Modulation of Human Muscle Spindle Discharge Figure 6. Principal component analysis (PCA) of 55 spontaneously active afferents in control (resting) conditions. Top panels display the first principal component of the time-domain analysis (explained variance = 48%), i.e. R-wave triggered discharge rate averages, extracting the modulation pattern common across afferents. Panel A shows the projection of the data with characteristic regions that are stable (p,0.01) across recordings depicted in gray. The time axis is normalized to the average RR-interval for each afferent before performing PCA. Panel B shows the histogram of the eigenvector coefficients representing the strength of this pattern in each afferent. Afferents with significant pulse wave modulation are depicted in black, non-significant afferents in grey. Lower panels display the first principal component of the frequency-domain analysis (explained variance = 43%), representing the common power spectrum across afferents (Panel C). The frequency axis is normalized to the average heart rate for each afferent before performing PCA. Panels D shows the histogram of the corresponding eigenvector coefficients. doi:10.1371/journal.pone.0035091.g006 comparison, for spontaneously active afferents in relaxed muscles the arterial pulsations accounted for 9.4% (n = 51) of the total variance. During active contraction the size of the net modulatory effect at the population level was considerable - at the peak it was 8.0% of the mean discharge rate. For comparison, the strength of discharge rate modulation in afferents responding to static stretch was 7.2%. Discussion We have shown that the majority of human muscle spindle afferents show a pulse-wave modulation of their discharge rate – both in relaxed muscles and during contraction. Pulse wave modulation was present in both primary and secondary muscle spindle afferents, although it was less frequent for secondary spindle endings. We describe three types of pulse wave influences on afferent discharge: (i) muscle spindles driven by arterial pulsations – afferents responding exclusively to the pulse wave with every spike phase-locked to the cardiac cycle, (ii) muscle spindles phase-locked by arterial pulsations – afferents in which spike timing of background activity becomes phase-locked to the cardiac cycle, and (iii) pulse wave modulated muscle spindles - afferents in which arterial pulsations increase or decrease the discharge rate of ongoing background activity while individual spikes are not phase-locked to the arterial pulsations. It has to be emphasised, though, that in certain conditions a given muscle spindle afferent may change its behaviour. For example, muscle spindles driven by arterial pulsations in relaxed muscle may become phase-locked or pulse-wave modulated when background activity is evoked by muscle stretch. Muscle spindles that were driven by arterial pulsations typically responded with one or two spikes at the time of the systolic pressure peak, while some afferents also responded to the dicrotic notch. In our sample of afferents we encountered spontaneously active Ia afferents in which the pulse wave caused ongoing spontaneous activity to phase-lock spikes to the cardiac cycle. This is the type of theoretical possibility McKeon and Burke [4] referred to as ‘‘resetting’’; however, they were unable to find any muscle spindle showing this kind of behaviour. No such afferents have been previously reported in the literature, indicating that they are not common; however, they may remain largely undetected unless they are specifically searched for. The importance here is that muscle spindle afferents showing this type of behaviour potentially may pose problems for the application of spike-triggered averaging methods used to determine the synaptic connectivity between muscle spindle afferents and alpha motor PLoS ONE | www.plosone.org 10 April 2012 | Volume 7 | Issue 4 | e35091