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1 Physiology, Emory University, Atlanta, GA, USA
2 Physiology, Emory University, Atlanta, GA, USA; Neurobiology, Univ School Physical Education, Poznan, Poland
* To whom correspondence should be addressed. E-mail: vhaftel{at}morehouse.edu.
Among the mechanisms that may result in modulation of the stretch reflex by the recent history of muscle contraction is the history dependence observed under some conditions in the response properties of muscle spindles. The present study was designed to test one report that in successive trials of muscle stretch-release, spindle afferent firing during stretch, i.e. the dynamic response shows no history dependence beyond the initial burst of firing at stretch onset. Firing responses of spindle afferents were recorded during sets of 3 consecutive trials of triangular stretch-release applied to triceps surae muscles in barbiturate-anesthetized rats. All 69 spindle afferents fired more action potentials (spikes) during the dynamic response of the first trial, excluding the initial burst, than in the following 2 trials. This reduced dynamic response (RDR) was nearly complete following trial 1 and amounted to an average of about 12 fewer spikes (16 pps slower firing rate) in trial 3 than in trial 1. RDR was sensitive to the interval between stretch sets, but independent of stretch velocity (4-32 mm/s). RDR was reflected in the synaptic potentials recorded intracellularly from 16 triceps surae 
-motoneurons: depolarization during muscle stretch was appreciably reduced following trial 1. These findings demonstrate history dependence of spindle afferent responses that extends throughout the dynamic response in successive muscle stretches and that is synaptically transmitted to motoneurons, with the probable effect, unless otherwise compensated, of modulating the stretch reflex.
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