MECHANICAL ACTIONS OF HETEROGENIC REFLEXES LINKING LONG TOE FLEXORS WITH ANKLE AND KNEE EXTENSORS OF THE CAT HINDLIMB

1. To study the means whereby ankle biomechanics are represented in th e interneuronal circuitry of the spinal cord we examined stretch-evoke d reflex interactions between the physiological extensors flexor hallu cis longus (FHL) and flexor digitorum longus (FDL) as well as their in teractions with gastrocnemius (G), soleus (S), and the quadriceps grou p (Q) in 34 unanesthetized decerebrate cats. To evoke stretch, DC moto rs provided ramp-hold-release length changes to tendons detached from their bony insertions. Semiconductor myographs measured resultant musc le force response. Reflexes were examined under both quiescent ( no ac tive force generation) and activated conditions; muscle activation was achieved through either crossed-extension or flexion reflexes. 2. FHL and FDL share mutual excitatory stretch-evoked interactions under mos t conditions examined. These interactions depended on muscle length, w ere asymmetric (with FHL contributing a larger magnitude of reflex exc itation onto FDL), and occurred at a latency of 16 ms. Mutual Ia syner gism previously described for these two muscles provides a basis for a ll of the above findings. Our data demonstrate that for this muscle pa ir, reflex connectivities revealed at the intracellular level can be e xtrapolated to cover the entire motoneuron pool; further, our data dir ectly demonstrate the net mechanical result of ensemble synaptic event s. 3. FHL was found to share strong, mutually inhibitory stretch-evoke d interactions with G, S, and Q. Stepwise regression statistical analy ses determined that these interactions depended on recipient muscle fo rce and donor muscle force. These reflex interactions all occurred at a latency of 28 +/- 4 (SE) ms. Further, the heterogenic inhibition bet ween FHL/G and FHL/S was attenuated by strychnine infusion (intravenou s) but unaffected by either mecamylamine, picrotoxin, or baclofen infu sion (intravenous, intrathecal). Disynaptic Ib inhibition previously d escribed among hindlimb extensors provides a basis for the above findi ngs; our data demonstrate that under certain conditions the ensemble a ctivity of this system can cause a dramatic decline in whole muscle fo rce output. 4. By contrast, FDL was found to share mutually inhibitory , stretch-evoked reflex interactions with G, S, and Q that were much w eaker than those observed between FHL and these same muscles. The smal l magnitude of inhibition observed in these interactions made it diffi cult to assess reflex latency or to determine the factor(s) that best predicted the heterogenic inhibition. 5. This study provides further e vidence of intrinsic differences in interneuronal organization between muscles whose activity occurs in a periodic manner during locomotion (''stereotypical'') and a muscle whose locomotor activity is character ized by both periodic and nonperiodic components (''facultative''). Mu scles with stereotypical activation patterns (FHL, Q, G, and S) share strong links through the force-dependent heterogenic inhibitory system described above. Conversely, FDL (a muscle activated in both stereoty pic and facultative manners) displayed weak mutual inhibition with ste reotypically active muscles. We propose that this organization prevent s facultative muscle activity from causing a reflex-mediated disruptio n of patterns forwarded to stereotypically active muscles.