Abdominal extension‐evoking interneurons in crayfish: Characteristics of the ganglionic driving networks

Abstract

AbstractSeveral different paradigms of extension‐evoking command fiber (CF) stimulation were used in order to characterize functional properties of the networks controlling abdominal posture in the crayfish. The types of connections between CFs and abdominal postural driving networks were first studied by stimulating several extension CFs from the same preparation at different frequencies. Differences in the recruitment order of extensor motoneurons imply that each CF makes unique synaptic connections with the abdominal ganglionic driving interneurons and/or the postural efferents. Furthermore, the ability of an extension CF to activate excitatory efferents after another extension CF has been driven to fatigue suggests that each CF has functionally distinct pathways to the extensor motoneuron pool. The second main objective of this paper was to determine if direct extension CF stimulation would produce integrative properties of the driving networks similar to those seen previously in the reflexively extending animal. As in reflex‐evoked behavior, the ganglionic driving networks are capable of spatially summating the input from two co‐activated CFs. In particular, low frequency co‐activation of two CFs recruited motoneurons which were inactive when single CFs were stimulated alone at the same frequency. Co‐stimulation of an extension CF with strong flexor‐biasing sensory input suppressed the flexors and resulted in a nearly pure extension motor program similar to that seen during reflex‐evoked extensions. Thus, overriding of antagonistic reflexes by CF activity argues for synaptic systems in each ganglion which rejects or reduces the gain of opposing relflexes, including those due to reafference during the commanded movements. These results strongly suggest a causal role for CFs in the generation of reflexive behavior in the freely behaving animal.