Spindle discharge in glucocorticoid-induced muscle atrophy

To see if the discharge behavior of muscle spindles is altered in glucocorticoid-induced muscle atrophy, the firing of Ia and II afferent units in deefferented medial gastrocnemius muscles of male cats given triamcinolone (4 mg/kg) for 10 to 12 days was monitored under static conditions of stretch and compared with results from untreated controls. No difference in mean rate of discharge from spontaneously firing units nor of nonspontaneous units at their length threshold was found. However, the incidence of spontaneous units in the treated animals was greater, and consistent with this the length threshold for nonspontaneous units was less. Position sensitivities were significantly reduced. Estimates made of volumes of discharge arising from total muscle populations of Ia and II afferent fibers indicated that they were increased throughout the range of muscle lengthening. In contrast with the control muscles, there was a greater volume of discharge from Ia than group II afferent fibers for most of the lengthening range. It is considered that these alterations were due to mechanical factors, rather than any direct effect on the sensory terminal.