- L. Rosalesa and D. Dresslerb
Keywords: botulinum toxin, dystonia, spindles
Dystonia may produce co-contractions and constant strain in numerous muscle fibers, including those of the muscle spindles. As proprioceptors, muscle spindles detect dynamic or static changes in muscle length and their afferent projections to the spinal cord play a central role in control of antagonistic muscles. Their parallel arrangement with extrafusal muscle fibers and association with the earlier recruited oxidative motor units allow them to conveniently sample the activity of all motor units and effectively modulate movement. At the same time, fusimotor muscle spindle innervation contracts the striated polar portions of the intrafusal muscle fibers and prevents their slackening during extrafusal muscle contractions. Botulinum toxin remains the most efficient therapy of dystonia. Its muscular mechanism of action is hinged on cholinergic blockade not only of extrafusal, but also of intrafusal muscle fibers. Besides being a targeted muscular therapy, the alteration of the corresponding sensory input following an effect of botulinum toxin on the intrafusal muscle fibers is pivotal in modulating loss of pre-synaptic inhibition in dystonia, including suppression of the tonic vibration reflex. Whether or not trans-synaptic botulinum toxin migration occurs, a modification of the central motor programming is bound to happen in dystonia, with botulinum toxin acting either as another _sensory trick_ or as a form of_short-term plasticity_. Knowledge of the muscle spindle anatomy and function is key to unify our understanding of abnormal movements and of effects of botulinum toxin therapy. Thus, in dystonia, overactivity of muscles and increased spindle sensitivity are germane to botulinum toxin targets of action.
Muscle hyperactivity links together dystonia, muscle spindles (MuS) and botulinum toxin (BT) therapy. The present manuscript attempts to summarize the intriguing information on MuS functioning in dystonia and in BT therapy as it arises from recent systematic studies.
Muscle spindles and movement
Spindle structure and function
The anatomical and physiological properties of MuS are detailed in the reviews by the groups of Banks , Kandel , and Pierrot-Deseilligny with Burke . MuS, being proprioceptors, are of highest density (40 MuS/gr muscle) in small neck muscles. Larger, more axial muscles have fewer MuS in relation to their bulk [3,4]. In neck muscles, complex MuS involving more than one spindle or a spindle/tendon organ combination, and spindle in tandem with two or more spindles end to end, are common . Contrary to previous assumptions, there are MuS in facial/jaw muscles and their existence can be quickly demonstrated by elucidation of typical stretch reflexes. In fact, spindle clusters in masseters have been found and only two cranial muscles in humans (lateral pterygoid and mylohyoid) are recorded by Voss  as having no MuS. Arranged in parallel with extrafusal muscle fibers (EF) the mammalian MuS contains two types of intrafusal muscle fibers (IF), namely nuclear bag and nuclear chain fibers. Both types of IF have in turn large diameter Group IA afferents from primary annulospiral endings, as well as the smaller diameter Group II afferents from flowerspray secondary endings. The mechanical responses of Group IA and II afferents differ in that the former is sensitive to changes in both muscle length and velocity, whereas the latter is mainly sensitive to instantaneous change in muscle length. These kinetic and static responses of both MuS afferents are modified by c-motorneurons which innervate both IF.