1. It has been suggested that a group of interneurones located in the midlumbar segments of the spinal cord might play a role in switching from the stance to swing phases of the step cycle during locomotion. We have further examined the input to these neurones from proprioceptive afferents to test whether the connections to these cells are consistent with this role. 2. Electrical stimulation of group I and group II afferents in branches of the femoral nerve which supply iliopsoas, the major hip flexor muscle, excited a large majority of intermediate zone midlumbar interneurones which receive input from quadriceps group II afferents. The central latencies and properties of the EPSPs indicate that both group I and group II afferents from iliopsoas make monosynaptic connections with many midlumbar interneurones. 3. Group II afferents from both the ankle flexor tibialis anterior and the digit dorsiflexor extensor digitorum longus excited midlumbar interneurones. Similarly, they were also excited by group II afferents from both of the two main anatomical divisions of the sartorius muscle. 4. The frequent and potent excitation of midlumbar neurones from group I and II afferents in iliopsoas suggests that they may be excited at the end of the stance phase of the step when these muscles are stretched. This possibility is discussed in relation to recent work on the functional control of the step cycle.
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