MECHANICAL ACTIONS OF HETEROGENIC REFLEXES AMONG ANKLE STABILIZERS AND THEIR INTERACTIONS WITH PLANTARFLEXORS OF THE CAT HINDLIMB

1. The stretch-evoked reflex organization of muscles whose major actio n is to abduct [peroneus brevis (PB); peroneus longus (PL)] and adduct [tibialis posterior (TP); flexor digitorum longus (FDL); flexor hallu cis longus (FHL)] the ankle, and their interactions with the hindlimb extensors gastrocnemius (G) and soleus (S), were studied in 27 unanest hetized decerebrate cats. Ramp-hold-release stretches of physiological amplitudes were applied to muscle tendons detached from their bony in sertion, and muscle force output was measured in response to these per turbations. Flexion and crossed-extension reflexes were used to modula te baseline force. 2. PB and TP shared strong, length-dependent, short -latency inhibitory reflexes prominent when the muscles were either ac tively generating force or quiescent. The mechanical characteristics o f this reflex suggest Ia reciprocal inhibition as the underlying mecha nism. Just as reciprocal inhibition between S and tibialis anterior st iffens the ankle joint against sagittal perturbations, we propose that reciprocal inhibition between PB and TP stiffens the ankle joint agai nst nonsagittal perturbations. 3. In all preparations (n = 7) and unde r all conditions examined, PB and PL shared well-demonstrated mutual e xcitation. The reflex responses were asymmetric (favoring excitation o f PL), length dependent, and occurred simultaneously with the stretch reflex at a latency of 16-18 ms. Mutual monosynaptic projections previ ously described between these two muscles explain all of the above fin dings. Our data further demonstrate that, under certain conditions, th e ensemble activity of this reflex interaction has a powerful effect o n the mechanical behavior of the muscle. 4. The heterogenic reflex org anization of the ankle adductors was as follows: FDL evoked a modest e xcitation on TP, whereas FHL evoked weak inhibition. Latency of the ex citation from FDL onto TP (24 ms) was greater than expected if the ref lex were mediated by heteronymous Ia afferents. In all preparations ex amined (n = 3), TP contributed no significant reflexes onto either FDL or FHL. 5. Mutual, asymmetric inhibition characterized interactions b etween 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 s tretch of PB. When this inhibition occurred, it dramatically decreased the S (or G) stretch response. Longer PB lengths evoked greater inhib ition of isometric S; regression analysis indicated that the model bes t predicting this inhibition contained muscle force and stiffness term s. No long-latency reflexes were noted from either G or S onto PB. The mechanism underlying long-latency inhibition is presently unknown: ho wever, features of this interaction suggest interneurons receive eithe r group II or group III afferent input. 6. G and TP shared short laten cy, mutually inhibitory, asymmetric reflexes favoring inhibition of TP . No long-latency interactions were noted, nor were there any mechanic ally significant interactions between S and TP. 7. Reflex interactions across the abduction/adduction axis thus favored inhibition of planta rflexion and adduction torques while emphasizing abduction torques: PB /S (or PB/G) interactions were mutual, asymmetric, and favored inhibit ion of G and S; TP/G interactions were mutual, asymmetric, and favored inhibition of TP; TP/PB interactions were approximately balanced. The overall mechanical outcome of these inhibitory interactions may partl y underlie the global corrective strategy seen in intact cats subjecte d to linear perturbations. 8. No significant reflex interactions were demonstrated between PL and TP, G, or S, nor were any long-latency ref lexes noted. Thus, whereas reflex interactions between the stereotypic ally activated PB and other stereotypically activated muscles (includi ng TP, G, and S) were strong and well-demonstrated, interactions betwe en the variably activated PL and these same muscles were far weaker. 9 . While a complete description of heterogenic reflex connectivity amon g muscles acting at the cat ankle joint defies simple summary, stateme nts regarding this organization can be made. In a broadening of the my otatic-unit concept, length-dependent interactions link muscles whose torque vectors are approximately parallel or in opposition to one anot her, regardless of whether the muscles act in sagittal or nonsagittal planes, Force-dependent interactions dominate the reflex organization of muscles required for antigravity support and/or muscles with solely stereotypic locomotor patterns. Complex interactions favoring the pro duction of abduction torque characterize the reflex interactions of sa gittal and nonsagittal muscles. Finally, muscles with solely stereotyp ical locomotor activity patterns tend to conform to the above rules; w hereas muscles with variable activity patterns have relatively weak re flex interactions with all other muscles except close Ia synergists.