Physiological adjustments in a reflex pathway following partial loss of target neurons.

This investigation was undertaken to study plasticity in a reflex pathway following partial elimination of target neurons. Adult cats were subjected to unilateral avulsion of the L7 spinal ventral root, which induces retrograde cell death among the motoneurons of the L7 segment. At 1, 3, 6 and 12 weeks after the lesion, the monosynaptic reflexes were recorded in the L6 and S1 ventral roots during stimulation of the L6, L7 and S1 dorsal roots. Since the group Ia muscle spindle afferents passing through these dorsal roots were deprived of their target motoneurons in the L7 segment, compensatory reflex changes were searched for in the remaining monosynaptic contacts with the intact target motoneurons of the L6 and S1 segments. The results indicate that a partial loss of target motoneurons triggers changes leading to increased monosynaptic reflexes of the remaining intact target motoneurons. On average, the reflexes had more than doubled their size at 12 weeks postoperatively. Possible mechanisms for this reflex potentiation are discussed.