Abstract
The terminations of motor axons in the toad glutaeus muscle show a course dependency on the segmental origins of the axons on the spinal cord. Rostral axons in spinal nerve 8 innervate muscle fibres near the ventral surface of the muscle, while caudal axons in spinal nerve 9 innervate fibres mostly towards the opposing dorsal surface. Axons originating between these extremes tend to innervate the central regions of the muscle. A similar topographic projection is reestablished after denervation and when regenerating axons reinnervate the muscle via entirely novel pathways (Brown and Everett [1991] J. Comp. Neurol. 309:495-506). The findings are compatible with the graded expression of a determinant within the glutaeus muscle that biases the formation of synapses between positionally matched muscle fibres and motor axons. In the present work, we provide strong support for this view by showing that when the muscle is reinnervated by axons arising from only one spinal nerve, they expand their projection and form synapses in the muscle in a topographically appropriate manner. In a second experiment, we tested whether a muscle that had regenerated from its resident myogenic (satellite) cell population would be similarly reinnervated. This experiment was prompted by the work of others (Donoghue et al. [1992] Cell 69:67-77) showing that the myogenic precursor cells in adult muscle are a repository of "positional memory." In our experiments, a glutaeus muscle was removed from adult toads and soaked in bupivacaine for a brief period to destroy the muscle fibres before being sutured back into its normal position in the limb. The distribution of motor units in the muscles was determined by glycogen depletion after allowing 3-4 months for the muscles to regenerate from their satellite cell population and to become reinnervated. We found that muscle fibres belonging to single motor units were dispersed widely in the regenerated muscles and showed no topographic organisation. We conclude that the positional cues that direct topographic map formation are available to motor axons when they reinnervate a denervated mature muscle, but play no role in the reinnervation of a regenerated muscle.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.