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).

 

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. 

Figure shows electrocardiograms in top panels and corresponding MSNA in lower panels. Representative subjects with normal and increased MSNA are shown.

 

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. 

 

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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Peripheral Vascular Properties Measured by Strain Gauge Plethysmography (SPG)
Laser Doppler Flowmetry (LDF)
Indicator Techniques to Measure Blood Volume and Cardiac Output
Heart Rate and Blood Pressure
Near Infrared Spectroscopy
Microneurography and Muscle Sympathetic Nerve Activity (MSNA)
Transcranial Doppler Ultrasound (TCD)
Impedance Plethysmography (IPG)
Skeletal Muscle Pump