Synaptic effects in lumbar alpha-motoneurones evoked from group II muscle afferents via two different interneuronal pathways in the cat.

Conditioning of monosynaptic reflexes was used to investigate group II excitation from quadriceps (Q) and sartorius (Sart) in posterior biceps-semitendinosus (PBSt) motoneurones and different lesions were made to analyze the interneuronal pathways. The purpose of the investigation was to find if the excitation from group II Q and Sart afferents, which enter the spinal cord in L4-L6, is relayed to motoneurones by interneurones located in these segments and/or in the same segments (L7-S1) as the PBSt motoneurones. In some experiments a transection of the dorsal column in L6, which interrupts the group II input from Sart and Q to interneurones in the L7-S1 segments, resulted in a marked decrease (by about 2/3) of group II excitation from these nerves. In other experiments the same lesion did not reduce the group II effect. In the latter cases the effect could be abolished by a second lesion of the lateral funiculus (LF) in L5-L6. It is postulated that potent effects from Q and Sart group II afferents can be evoked by interneurones located in L7-S1, i.e. in the same segments as the PBSt motoneurones (L7-S1), and/or via interneurones in more rostral segments. The axons of the more rostrally located group II interneurones appear to descend mainly in the middle part of the LF.