Reflex transmission to lumbar α-motoneurones in the mouse similar and different to those in the cat

• Motoneuronal recordings and monosynaptic reflexes were performed in mice in vivo.
• Threshold and latency of group I–III EPSPs in mice were similar to those in the cat.
• Different to cat, GS nerve reflex in mice was facilitated by flexor reflex afferents.
• Foot sole nociceptive afferents inhibited ipsilateral but facilitated contralateral peroneal.

Investigation and interpretation of defective motor circuitries in transgenic mice required further basic results from wild-type mice. Therefore, we investigated the lumbar motor reflex pattern in anaesthetised mice using intracellular motoneuronal recording and monosynaptic reflex testing. Thresholds and latencies in mice were similar to those in cats: thresholds for monosynaptic (group I) EPSPs were slightly above 1 T (T = threshold for the lowest threshold fibres), around 1.5 T for group II EPSPs and above 10 T for group III EPSPs; group I EPSPs were maximal with a stimulus strength around 2 T, group II EPSPs were maximal with 5–8 T; latencies to the group I incoming volley were below 1 ms for monosynaptic group I EPSPs, around 3 ms for polysynaptic group II EPSPs and above 4 ms for polysynaptic group III EPSPs. In contrast to reflex actions in the cat, monosynaptic gastrocnemius-soleus reflexes were facilitated by conditioning stimulation of the peroneal, sural and tibial nerves, i.e. by a variety of different, probably flexor reflex afferents. This facilitation persisted after high lumbar spinalisation indicating an independency to supraspinal influences. Nociceptive muscle afferents facilitated the peroneal monosynaptic reflex while nociceptive cutaneous afferents from the foot sole inhibited the ipsilateral but facilitated the contralateral peroneal reflex.