Nodal and terminal sprouting from motor nerves in fast and slow muscles of the mouse.

1. A study of nodal and terminal sprouting in fast and slow muscles of the mouse hind limb has been made using the zinc iodide and osmium tetroxide stain. 2. The terminal sprouting normally elicited by botulinum toxin injection can be prevented by regular and frequent direct electrical stimulation of the muscle fibres. But the number of end-plates innervated by nodal sprouts in partly denervated spinal preparations was not reduced by direct muscle stimulation. 3. In leg muscles given varying doses of botulinum toxin the amount of terminal sprouting was linearly related to the degree of paralysis. In partly denervated muscles neither the amount of terminal sprouting nor the amount of nodal sprouting was correlated with the degree of denervation. 4. Partial denervation causes relatively more nodal sprouting in the fast muscles peroneus tertius and extensor digitorum longus than in the slower soleus muscle, which itself has considerably more terminal sprouting than the others. The fast muscles can develop as much terminal sprouting as the soleus only in response to full paralysis with botulinum toxin. 5. No evidence could be found for a sprouting signal generated or spreading within the spinal cord. 6. It is concluded in confirmation of earlier work (Duchen & Strich, 1968; Brown & Ironton, 1977 a) that the source of the signal for terminal sprouting is denervated or otherwise inactivated muscle fibres, whose action is boosted by the presence of degenerating nervous tissues. It is suggested that fast muscles probably have less terminal sprouting when partly denervated than slow muscles (a) because of the longer time it takes a fast muscle to undergo the changes associated with inactivity and (b) because of their higher resistance to the effects of nerve degeneration. It does not seem that the signal for nodal sprouting comes from the muscle fibres but further experimentation is needed to establish this firmly.