Experimental evidence for saltatory propagation of the Mauthner axon impulse in the tench spinal cord

External longitudinal current recording was applied in situ to the exposed spinal cord of the tench (Tinca tinca) for the study of impulse propagation in the Mauthner axon, a giant nerve fibre whose myelin sheath is not interrupted by nodes of Ranvier. Impulses in the antidromically excited Mauthner axon were recorded from the dorsal surface of the spinal cord; the time-lag between the main peaks of the bipolarly recorded current signal and unipolarly recorded reference signal was measured at regular intervals along the cord. Using a slot width of the bipolar electrodes, d = 0.65 mm and electrode displacement s = 0.5 mm, the latency plotted as a function of distance showed small fluctuations, but no clear-cut steps over a 12.5 mm stretch of spinal cord. However, with improved spatial resolution (d = 0.24 mm, s = 0.1 mm) and electrical insulation of the spinal cord from the underlying tissues, it was possible to demonstrate steps in the latency plot occurring at 0.5--0.3 mm intervals and indicating a saltatory propagation of the Mauthner axon impulse. The distances between the latency steps and their distribution was comparable to the known distribution of the Mauthner axon collaterals suggesting that the myelin-free regions of the collaterals may be quivalents of Ranvier nodes.