Meeting Abstract

S1.3  Tuesday, Jan. 4  Spinal Circuits That Generate Multiple Kinds of Rhythmic Limb Movements in Turtles BERKOWITZ, A.*; HAO, Z.-Z.; University of Oklahoma; University of Oklahoma ari@ou.edu

Are different kinds of rhythmic limb movements that use the same muscles produced by separate or shared networks of CNS neurons? The turtle spinal cord is a model system to address this. Even without brain input and movement-related sensory feedback, it can generate the basic patterns of motor neuron activity (i.e., fictive motor patterns) underlying 3 forms of site-specific scratching, plus forward swimming. In one study, we delivered scratch-evoking and swim-evoking stimuli at overlapping times. We expected that if the CNS rhythm-generating networks are entirely separate, then motor neuron activity would be a superimposition of the two rhythms. Instead, 1) a suprathreshold scratch stimulus and a subthreshold swim stimulus combined to produce a normal swim, 2) a suprathreshold scratch stimulus and a suprathreshold swim stimulus combined to produce a swim that was faster than the rhythm with either stimulus alone, and 3) the combined stimuli sometimes disrupted rhythm generation entirely. These effects suggest that there either is one rhythm-generating network or are strong interactions between two. In another study, we recorded intracellularly from spinal interneurons and filled each with a dye. We identified a morphological category of neurons (transverse interneurons or T neurons) with dendrites that are extensive in the transverse plane but short rostrocaudally. T neurons were rhythmically activated during both scratching and swimming. On average, they had larger membrane potential oscillations and fired action potentials at a higher peak rate than other scratch-activated interneurons. Some had ventral horn axon terminals, as expected if they affect motor output relatively directly. T neurons are thus good candidates to be shared rhythm-generating network components and/or last-order premotor interneurons.