Long range afferents in rat spinal cord. III. Failure of impulse transmission in axons and relief of the failure after rhizotomy of dorsal roots.

Dorsal root afferents entering the spinal cord form a T-junction with a caudal branch descending many segments and giving off side branches terminating in the dorsal horn. This anatomical finding contrasts with the physiological observation that the postsynaptic effects of arriving afferents in the dorsal horn are limited to a few segments on either side of the root carrying the input. This paper explores the possibility that one explanation for this paradox is that orthodromic impulse conduction fails to penetrate the long range parts of the caudal branch. The experiments show that when all roots are intact, very few fibres can be detected carrying orthodromic impulses as far as 20 mm caudal to the entry point. After section of neighbouring dorsal roots, however, large numbers of conducting fibres can be recorded at that point. Signs of orthodromic conduction begin 7 days after root section. These results were confirmed by another method which compared the relative refractory period of the membrane of the descending branch produced either after a local stimulus had evoked an action potential or after a rostral distant stimulus had produced an orthodromic action potential. Again, in the intact cord, the results indicate that impulses fail to penetrate long distances into the descending branches but that, as soon as 2 days after rhizotomy in the area of suspected conduction failure, orthodromic conduction is restored. It is proposed that the failure and release of conduction may depend on the control of membrane potential in the primary afferents, which would form a pre-presynaptic control mechanism.