Muscle sympathetic nerve activity

What is MSNA?

Muscle sympathetic nerve activity (MSNA) is recorded through the use of microneurography. This is the process of percutaneously inserting a tungsten microelectrode into a nerve to record action potentials. This is typically done in accessible peripheral nerves such as the peroneal or median nerves.

The MSNA toolkit supports the calculation of common parameters associated with this technique.

MSNA extraction

Following the aggregate phase, this video demonstrates how quickly the important metrics can be found in your data.

Parameter outputs

ParameterDefinition
Total activityDetermines the average area under the curve for detected MSNA bursts.
Bursts/100 heart beatsBurst count per 100 heart beats.
Bursts/minuteBurst count per minute.
Burst area/minuteBurst area per minute.
Baroreflex gainSlope relating diastolic blood pressure (in bins) and MSNA activity. Also referred to in the literature as baroreflex sensitivity (BRS).
This would be expected to be a negative number, but its absolute value is sometimes quoted as the uncertainty in the T50 (below).
T50The diastolic blood pressure at which 50% of R-R intervals have an associated MSNA burst.
Error SignalThis is the T50 after the mean diastolic blood pressure has been subtracted.

Control definitions

ParameterDefinition
Nerve signal selectDefines the channel that corresponds to the integrated MSNA neurogram. Please ensure accurate channel labelling during Aggregate.
Artifact thresholdAdjusts the step change between adjacent samples
that causes a peak to be rejected from being a MSNA burst (most likely
a noise spike). In effect this is controlling the allowed maximum
SLOPE of the signal. The HIGHER this control is set, the more signals
will be considered to be MSNA bursts.
Burst thresholdThreshold for determining what constitutes a valid burst in the neurogram. The criteria for burst identification are based on amplitudes and the slopes of rise and fall of candidate bursts compared against critical values (1). The height, rise slope, and fall slope are scaled proportionally such that if 2 of the 4 criteria are met, the area surrounding the burst is set to zero, allowing for beat-by-beat detection of MSNA bursts in the neurogram.

The threshold is applied to a filtered version of the signal that you
can't see, but the levels will correspond approximately to the scaled
waveform that you can see. The LOWER this control is set, the more
signals will be considered to be MSNA bursts.
Pressure bin sizeSize of bins for grouping diastolic blood pressures. Default is 3 mmHg.
Remove artifactsRemoves MSNA signal artifacts.
Burst areaSelects the method for calculating burst areas. No criterion = Areas for all the cardiac cycles, regardless of whether a burst has been detected or not; Criterion = Areas of all the bursts (value of zero attributed to cardiac cycles where it has been decided that there is no burst).
BRS methodSelects the method for calculating BRS slopes.
1 = plots the mean MSNA area/heart beat (y-axis) vs. diastolic blood pressure bins (x-axis).

2 = same as option 1 but slopes are statistically weighted by the number of cardiac cycles in each diastolic blood pressure bin. This technique is equivalent to the technique described by Halliwill, 2001.

3 = plots the burst count(burst area is ignored; y-axis) vs. diastolic blood pressure bins (x-axis).

4 = same as option 3 but slopes are statistically weighted by the number of cardiac cycles in each diastolic blood pressure bin. This technique is equivalent to the technique described by Kienbaum et al., 2001.
NormaliseBy default, this control is ON, so that the signal is normalised to between 0 and 1 (technically it is scaled and shifted to that the first and 99th percentile are at 0 and 1 respectively, to make the normalisation process robust against noise spikes). But if you want different sections of the file (e.g., 15 second blocks) to all be normalised to the same level, you can apply the normalisation back at the Aggregate stage (by checking the box labelled "Scale" on the Extras screen), and then turning OFF the normalisation on this MNSA screen.

Graph definitions

GraphDefinitions
Integrated nerve inputx-axis is time in seconds.

y-axis is integrated MSNA neurogram.
Burst area (criterion)x-axis is time in seconds.

y-axis is the burst area in AUR/heart beat or bursts/100 heart beats depending on the method used.
Baroreflex sensitivityx-axis is diastolic blood pressure in mmHg.

y-axis is AIU/heart beat (BRS method 1 or 2), or bursts/100 heart beats (method 3 or 4).
Burstsx-axis is time in seconds.

y-axis is burst amplitude.

References

(1) Hamner JWTaylor JA. Automated quantification of sympathetic beat-by-beat activity, independent of signal quality.  2001 Sep;91(3):1199-206.

(2) Kienbaum PKarlssonn TSverrisdottir YBElam MWallin BG.Two sites for modulation of human sympathetic activity by arterial baroreceptors? 2001 Mar 15;531(Pt 3):861-9.

(3) Halliwill JR. Segregated signal averaging of sympathetic baroreflex responses in humans.  2000 Feb;88(2):767-73.

Autoregulatory index

Cerebral autoregulation can be evaluated by measuring relative blood flow changes in response to a steady-state change in the blood pressure (static method) or during the response to a rapid change in blood pressure (dynamic method).