Elisabeth eLambert

Elisabeth Lambert

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Sympathetic nervous system activity in health and disease—advances with microneurographic recordings

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Frontiers in Physiology

EDITORIAL
published: 157        December 2012
doi: 10.3389/fphys.2012.00470
Sympathetic nervous system activity in health and
disease—advances with microneurographic recordings
Elisabeth Lambert 1,2*
1 Human Neurotransmitters Laboratory, Baker IDI Heart and Diabetes Institute, Melbourne, VIC, Australia
2 Department of Physiology, Monash University, Melbourne, VIC, Australia
*Correspondence: elisabeth.lambert@bakeridi.edu.au
Edited by:
Geoffrey A. Head, Baker IDI Heart and Diabetes Institute, Australia
Reviewed by:
Geoffrey A. Head, Baker IDI Heart and Diabetes Institute, Australia
The recent development and implementation of a catheter-based
approach to denervate the renal sympathetic nerves in patients
with resistant hypertension has ensured that the sympathetic
nervous system (SNS) remains on center stage in cardiovas-
cular medicine (Krum et al., 2009). Given this current inter-
est in the field, the research topic “SNS activity in health and
disease—advances with microneurographic recordings” is timely
as it highlights the technique of microneurography, the only
method available that directly assesses sympathetic nerve firing
in the clinical setting. Microneurography permits the assessment
of muscle (vasoconstrictor) sympathetic nerve activity (MSNA)
in humans and was introduced over 40 years ago (Hagbarth
and Vallbo, 1968). The eight articles compiled demonstrate how
the technique has evolved and, importantly, how it has facili-
tated the understanding of sympathetic regulation in the healthy
state as well as the sympathetic neural abnormalities underlying
a number of conditions ranging from poor orthostatic control to
chronic diseases linked with elevated cardiovascular risk.
Sympathetic neural drive is dependent on a range of factors
including race, sex, and age. This idea is developed further in the
review of Fu (2012) who points out that the sex and age-related
differences in the prevalence of hypertension and cardiovascular
disease in humans may be determined, at least in part, by
differences in sympathetic control. Evidence derived from studies
examining MSNA during phases of the menstrual cycle, pre- and
post-menopause or during pregnancy highlights the role of sex
hormones in neural control in women. Fu also draws attention
to a number of conditions such as postural tachycardia, obe-
sity, polycystic ovary syndrome, hypertensive pregnancy, essential
hypertension, heart failure, and myocardial infarction where ele-
vated sympathetic tone plays a major role in disease severity and
prognosis. In line with the concept of age and sex differences in
the control of SNS activity, the original contribution from Barnes
et al. (2012) reveals distinct differences between young women,
young men, and older women with regards to sympathetic and
cardiac baroreflex sensitivity in response to a hypotensive chal-
lenge, again highlighting the potential importance of female sex
hormones in sympathetic control. Multiunit MSNA has also
been extensively used to investigate conditions linked with ortho-
static intolerance. Ichinose and Nishiyasu (2012) and Ryan et al.
(2012) extensively studied neural responses during syncope or
until the point of decompensation induced by haemorrhage.
Ichinose and Nishiyasu (2012) used multiunit MSNA to assess
the arterial baroreflex (ABR) control under orthostatic stress and
proposed that enhancement of the ABR control under ortho-
static stress is an excellent defence against orthostatic hypotension
and suggest that impairment of ABR control over sympathetic
vasomotor activity leads to the severe hypotension associated
with orthostatic syncope. Ryan et al. (2012) discuss the possible
mechanisms involved in eliciting alterations in sympathetic acti-
vation to explain hemodynamic decompensation during severe
hypovolemia. Altogether, these articles clearly indicate that mul-
tiunit MSNA provides valuable insight into the role of the SNS
in the maintenance of blood pressure during various challenges.
Nonetheless, the use of multiunit MSNA is not limited to this
type of investigations and, as reviewed by Bruno et al. (2012),
recent studies using multiunit MSNA recordings have highlighted
that the SNS may be critically influenced, at the central and
at the peripheral level, by factors regulating vascular function
such as nitric oxide, reactive oxygen species, endothelin, and the
renin-angiotensin system. This is an important issue and certainly
merits further investigation as the interaction between the SNS
and endothelial functionmay be of important clinical significance
(Sverrisdottir et al., 2010).
Three articles acknowledge the limitations of multiunit MSNA
and emphasize the increased information associated with record-
ing single unit MSNA. Single unit recording is technically more
