Exp Brain Res (1992) 90:557 566 BrainResearch 9Springer-Verlag 1992 Entrainment of the locomotor rhythm by group Ib afferents from ankle extensor muscles in spinal cats K.G. Pearson, J.M. Ramirez*, and W. Jiang** Department of Physiology, Universityof Alberta, Edmonton, Canada Received August 22, 1991 / Accepted March 16, 1992 Summary. 1. Previous studies have concluded that the timing of the locomotor rhythm can be strongly in- fluenced by input from group Ib afferents from leg exten- sor muscles (Duysens and Pearson 1980; Conway et al. 1987). The main objective of the present study was to obtain additional evidence for this conclusion by examin- ing the characteristics of entrainment of the locomotor rhythm by rhythmic stimulation of group I afferents and by rhythmic force pulses in the ankle extensor muscles. 2. A reduced, non-immobilized preparation was de- veloped in spinal cats that allowed isometric contractions of ankle extensor muscles to be elicited by ventral root stimulation during the expression of locomotor activity. The same preparation was used to examine the influence of electrically stimulating group I afferents from the ankle extensors and the effect of rhythmically stretching these muscles. The locomotor rhythm was initiated by sustained mechanical stimulation of the perineum following the administration of Clonidine and, in some preparations, Naloxone. 3. The timing of the onset of flexor burst activ- ity was examined during entrainment with saw-tooth and ramp-and-hold stretches of the ankle extensor muscles. Flexor bursts were initiated about 200 ms following the release from the stretch, and this latency was independent of the entrainment frequency. 4. The locomotor rhythm was readily entrained by rhythmic contractions of the ankle extensor muscles produced by ventral root stimula- tion provided the magnitude of the contractions was greater than about 10N. Repetitive stimulation of group I muscle afferents from the ankle extensors also entrained the locomotor rhythm, with the timing of motor activity being similar to that during entrainment with rhythmic muscle contractions. Burst activity in the ipsilateral exten- sors was coincident with the stimulus trains in both cases. This similarity argues for entrainment being produced * Present addresses: Fachbereich Biologic, Universit/it Kaiser- slautern, W-D-6750 Kaiserslautern, Federal Republic of Germany ** CRSN, Physiologic, Facultede Medecine,C.P. 6128, succursalA, Montreal, Quebec H3C 3J7 Correspondence to: K. Pearson mainly by input from group Ib afferents. 5. The functional implication of the results of this and previous studies is that input from group Ib afferents during the stance phase of walking acts to inhibit generation of flexor burst activ- ity and to promote extensor activity. The proposal that a decline in Ib activity near the end of the stance phase is involved in regulating the stance to swing transition is discussed. Key words: Locomotion- Cat walking Rhythmic motor activity - Entrainment Introduction There is now considerable evidence that sensory feedback has an important role in establishing some features of the motor pattern for walking in the cat (Smith 1986; Grillner and Dubuc 1988; Rossignol et al. 1988). The clearest example of afferent regulation of the step cycle is the adaptation of stepping in decerebrate and spinal cats to the speed of the treadmill upon which they are walking. One source of afferent information involved in regulating the stance-to-swing transition in the hind legs may be tendon organs in leg extensor muscles. This possibility was first suggested by the finding in walking decerebrate cats that the locomotor rhythm in the isolated ankle extensor muscles of one hind leg can be inhibited by maintained stretches of the muscles. The generation of rhythmic flexor burst activity is suppressed so long as the muscle force is maintained above about 40N regardless of the length of the extensor muscles (Duysens and Pearson 1980). Subsequently Conway et al. (1987) reported that the fictive locomotor rhythm in DOPA/Nialamide treated spinal cats can be entrained by rhythmically stretching the ankle extensor muscles with the ipsilateral extensor activ- ity coinciding with peak tension in the extensor muscles. Both these observations indicate that the initiation of flexor burst activity during walking requires unloading of