Figure 3

Dorsal versus ventral injury current. Data obtained from "Step-back" experiments on two guinea pig spinal cord crush injury sites. A shows recordings made near the dorsal portion of spinal cord. The first measurement was taken 10 minutes after creating the crush injury at a distance of 30 μm from the injury site. B shows similar recordings obtained near the ventral portion of a spinal cord. The first measurement was recorded also approximately 30 μm from the injury site, and 5 mins after the injury was inflicted. Note the differences in scale of magnitude. The vibrating probe was then stepped back at smaller distances at first and then larger distances afterwards, until the current decayed to a steady level. As explained in the text, an "exponential linear combination" model was used for curve fitting and has been extended to show the theoretical current value at the injured spinal cord surface (x = 0). This model is different from the artificial source model because of obvious differences in the current source geometry (point source as compared to the complicated geometry of spinal cord injury site). The step-back data was corrected for current decay due to time using the time adjustment model and is plotted as well (blue dots). Note that now the current density at distance of up to 2 mm from injury site is much higher. If this plot were to be extended, it would touch the x-axis "cms" away from the injury site. This implies that the injury current field is much larger than expected and might explain the extent to which a focal injury can propagate in distance and time.