensity.
Limitations
Several fundamental assumptions and limitations were
involved in this study. First, skin blood flow over the VL
muscle was not recorded in this study, because we have
already reported that no changes in skin blood flow were
observed during voluntary one-legged cycling (Ishii et al.
2012). This is in agreement with the finding that skin vas-
cular conductance of the forearm or leg unchanged dur-
ing isometric exercise (Saad et al. 2001). On the other
hand, some studies (Taylor et al. 1988; Kellogg et al.
1991; Vissing et al. 1991) reported a decrease in skin
blood flow and an increase in skin sympathetic nerve
activity during exercise. However, even if skin blood flow
in nonexercising limb decreases during exercise, the blood
flow change cannot explain the increase in Oxy-Hb dur-
ing exercise. Thus, it is likely that the contribution of skin
blood flow to the Oxy-Hb signal was the minimum.
Second, the moving averages of the NIRS signals in the
contracting VL were calculated to cancel a phasic change
in the NIRS signals due to movement artifact. Nonethe-
less, there was a transient drop in Oxy-Hb immediately
after the onset of exercise. However, a difference of the
DOxy-Hb
atr
between the Oxy-Hb response in the control
and atropine conditions canceled the movement-related
change in the Oxy-Hb. Finally, as we did not administrate
atropine intra-arterially, atropine was infused into sys-
temic circulation. The dose of atropine administered in
this study was so low as to have almost no detectable
effect on the central nervous system (Brown and Taylor
2001). Indeed, atropine did not change both the RPE
during exercise and vividness score during mental imag-
ery. However, atropine tended to blunt the initial increase
in MAP during exercise, although there was no significant
difference (P>0.05). Such subtle decrease in the initial
pressor response is unlikely to blunt the initial Oxy-Hb
response to one-legged cycling in the atropine condition,
because the Oxy-Hb response of the noncontracting mus-
cle did not always follow the change in MAP. Further-
more, motor imagery induced cholinergic vasodilatation
in bilateral muscles without changing MAP. Taking these
results into consideration, the increased blood flow
response observed at the start of voluntary one-legged
exercise is not simply due to an increase in systemic per-
fusion pressure but is associated with the neurally medi-
ated cholinergic vasodilatation induced by central
command.
In conclusion, we have provided for the first time new
evidence that central command evokes cholinergic vasodi-
latation in skeletal muscle not only during motor imagery
but also at the early period of voluntary one-legged exer-
cise in humans and that the centrally induced vasodilator
signal is likely to be transmitted equally to bilateral skele-
tal muscles. Lee et al. (2007) reported using pseudorabies
virus injection into the rat hind limbs that sympathetic
premotor neurons in the medulla had dual innervation of
bilateral skeletal muscles. If this is also true in humans,
the bilateral cholinergic vasodilatation may be relayed via
such sympathetic premotor neurons. However, precise
neural pathways responsible for the cholinergic vasodila-
tation remain to be studied.
Conflict of Interest
None declared.
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