difficult; however, it provides a more precise description of the
way vasoconstrictor neurons behave during a sympathetic burst.
Upon our previous findings, we (Lambert et al., 2012) highlight
the fact that in conditions associated with anxiety disorders, single
unit recording uncovers an atypical firing pattern of vasoconstric-
tor neurons even though multiunit MSNA is normal. We discuss
the role of the firing pattern in sympathetic neural adaptation to
long-term sympatho-inhibition seen during weight loss and the
possible implication of aberration in the firing pattern in car-
diovascular health taking into consideration that multiple firing
during a sympathetic burst may carry adverse end organ conse-
quences. Similarly, in their review, Murai et al. (2012) emphasize
that in congestive heart failure (CHF) patients, the level of mul-
tiunit MSNA is nearly maximal at rest, so the frequency of pulse
synchronized MSNA cannot increase further. Thus, single unit
MSNA analysis is more useful for determining actual sympathetic
neural firing within one cardiac interval. For example, they show
that the firing of multiple spikes within one cardiac interval is sig-
nificantly augmented during the Valsalva maneuver and exercise.
www.frontiersin.org December 2012 | Volume 3 | Article 470 | 1
Lambert Advances with microneurographic recordings
This response is thought to influence strong effector organ
responses, increasing peripheral vascular tone and may be seen
as a contributor to exercise intolerance in these patients. This
idea is reinforced by the findings that aberrant sympathetic fir-
ing pattern is associated with large noradrenaline release from the
heart (Lambert et al., 2011). Macefield et al. (Macefield, 2012)
reinforces the idea of the superiority of single-unit recordings
compared to multiunit MSNA in conditions such as obstruc-
tive sleep apnea, chronic obstructive pulmonary disease, and
bronchiectasis. These conditions are all associated with an
elevation in multiunit MSNA to the same extent to that com-
monly seen in CHF, yet only single-unit MSNA reveals great
differences in the sympathetic firing pattern in these respiratory
diseases.
The SNS has long been recognized to play a central role in
cardiovascular regulation, and targeting sympathetic activity is
certainly a therapeutical approach in the treatment of many car-
diovascular diseases. Thus, a reliable tool to assess SNS activity
such as microneurography remains essential in order to com-
prehend the physiological mechanisms underlying disturbed SNS
activity and to assess the effects of therapeutical strategies. It is
anticipated that assessment of both multiunit and single unit will
remain central in future investigations related to cardiovascular
health.
REFERENCES
Barnes, J. N., Matzek, L. J.,
Charkoudian, N., Joyner, M. J.,
Curry, T. B., and Hart, E. C.
(2012). Association of cardiac
baroreflex sensitivity with blood
pressure transients: influence
of sex and menopausal status.
Front. Physio. 3:187. doi: 10.3389/
fphys.2012.00187
Bruno, R. M., Ghiadoni, L., Seravalle,
G., Dell’oro, R., Taddei, S., and
Grassi, G. (2012). Sympathetic
regulation of vascular function
in health and disease. Front.
Physio. 3:284. doi: 10.3389/fphys.
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Hagbarth, K. E., and Vallbo, A.
B. (1968). Pulse and respira-
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Ichinose, M., and Nishiyasu, T. (2012).
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Krum, H., Schlaich, M., Whitbourn,
R., Sobotka, P. A., Sadowski, J.,
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Lambert, E., Hering, D., Schlaich, M.,
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Lambert, E. A., Schlaich, M. P.,
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Murai, H., Takamura, M., and Kaneko,
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fphys.2012.00109
Ryan, K. L., Rickards, C. A., Hinojosa-
Laborde, C., Cooke, W. H.,
and Convertino, V. A. (2012).
Sympathetic responses to central
hypovolemia: new insights from
microneurographic recordings.
Front. Physio. 3:110. doi: 10.3389/
fphys.2012.00110
Sverrisdottir, Y. B., Jansson, L. M.,
Hagg, U., and Gan, L. M. (2010).
Muscle sympathetic nerve activity
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Received: 13 November 2012; accepted:
30 November 2012; published online: 17
December 2012.
Citation: Lambert E (2012) Sympathetic
nervous system activity in health and
disease—advances with microneuro-
graphic recordings. Front. Physio. 3:470.
doi: 10.3389/fphys.2012.00470
This article was submitted to Frontiers
in Integrative Physiology, a specialty of
Frontiers in Physiology.
Copyright © 2012 Lambert. This is
an open-access article distributed under
the terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in other
forums, provided the original authors
and source are credited and subject to any
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party graphics etc.
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