Mechanical actions of heterogenic reflexes among ankle stabilizers and their interactions with plantarflexors of the cat hindlimb

S. J. Bonasera and T. R. Nichols Department of Physiology, Emory University, Atlanta, Georgia 30322, USA. 1. The stretch-evoked reflex organization of muscles whose major action is to abduct [peroneus brevis (PB); peroneus longus (PL)] and adduct [tibialis posterior (TP); flexor digitorum longus (FDL); flexor hallucis longus (FHL)] the ankle, and their interactions with the hindlimb extensors gastrocnemius (G) and soleus (S), were studied in 27 unanesthetized decerebrate cats. Ramp-hold-release stretches of physiological amplitudes were applied to muscle tendons detached from their bony insertion, and muscle force output was measured in response to these perturbations. Flexion and crossed-extension reflexes were used to modulate baseline force. 2. PB and TP shared strong, length-dependent, short-latency inhibitory reflexes prominent when the muscles were either actively generating force or quiescent. The mechanical characteristics of this reflex suggest Ia reciprocal inhibition as the underlying mechanism. Just as reciprocal inhibition between S and tibialis anterior stiffens the ankle joint against sagittal perturbations, we propose that reciprocal inhibition between PB and TP stiffens the ankle joint against nonsagittal perturbations. 3. In all preparations (n = 7) and under all conditions examined, PB and PL shared well-demonstrated mutual excitation. The reflex responses were asymmetric (favoring excitation of PL), length dependent, and occurred simultaneously with the stretch reflex at a latency of 16-18 ms. Mutual monosynaptic projections previously described between these two muscles explain all of the above findings. Our data further demonstrate that, under certain conditions, the ensemble activity of this reflex interaction has a powerful effect on the mechanical behavior of the muscle. 4. The heterogenic reflex organization of the ankle adductors was as follows: FDL evoked a modest excitation on TP, whereas FHL evoked weak inhibition. Latency of the excitation from FDL onto TP (24 ms) was greater than expected if the reflex were mediated by heteronymous Ia afferents. In all preparations examined (n = 3), TP contributed no significant reflexes onto either FDL or FHL. 5. Mutual, asymmetric inhibition characterized interactions between PB and the plantarflexors S and G. Most remarkable was a novel, long-latency (72-74 ms) reflex inhibition evoked on both S and G by stretch of PB. When this inhibition occurred, it dramatically decreased the S (or G)