Stretch and H reflexes in triceps surae are similar during tonic and rhythmic contractions in high decerebrate cats.

During locomotion in decerebrate and spinal cats the group Ia afferents from hind leg muscles are depolarized rhythmically. An earlier study concluded that this locomotor-related primary afferent depolarization (PAD) does not contribute to modulation of monosynaptic reflex pathways during locomotion. This finding indicated that the neural network generating the locomotor rhythm, the central pattern generator (CPG), does not presynaptically inhibit monosynaptic reflexes. In this investigation we tested this prediction in decerebrate cats by measuring the magnitude of reflexes evoked in ankle extensor muscles during periods of tonic contractions and during sequences of rhythmic contractions. The latter occurred when the animal was induced to walk on a treadmill. At the similar levels of activity in the soleus muscle there was no significant difference in the magnitude of the soleus H reflex in these two behavioral situations. Similar results were obtained for reflexes evoked by brief stretches of the soleus muscle. We also examined the reflexes evoked by ramp-and-hold stretches during periods of rhythmic and tonic activity of the isolated medial gastrocnemius (MG) muscle. At similar levels of background activity, the reflexes evoked in the MG muscle were the same during rhythmic and tonic contractions. Our failure to observe a reduction in the magnitude of H reflexes and stretch reflexes during rhythmic contractions, compared with reflexes evoked at the same level of background activity during tonic contractions, is consistent with the notion that the CPG for stepping does not presynaptically inhibit monosynaptic reflexes during the extension phase of locomotor activity. Our results indicate that presynaptic inhibition of the monosynaptic reflex associated with normal locomotion in cats or humans arises from sources other than the extensor burst generating system of the central pattern generator.