Figure 1. Effects of sympathetic activation on a spindle secondary ending.

Recording from a spontaneously active secondary muscle spindle ending in tibialis anterior during passive dorsiflexion of the ankle, which unloads the spindle (A), and a sustained maximal inspiratory breath-hold, which increases muscle sympathetic drive but does not change the spindle firing (B). Superimposed spikes are shown in the inset of A. The inset of B shows an expanded section and illustrates the far-field muscle sympathetic activity detected from the same microelectrode (RMS-processed nerve signal). BP, blood pressure.