EXCITATION OF INTRASPINAL MAMMALIAN AXONS BY NERVE IMPULSES IN ADJACENT AXONS

Abstract

The dorsal column of the spinal cord contains the ascending branches of sensory fibers which enter the cord over the ipsilateral dorsal roots. For a period of time after transec-tion of the dorsal column, at a level cephalad to the entry of a stimulated dorsal root, impulses in the ascending branches of the active fibers directly excite adjacent axons. The impulses in the secondary axons then travel anti-dromically (caudally) and emerge as a centrifugal discharge in dorsal root fibers adjacent to those which carried the centripetal volley. The secondary impulses are initiated by processes contemporaneous with the arrival of primary impulses at the region caudad to the cut, and before post-synaptic spinal neurons become active. Subthreshold increases in the excitability of tested dorsal column axons are produced by a primary volley which does not actually initiate secondary impulses. The increase in excitability is greatest 2-3 mms. caudad to a cut. At this locus the maximal excitability coincides with the time at which the conditioning impulses produce the greatest relative negativity. Before section of the dorsal column, the excitability of tested axons is decreased as impulses pass by in adjacent axons. Transection of the dorsal column produces an immediate increase in the size of the motor discharges that are evoked by dorsal root volleys.