Mechanisms of functional recovery and regeneration after spinal cord transection in larval sea lamprey.

1. Large sea lamprey larvae, close to metamorphosis, regained swimming coordination after several weeks following complete spinal cord transection. Recovery was much faster when animals were kept at 23 than at 12 degrees C. 2. The behavioural recovery involved a regenerative mechanism in the spinal cord, since stimulation of the head resulted in tail curling, even when all tissue other than spinal cord and notocord was stripped away for several cm above and below the transection. 3. Following complete behavioural recovery, stimulation of the rostral cord evoked electrical signals recorded from the cord dorsum for only 10 mm below the transection. 4. Dorsal cells and giant interneurones, which normally project to the brain, could not be antidromically activated across the transection zone. However, giant interneurones could be activated polysynaptically by descending volleys. 5. Twelve of eighteen large reticulospinal axons followed in serial sections regenerated across the glial-ependymal scar, but branched abnormally and migrated away from their customary locations. They became smaller, and were finally lost within 4 mm of the centre of the transection zone. 6. These data suggest that behavioural recovery does not involve long axon tract regeneration. An alternate hypothesis, that short distance sprouting of axons across the transection zone may result in synapse formation with propriospinal interneurones which relay the necessary information, is discussed.