Discharges of Single Hindlimb Afferents in the Freely Moving Cat A. PROCHAZKA, R. A. WESTERMAN, AND S. P. ZICCONE Department of Physiology, Monash University, Clayton, Victoria, 3168, Australia SUMMARY AND CONCLUSIONS I. Implanted semimicroelectrodes were used to record single afferent fiber discharges from L 7 dorsal roots during unrestrained walking in the conscious cat. 2. A series of tests were used to identify an afferent during a short period of anesthesia fol- lowing each recording session. The majority of afferents were from muscle spindle primary end- ings in hindlimb muscles. 3. Ankle extensor spindle primaries generally showed their highest firing rates during that phase of stepping in which they were passively stretched. During active muscle contraction there was evidence of fusimotor drive, although this was not usually sufficient to entirely over- come the unloading effect of rapid muscle short- ening. The variability of firing rate from cycle to cycle was considerably greater for the phase of active muscle contraction. The EMG response to brisk stretches of the ankle extensor muscle indicated a rapid (disynaptic or trisynaptic) reflex arc in the conscious animal. 4. Knee flexor spindle primaries showed simi- larly higher firing rates during passive muscle stretching in the step cycle. The shorter periods of presumed a-y coactivation corresponded to the much more phasic role of these muscles in stepping. 5. Tendon organs in the physiological exten- sors of the toes were mainly active during stance, although some discharges were usually seen during the swing phase. It is suggested that previous experiments on mesencephalic prep- arations may have led to an exaggerated view of the degree of a-y coactivation during n stepping movement S. INTRODUCTION ormal Received for publication January 19, 1976. cle spindle afferents in the jaw muscles of con- scious cats (27) and monkeys (5, 15) seems to differ from that expected from current theories. The bulk of the afferents showed an unexpec- tedly high firing rate during movements where their muscle was passively stretched, and gener- ally a surprisingly low degree of apparent fusi- motor drive during active contraction. This is at variance with data from anesthetized (4, 23) and decerebrate preparations (24, 25) where the large majority of muscle spindles were active mainly during muscle contraction. It is similarly at variance with the discharge patterns of presumed spindles in conscious hu- mans (26, 28, 29). However very few records of spindle discharge during actual movement have been presented by these latter workers. The joint movements never exceeded more than a few degrees. It should be pointed out that in most of the conscious recordings to date, the identification tests have generally been rather cursory and, therefore, at least some of the affer- ents may have innervated other types of recep- tors. In the present study, recordings of single fi- ber discharges were obtained from the L, dor- sal roots of fully conscious cats during normal walking. This involved the implantation of dor- sal root semimicroelectrodes, which have been described in preliminary reports ( 18, 20). An important aspect of the experiments was the identification procedure, which took place during short periods of anesthesia immediately after each afferent recording session. This al- lowed physiological and pharmacological testing of the given afferent under controlled condi- tions. Due to the relatively large electrode tips, the recordings heavily favored group I afferents. The electrodes were “free-floating” and, there- fore, single-fiber recordings relied on chance movements of the electrode tips into stable loca- tions near dorsal root axons. Fortunately, this was generally found to occur at least two or three times over the course of a few days. As a result, recordings were obtained of the activity