The effect of knee joint afferent discharge on transmission in flexion reflex pathways in decerebrate cats.

1. Changes in excitability of reflex arcs mediating flexion withdrawal ad crossed extensor reflexes have been examined in decerebrate cats. 2. The excitability of flexion withdrawal and crossed extensor reflexes was shown to be modulated by knee joint position. Flexion withdrawal reflexes were most easily elicited when the knee was extended and crossed extensor reflexes were most easily elicited when the knee was flexed. 3. The modulation of transmission was not confined to reflex pathways to muscles acting at the knee but also included pathways to muscles acting at the hip and ankle, as well as pathways to muscles in the contralateral limb. 4. The changing excitability of reflex pathways caused by movement of the knee joint was unrelated to the stretch applied to muscles acting at the knee and to cutaneous afferent discharge. Modulation of reflex excitability by joint movement was totally abolished by local anaesthesia of the knee joint in an otherwise intact limb. 5. The results of the present experiments indicate that transmission in flexion reflex pathways can be inhibited by knee joint afferent discharge.