Testing the classification of static gamma axons using different patterns of random stimulation

Testing the classification of static gamma axons using different patterns o f random stimulation. J. Neurophysiol. 81: 2823-2832, 1999. The possibility of using randomly generated stimulus intervals (with a Poisson distributio n) to identify the type(s) of intrafusal fiber activated by the stimulation of single static gamma axons was tested in Peroneus tertius muscle spindle s of anesthetized cats. Three patterns of random stimulation with different values of mean intervals [20 +/- 4.47, 30 +/- 8.94, and 40 +/- 8.94 (SD) m s] were used. Single static gamma axons activating, in single spindles, eit her the bag, fiber alone or the chain fibers alone or both types of intrafu sal fiber were prepared. Responses of spindle primary endings elicited by t he stimulation of gamma axone were recorded from Ia fibers in cut dorsal ro ot filaments. Cross-correlograms between stimuli and spikes of the primary ending responses, autocorrelograms: interval histograms of responses, and s timulations were built. The characteristics of cross-correlograms were foun d to be related not only to the type of intrafusal muscle fibers activated but also to the parameters of the stimulation. Moreover some cross-correlog rams with similar characteristics were produced by the activation of differ ent intrafusal muscle fibers. It also was observed that, whatever the type of intrafusal muscle fiber activated, cross-correlograms could exhibit osci llations after an initial peak, provided the extent in frequency of the pri mary ending response was small; these oscillations arise in part from the a utocorrelation of the primary ending responses. Therefore, cross-correlogra ms obtained during random stimulation of static gamma axons cannot be used for unequivocally identifying the type(s) of intrafusal muscle fiber these axons supply.