NEW OBSERVATIONS ON COUPLING BETWEEN GROUP-II MUSCLE AFFERENTS AND FELINE GAMMA-MOTONEURONS

1. Extra- or intracellular recordings were made from seventy-six gamma -motoneurones of hindlimb muscles in chloralose anaesthetized cats to re-assess the coupling between secondary muscle spindle afferents (gro up II muscle afferents) and these neurones. The latencies of a number of responses evoked by group II muscle afferents in gamma-motoneurones were shorter than minimal latencies of responses induced disynaptical ly in other spinal neurones. These latencies are therefore compatible with monosynaptic coupling between muscle spindle secondaries and gamm a-motoneurones. 2. Responses fulfilling criteria for monosynaptically evoked responses were seen in about one third of gamma-motoneurones wi th input from the group II muscle afferents tested (in 6 of 18 motoneu rones recorded intracellularly and in 26 of 74 motoneurones recorded e xtracellularly). They were usually evoked from only one of the stimula ted nerves, stimulation of group II afferents of other nerves being fo llowed by responses at longer latencies. 3. Most gamma-motoneurones we re excited by group II afferents from several muscles, both flexors an d extensors. However, a comparison of group II input to gamma-motoneur ones innervating medial gastrocnemius and four other hindlimb muscles revealed differences in both incidence and sources. 4. This study exte nds results of previous studies by providing evidence that some synapt ic actions of group II afferents, including afferents from the same mu scle, are evoked monosynaptically, and may assist in sustaining the ac tivation of gamma-motoneurones by positive feedback.