The effect of the local anaesthetic bupivacaine on the muscle spindle of rat.

The effects of a long-lasting local anaesthetic, bupivacaine, in combination with hyaluronidase, on the intrafusal muscle fibres of adult rat muscle spindles have been investigated by light and electron microscopy. Necrotic changes in the axial bundle are present within 4 h of the combined drug injection and are widespread by 2 days. The equatorial nucleation of the spindle is lost owing to the necrosis of the myonuclei. Satellite cells, however, appear to survive these changes, even where the underlying muscle fibres are grossly necrotic. Sensory nerve terminals undergo necrosis during this period and plate-type fusimotor nerve terminals withdraw from the degenerating muscle fibres. Macrophage infiltration and early regeneration of the axial bundle is apparent by the third day. Myoblasts first appear at the periphery of the affected muscle fibres, but decrease in number as regeneration proceeds. By the end of the third week, regeneration of the muscle component is complete and re-innervating fusimotor nerve terminals established. The encapsulated regions of the spindle remain abnormal, without recognizable bags or chains of nuclei, although the muscle fibres differ both in size and ultrastructure. Regenerating sensory axons make contacts with the intrafusal bundle that differ in their configuration and ultrastructure from normal terminals. The production of enucleated spindles is discussed in the light of the morphogenetic influences of the spindle nerve terminals, as well as the possible use of such a model in discovering the function of nuclear bags and nuclear chains.