J Neurophysiol (April 1, 2003). 10.1152/jn.00952.2002
Submitted on Submitted 19 November 2002; accepted in final form 12 December 2002

Magnitude of Oscillations in the Response of Ia Muscle Spindle Endings Under a Static  Stimulation of Increasing Frequency

Department of Neurophysiology (OE 4230), Hannover Medical School, D-30625 Hannover, Germany

Schäfer, S. S., B. Berkelmann, and F. Dadfar. Magnitude of Oscillations in the Response of Ia Muscle Spindle Endings Under a Static  Stimulation of Increasing Frequency. J. Neurophysiol. 89: 1748-1760, 2003. Under static  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  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  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  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  stimulations were identified. The first group of  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  stimuli. According to this observation, the highly irregular discharges result from the fact that the Ia afferent generates more than one action potential per  stimulus. The second group of  stimulation leads in the Ia response either to driving of the action potentials in the rhythm of the  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  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.