Nerve sheaths and motoneurone collateral sprouting

Abstract

When disease or injury causes partial loss of innervation from a muscle, the remaining axons sprout and form new connections to the denervated muscle fibres¹. Sprouting can occur in two ways: from axon terminals (terminal sprouting) or from the intramuscular axons themselves, probably from the nodes of Ranvier (collateral sprouting)^1,2. Terminal sprouting has been induced experimentally using various methods, including partial denervation², nerve conduction block³ and nerve transmission block^4,5. A common factor in the induction of terminal sprouting seems to be changes in the surface membrane of muscle fibres^6,7; these changes and terminal sprouting are prevented by direct stimulation of the muscle⁵. Collateral sprouting has been induced only by partial denervation and is not prevented by direct stimulation⁵. This has been taken as evidence⁶ for an earlier suggestion² that products of nerve or axon degeneration may be a direct stimulus for collateral sprouting. We report here that axon degeneration products alone are probably not the stimulus for collateral sprouting.