Spinal motoneuronal excitability in hyperkinesis: effects of the Jendrassik manoeuvre.

H-reflex recovery functions and homosynaptic depression curves were studied in unmedicated hyperkinetic and control children in the presence and absence of the Jendrassik manoeuvre. The Jendrassik manoeuvre failed to affect recovery function reflex amplitude across or within groups, but reflex amplitude was augmented in the homosynaptic depression procedure for hyperkinetic children who failed to show recovery function facilitation. The absence of Jendrassik-associated reflex facilitation in children during the recovery function and during selective homosynaptic depression rates may reflect developmental influences on processes underlying this phenomenon. However, reflex augmentation in non-facilitating hyperkinetic subjects suggests that these processes are present in children and can be revealed under conditions of decreased motoneuronal excitability. These findings suggest that reduced gamma-efferent drive may contribute to decreased motoneuronal excitability observed in hyperkinetic children, but the possible role of other Jendrassik-activated influences, such as coactivation of alpha and gamma motoneurons or selective activation of alpha motoneurons by processes independent of increased gamma-efferent drive, cannot be excluded.