Activation ofmidlumbar neurones byafferents fromanterior hindlimb muscles inthecat

1. Ithasbeensuggested that a groupofinterneurones located inthemidlumbar segments of thespinal cordmightplay a role inswitching fromthestance toswingphases ofthestep cycle during locomotion. We havefurther examined theinput tothese neuronesfrom proprioceptive afferents totest whether theconnections tothese cells areconsistent withthis role. 2. Electrical stimulation ofgroup Iandgroup IIafferents inbranches ofthefemoral nerve whichsupply iliopsoas, themajor hipflexor muscle, excited alarge majority ofintermediate zonemidlumbar interneurones whichreceive input fromquadriceps groupIIafferents. The central latencies andproperties oftheEPSPsindicate thatbothgroup Iandgroup II afferents fromiliopsoas make monosynaptic connections withmany midlumbar interneurones. 3. GroupIIafferents fromboththeankleflexor tibialis anterior andthedigit dorsiflexor extensor digitorum longus excited midlumbar interneurones. Similarly, theywere also excited bygroupIIafferents frombothofthetwomainanatomical divisions ofthesartorius muscle. 4. Thefrequent andpotent excitation ofmidlumbar neuronesfrom group IandIIafferents in iliopsoas suggests thatthey may beexcited attheendofthestance phase ofthestepwhen these muscles arestretched. Thispossibility isdiscussed inrelation torecent workon the functional control ofthestepcycle.