BOTULINUM-A TOXIN EFFECTS ON RAT JAW MUSCLE-SPINDLES

Botulinum A toxin (Botox) is used for the treatment of many muscular d ystonias. However, the relief of the sustained and abnormal postures i nduced by Botox administration is not fully explained. In this work th e possibility was considered that Botox can produce a block not only a t the alpha motor endings, but also at the gamma motor endings, conseq uently reducing the spindle inflow to the alpha motoneurons, which hav e a great role in maintaining the tonic myotatic reflex. Jaw muscle sp indle discharge was recorded before and after Botox injection in the d eep masseter muscle. The drug consistently reduced the spindle afferen t discharge. Such an effect is suggested to be direct on gamma endings as: i) muscle tension was not modified by Botox during the recording time; ii) saline administration never changed the spindle discharge. T he Botox effect on muscle spindles suggests that the relief from dysto nias could be due not only to a partial motor paralysis, but also to a decrease of the reflex muscular tone.