Tonic ankle reflex in parkinsonian rigidity and in spasticity. The role of the fusimotor system.

Abstract

The size of the tonic ankle reflex response to a constant submaximal mechanical stimulus was recorded electrically and mechanically. Electrically the response was measured by integrating, voltage-to-frequency converting, and counting the electromyogram. Mechanically the response was measured by the extension of the triceps surae, as indicated by the potentiometer of a myograph, giving the degree of dorsiflexion of the ankle joint at different loads. Ten normal controls, 20 Parkinsonian patients, and 10 spastics were examined.—In normal subjects the sustained muscle stretch resulted in no significant tonic reflex response. In all Parkinsonians the same muscle stretch elicited a definite tonic reflex. In spastics the mean value was intermediate to the two groups.—A peripheral selective blockade of small sized nerve fibres in three Pakinsonian patients caused a temporary decrease in the augmented tonic reflex contraction without any blocking effect of the alpha motor or muscle spindle Ia afferent fibres.—The increased position sensitivity and the modest velocity sensitivity to length increment of the Parkinsonian muscle, together with the effects of the peripheral selective nerve blockade, suggest that an increased fusimotor innervation to the static muscle spindle system contributes to the rigidity in Parkinsonians.