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Microneurography (MSNA )
is a real-time measure of sympathetic nerve activity. Multiunit recordings of
efferent postganglionic MSNA will be obtained with a tungsten microelectrode
into a muscle fascicle of the peroneal nerve, posterior to the fibular head 180.
Recording will be done randomly in either of the legs using an unipolar tungsten
electrode (uninsulated tip diameter 1 to 5 μm, shaft diameter 200 μm;
Frederick Haer & Co.).Nerve activity will be amplified with a gain of
100,000, band pass filtered (0.7 to 2 kHz), and integrated using a 0.1 sec lag
(University of Iowa, Iowa City). A low impedance reference electrode will be
inserted a few cms away. After acquiring a stable site, resting MSNA is
recorded. The integrated MSNA appear as upright “bursts”. Bursts identified
by inspection of the neurogram will be expressed as burst frequency (bursts per
min)and burst incidence (bursts per 100 heart beats). Criteria for adequate MSNA
recording will include: (1) pulse synchrony; (2) facilitation during Valsalva
straining and suppression during the hypertensive overshoot after release; (3)
increases in response to breath-holding; and (4) insensitivity to startle (i.e.,
loud noise).
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| Amplifier and tungsten electrode inserted in
the peroneal nerve. A separate grounding electrode is in place. This
enables multi-unit recordings of sympthetic nerve recordings from the
muscle. Separate skin nerve recordings can also be accomplished if
needed. |
Central sympathetic effects on vasoconstriction are obtained.
Combined with measures such as vascular ultrasound to measure blood flow, one
can distinguish contributions of sympathetic activity from contributions
of neurovascular transduction to vasoconstriction.
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| Figure shows electrocardiograms in
top panels and corresponding MSNA in lower panels. Representative subjects
with normal and increased MSNA are shown.
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| Using spontaneous
beat-to-beat changes in MSNA and femoral ultrasound velocity (middle panels)
we generate linearized neurovascular transduction relations in the right
panel. LFP subjects with normal resting MSNA are in black while LFP subjects
with increased resting MSNA are in gray.
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| Using the modified Oxford method (vasodilation and
decreased BP with sodium nitroprusside followed by vasoconstriction with
phenylephrine) to produce changes in blood pressure, RR-interval, and MSNA
we generated sympathetic (MSNA vs diastolic BP, upper panel) and cardiovagal
( R-interval vs systolic BP, lower panel) baroreflex function curves. Using
spontaneous beat-to-beat changes in MSNA and femoral ultrasound flow
velocity Subjects with normal resting MSNA are in black while subjects with
increased resting MSNA are in gray. There is flattening of sympathetic and
cardiovagal baroreflex curves near the operating blood pressure in subjects
with increased resting MSNA (gray) while the transduction slope is normal.
This indicates decreased baroreflex sensitivity at a CNS level. On the other
hand there is normal baroreflex sensitivity and set point with increased
transduction slopes in other LFP patients (black) indicating normal MSNA,
and increased transduction-mediated vasoconstriction.
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Microneurography and Muscle Sympathetic Nerve Activity (MSNA)