Long-lasting facilitation of a monosynaptic pathway as a result of "partial" axotomy of its presynaptic elements.

We investigated the long-lasting effects of severing an axonal branch on the function of the synapses made by the remaining branches of the "partially" transected axon. The experimental model used was the monosynaptic reflex pathway of the cat lumbar cord. Preliminary surgical procedures consisted of a dorsal column (DC) hemisection at the L3--L4 level on the left side under aseptic conditions. During the final recording experiments, the monosynaptic reflex (MSR) of the operated and normal (control) side was studied in motorpools in the L7 and S1 segments. The results indicate that the DC operation produced on the operated side (a) an enhancement of the MSR and (b) a more powerful postetanic potentiation, both of the "early" and "late" varieties. These effects were seen as early as 34 days and as late as 121 days after the original DC hemisection. Control experiments were carried out to exclude denervation supersensitivity, supraspinal effects, and normal asymmetries between sides as the mechanism underlying these effects. A presynaptic change in the operated axons, resulting in an increased synaptic efficiency, is proposed as the possible mechanism.