Action potential patterns of intrafusal gamma and parasympathetic motoneurons, secondary muscle spindle afferents and an oscillatory firing alpha 2-motoneuron, and the phase relations among them in humans.

1. Single-fibre action potentials (APs) were recorded from a S4 root of a brain-dead human and a patient with a spinal cord lesion, with 2 pairs of wire electrodes, and distribution histograms of conduction velocity frequencies were constructed. By plotting the intrafusal velocity main gamma peak on a logarithmic scale, the peak splitted into one dynamic ad two static gamma-peaks and an additional peak of parasympathetic fibres. 2. Apart from the doublet firing, a single gamma 1 (dynamic) and a single gamma 21-motoneuron (static) fired with shortest interspike intervals of 80 msec, similarly as a simultaneously firing secondary spindle afferent fibre; with 13 msec the doublet interspike interval of the gamma 1-motoneuron was very similar to that of the spindle afferent fibre. The intrafusal motoneurons fired with patterns very similar to those of secondary spindle afferents, and they did not fire in the oscillatory mode for high activations as alpha 2 and alpha 3-motoneurons do. 3. The stability of the oscillatory firing of a sphincteric alpha 2-motoneuron increased with the increasing phases of approx. 50 msec between the APs of the gamma-motoneurons and the impulse train of the motoneuron following bladder catheter pulling. Moreover, in a certain phase range the number of phases between the APs of a gamma 1-motoneuron and a secondary spindle afferent fibre increased upon stimulation and, as shown earlier, also the number of phase correlations between the APs of some secondary spindle afferents and the impulse train of the oscillatory firing alpha 2-motoneuron increased. Phase relations between the APs of two spindle afferents also increased upon the bladder catheter pulling. During the oscillatory firing of the alpha 2-motoneuron a phase loop seemed to exist from the APs of the alpha 2-motoneuron to the APs of certain gamma-motoneurons, to the APs of certain secondary spindle afferent fibres and back to the APs of the alpha 2-motoneuron, in similarity to the phase recoupling of electronic oscillators. 4. Single gamma-motoneurons changed their activity levels rhythmically, similarly as the secondary spindle afferents, often with a frequency of 0.3 Hz. Parasympathetic intrafusal motoneurons fired at a constant activity level for about 10 sec, and activated up to 6 single encoding sites of a parent secondary spindle afferent fibre. Shortly after the recruitment of a new encoding site, the interspike intervals of the parent fibre were similar again.(ABSTRACT TRUNCATED AT 400 WORDS)