THE CONCEPT OF NERVE‐MUSCLE SPECIFICITY

Summary (1) The concept of nerve‐muscle specificity postulates the existence of an inherent, specific attraction between given striated muscle fibres and given nerve fibres or neurons. (2) The chief theories of nerve‐muscle specificity, those of Furbringer and Cunningham, are considered. Furbringer's theory–that there is an unvarying, specific relationship between particular macroscopic, peripheral nerve branches and particular muscles–is founded upon a false embryological concept and is completely negated by both comparative and experimental data. Cunningham's theory–that there is a constant relationship between given neurons and given muscles but which recognizes variability in pathways adopted by axons and hence in macroscopic innervation –has some comparative anatomical support but is denied by the findings of experimental embryology. The theory that muscular nerve supply is inconstant, and that there may be secondary innervation, especially in phylogeny, is more in accordance with known facts. (3) Existing knowledge of the development of nerve‐muscle relationships–the outgrowth of nerve fibres, the formation of peripheral nerve patterns, and the establishment of terminal connexions–is reviewed. Although a number of important questions remain to be answered, there is nothing to indicate the existence of anything approaching a true nerve‐muscle specificity. (4) It is concluded that present evidence is against the concept of nerve‐muscle specificity. Thus resemblances in muscular nerve supply must merely reflect general similarity in development. Because of this, innervation is of great aid in homo‐logizing the muscles of animals belonging to one vertebrate class, but not in homologizing the muscles of animals of different classes. It is shown that urodeles so differ from other tetrapod vertebrates respecting nerve‐muscle relationships that they form a group unto themselves.