Recovery of locomotion correlated with axonal regeneration after a complete spinal transection in the eel

This research has examined the relationship between axonal regeneration and the return of normal movement following complete transection of the spinal cord. We made measurements of tail beat frequency and amplitude of the caudal body wave from video recordings of eels (Anguilla anguilla) swimming in a water tunnel at several speeds. Each eel was then anaesthetised and the spinal cord cut caudal to the anus; in some animals the resulting gap was filled with a rubber block. All animals were kept at 25 degrees C for recovery periods ranging from 7 to 128 days, during which their swimming performance was monitored regularly. Each fish was then re-anaesthetised and perfused with fixative and the regrowing descending axons labelled with 1,1'-diotadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate. For all animals and at all speeds after surgery, tail beat frequency increased, while amplitude decreased. In non-blocked animals, an improvement in performance was first seen from 8 days following transection and thereafter tail beat frequency decreased progressively until it had returned to normal after 35 to 45 days, while amplitude remained below baseline until at least 45 days. In these animals, few axonal growth cones had penetrated the caudal stump by 7 days, but some had extended as much as 3 mm by 15 days. Many had reached as far as 6 mm between 25 and 36 days, while by 128 days they had progressed up to 10.5 mm. Contralateral crossing was never observed. Functional recovery was never witnessed in animals in which the cord had been blocked and these eels swam at all times with elevated tail beat frequency and reduced caudal amplitude. No labelled axons could be traced into the caudal spinal cord at any recovery stage in such animals. We conclude that re-innervation of only 1-2 segments caudal to the injury is necessary for functional recovery, although continued axonal growth may be important for the refinement of some aspects of movement.