An assessment of the spread of the signal for terminal sprouting within and between muscles.

Certain muscles of the mouse and rat have been studied in order to assess how far a signal from denervated muscle can spread to elicit terminal sprouting from intact endplates. Denervation of the muscles surrounding the rat foot 4th lumbrical muscle caused no terminal sprouting in the 4th lumbrical itself. In the hemidenervated mouse gluteus maximus terminal sprouting was restricted to the central region of the muscle where innervated and denervated fibres intermingle. There was no enhancement of such sprouting if the underlying and closely apposed gluteus medius was simultaneously denervated. Hemidenervation of the mouse diaphragm and interscutularis, where intact endplates lie near to denervated muscle fibres, produced no terminal sprouting. Hemidenervation of the mouse platysma, where intact endplates often lie adjacent to denervated muscle fibres, similarly produced no significant response. However, all muscles were capable of producing extensive terminal sprouting in response to paralysis induced by botulinum toxin. The stimulus for terminal sprouting produced by an inactive muscle fibre must therefore be effective only on the fibre's own terminal or immediately adjacent terminals.