Activity in muscle spindle secondary (group II) afferents can have powerful actions on the step cycle, with some afferents (e.g. those from tibialis anterior, TA) causing a prolongation of ongoing flexion and others (e.g. extensor digitorum longus, EDL) advancing the onset of extension (Perreault et al. 1995; McCrea et al. 2000). In decerebrate cats with intact spinal cords, neither of these actions appears to be a part of the flexion reflex. Here we report on interneurones that are excited by group II muscle afferents located in lower lumbar (L6-L7) segments, during fictive locomotion (n = 19) or fictive scratch (n = 3) in four decerebrate cats.

Adult cats were anaesthetised with halothane and subsequently decerebrated at a precollicular level. Fictive locomotion was induced by electrical stimulation of the mesencephalic locomotor region (MLR; e.g. see Perreault et al. 1995) and fictive scratch was produced by topical application of curare to the cervical dorsal roots (Feldberg & Fleischhauer, 1960). Electrical stimuli (up to 5 T) were applied to peripheral nerves to identify interneurones and perturb the step cycle. Rhythmic EMG activity was recorded from hindlimb muscle nerves and recordings made from interneurones using glass microelectrodes. All protocols followed conformed to local guidelines (for full details of the preparation, see Perreault et al. 1995).

Half (11/22) of the group II excited interneurones were either completely or partially inhibited in both the flexion and extension phases. Five interneurones were active during flexion, three during extension, while three fired action potentials through the transition between flexion and extension or vice versa. Two of the interneurones active through the transition, between extension and flexion, during locomotion were active only in the extension phase during scratch. Activity of group II interneurones during fictive behaviours in the absence of afferent input indicates that some of the interneurones are part of the central pattern generating network. In addition, the excitability of some interneurones with input from ankle flexor group II afferents is centrally regulated to relay proprioceptive spindle secondary input to the spinal cord at particular times. Identification of the targets and actions of these interneurones will be an important goal for further understanding how these systems regulate stepping.

This work was supported by the Canadian Institutes of Health Research and National Institute of Health.