Extent of branching and conduction velocities of ascending and descending primary afferents in isolated spinal cord of the rat and hamster

Wall and Shortland (1991) have shown that afferent fibers entering the cord in thoracic and lumbar roots of adult rats have branches that may penetrate up to 11 segments caudally from the root entry zone. We have investigated the extent of branching and conduction velocities of ascending and descending branches of lumbar and thoracic primary afferents in isolated spinal cords of adult hamsters (60-110 g) and juvenile rats (30-46 g). Hemisected spinal cords were maintained at cold artificial cerebrospinal fluid (25-27 °C) in which the Ca2+ had been replaced by Mn2+ (2 µM) to block synaptic activity. Hamsters' antidromic conducted responses were obtained up to 10 segments caudal to dorsal roots T6- T7 and 17 segments rostral to L3. The mean conduction velocity was 3.7 m/s (S.E.M. ± 0.4) for descending primary afferents (n=2l) and 9.9 ± 1.2 m/s for ascending afferents (n=26). In the rat, antidromic responses were measured over 4 segments caudal and 17 segments rostral to the L3 dorsal root. Conduction velocities were 4.4 ± 0.5 m/s and 7.7 ± 0.7 m/s for descending and ascending branches, respectively (n=4). The results were similar to those reported by Wall and Shortland, although the temperature difference between the in vitro and in vivo preparations would have resulted in approximately halving of the velocity in isolated cord preparation. The conduction velocity of descending fibers was about half of ascending fibers which may be related to the differences within the branches.