Botulinum A toxin effects on rat jaw muscle spindles.

Botulinum A toxin (Botox) is used for the treatment of many muscular dystonias. However, the relief of the sustained and abnormal postures induced by Botox administration is not fully explained. In this work the possibility was considered that Botox can produce a block not only at the alpha motor endings, but also at the gamma motor endings, consequently reducing the spindle inflow to the alpha motoneurons, which have a great role in maintaining the tonic myotatic reflex. Jaw muscle spindle discharge was recorded before and after Botox injection in the deep masseter muscle. The drug consistently reduced the spindle afferent 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. The Botox effect on muscle spindles suggests that the relief from dystonias could be due not only to a partial motor paralysis, but also to a decrease of the reflex muscular tone.