Magnitude of oscillations in the response of Ia muscle spindle endings under a static gamma stimulation of increasing frequency.

Under static gamma stimulations, Ia afferents may discharge in a highly irregular way or may be driven. However, the genesis of the highly irregular form of discharge is unclear. We offer an interpretation of irregular discharge behavior. Twenty-three primary (Ia) muscle spindle afferents from the tibial anterior muscle of the cat were subjected to static gamma stimulation, the stimulation frequency increasing linearly from 2 to 110/s. In addition, 17 of the spindle afferents were subjected to two different prestretch values of the muscle while the static gamma fiber was now subjected to constant frequency stimulation at five different stimulation frequencies ranging from 9.4 to 95/s. The responses of the Ia afferents to the static gamma stimulation were presented through discharge patterns that were constructed by the frequencygram method and were subjected to computer analysis, by means of which the Ia responses were evaluated quantitatively. Two groups of static gamma stimulations were identified. The first group of gamma stimulations leads in the Ia response to highly irregular discharging within a broad discharge band. This highly irregular discharging resolves into regular oscillatory responses of large magnitude occurring in the rhythm of the gamma stimuli. According to this observation, the highly irregular discharges result from the fact that the Ia afferent generates more than one action potential per gamma stimulus. The second group of gamma stimulation leads in the Ia response either to driving of the action potentials in the rhythm of the gamma stimulation frequency or of submultiples of it or to irregular discharging within a smaller discharge band. Under the two latter conditions, oscillatory Ia responses of small magnitude occurring in the rhythm of the gamma stimuli are proved to be generated by the Ia afferents. The results are explained in terms of the strength of contraction of the polar parts and the resulting stretch of the sensory parts of the intrafusal muscle fibers that are responsible.