A new simulation method to deduce fusimotor activity from afferent discharge recorded in freely moving cats

Direct recordings from identified and classified fusimotor gamma-motoneurons in freely moving animals have not so far been achieved, and present knowledge on fusimotor activity during natural movements is based on qualitative inferences made from spindle afferent discharge. In order to put such deductions on a firmer basis, a simulation method has been developed, which provides quantitative estimates of the fusimotor drive that shaped the spindle afferent discharge, as recorded in chronically implanted cats during voluntary or imposed movements. Simulations are performed in acute experiments on anesthetized cats, whereby variations in muscle length and EMG envelopes are reproduced by an electromagnetic servo from digitally stored segments of the original records. The responses of spindle afferents to the simulated movements are then examined, both in the absence of fusimotor action and during concomitant stimulation of functionally single gamma-motoneurons, rate modulated according to a variety of stored functions (including the original EMG envelope).