By recording single-fibre action potentials (APs) from human sacral nerve roots, the impulse patterns of 6 secondary spindle afferents could be identified, analysed and compared with the impulse trains of oscillatory firing alpha 2-motoneurons. 2. Secondary spindle afferents fired with single APs, doublets, bursts and equidistant APs. On the assumption that the most simple firing pattern of a single fibre with a shortest interspike interval (II) of 80 msec was generated by a single encoding site, the firing patterns of the other afferents could be splitted into firing patterns of single encoding sites with similar II distributions and also with a shortest II of about 80 msec. 3. Successive IIs also reflected the different ways of firing of the secondary muscle spindle afferents. A fibre from a paraplegic with bladder dyssynergia seemed to show a higher activity than a comparable probably normal fibre. The doublet IIs of two secondary spindle afferents ranged from about 9 to 14 msec with peaks at 10.2, 11.2 and 13 msec in the distributions. Multiple encoder sites increased discharge rates of parent fibres, but did not always regularize the output discharge. 4. The simultaneously plotted impulse patterns of 4 secondary spindle afferents showed coordinated firing: they did not fire simultaneously but regularized the summed activity. A phase of 50 msec (range 30 to 70 msec) often occurred between the spindle afferent APs and the impulse trains of an oscillatory firing alpha 2-motoneuron. An afferent drive of the oscillator with a certain phase is likely.
