Different transgenic mice have been generated with clinically relevant motor disorders, in which an analysis of the motor system is still missing. In order to establish a basis for further pathophysiological investigations we started to investigate spinal motor control in mice (anaesthesia: pentobarbital 70 mg/kg i.p./ methohexital 60-70 mg/kg/h i.v.). Lumbar reflex activity was recorded intracellularly from motoneurones innervating the tibial nerve (Tib., including gastrocnemius soleus, GS, and flexor digitorum longus) or with monosynaptic reflex testing of GS.

In tibial motoneuones monosynaptic group I EPSPs occurred with a latency of 0.4-0.6 ms from the group I incoming volley. Homonymous or synergistic group II EPSPs (no group II IPSPs were observed) occurred with a threshold of 1.7-2 T (T: threshold in relation to lowest threshold fibres), were maximal with 7-10 T and had a latency to group I incoming volleys of around 2.5 ms. Mean difference between group I and group II incoming volleys was 0.41 ms, indicating an at least disynaptic transmission from group II afferents.

Monosynaptic reflexes of GS were facilitated by conditioning stimulation of the common peroneal (with 5 T), the sural (2 T) and the tibial nerve (5 T), i.e. by a variety of different, probably flexor reflex afferents. This facilitation persisted after high lumbar spinalization.

The results revealed latencies and thresholds of group I and II EPSPs in mice which are comparable to those observed in cats. However, in contrast to reflex actions in the cat flexor reflex afferents induce predominantly extensor facilitation in the ankle and foot extensors.

I.K. was supported by STINT.