Receptor mechanisms underlying heterogenic reflexes among the triceps surae muscles of the cat

The soleus (S), medial gastrocnemius (MG), and lateral gastrocnemius (LG) m uscles of the cat are interlinked by rapid spinal reflex pathways. In the d ecerebrate state, these heterogenic reflexes are either excitatory and leng th dependent or inhibitory and force dependent. Mechanographic analysis was used to obtain additional evidence that the muscle spindle primary ending and the Golgi tendon organ provide the major contributions to these reflexe s, respectively. The tendons of the triceps surae muscles were separated an d connected to independent force transducers and servo-controlled torque mo tors in unanesthetized, decerebrate cats. The muscles were activated as a g roup using crossed-extension reflexes. Electrical stimulation of the caudal cutaneous sural nerve was used to provide a particularly strong activation of MG and decouple the forces of the triceps surae muscles. During either form of activation, the muscles were stretched either individually or in va rious combinations to determine the strength and characteristics of autogen ic and heterogenic feedback. The corresponding force responses, including b oth active and passive components, were measured during the changing backgr ound tension. During activation of the entire group, the excitatory, hetero genic feedback linking the three muscles was found to be strongest onto LG and weakest onto MG, in agreement with previous results concerning the stre ngths of heteronymous Ia excitatory postsynaptic potentials among the trice ps surae muscles. The inhibition, which is known to affect only the soleus muscle, was dependent on active contractile force and was detected essentia lly as rapidly as length dependent excitation. The inhibition outlasted the excitation and was blocked by intravenous strychnine. These results indica te that the excitatory and inhibitory effects are dominated by feedback fro m primary spindle receptors and Golgi tendon organs. The interactions betwe en these two feedback pathways potentially can influence both the mechanica l coupling between ankle and knee.