Neuronal repair and avoidance behavior in the flatworm, Notoplana acticola.

In Notoplana avoidance behavior is lost after bisection of the brain or removal of one of its lobes. Behavioral recovery usually occurs within 3-10 days. Recovery of individuals may be gradual or abrupt. Grouped data shows gradual linear repair of turning behavior. Most animals with all connectives between the two lobes of the brain severed recovered preoperative responses, while those with one lobe of the brain removed averaged about 60% of the preoperative level of response. Some individuals in both groups recovered completely. Histological evidence of neuronal repair was found in all animals. Where the lobes of the brain were separated, connectives between them appeared to re-form. In worms with one lobe of the brain removed, the nerves disconnected by the excision joined the remaining lobe. Action potentials are conducted across repaired tissue in both split-brain and half-brain worms in both seawater and Mg2+-rich solutions. CNS repair appears to involve functional synaptic contacts. Notoplana does not replace ganglionic tissue but does compensate adequately for CNS damage.