215 DEVELOPMENTAL APPROACHES TO THE ANALYSIS OF VERTEBRATE CENTRALPATTERNGENERATORS M.J. O'DONOVAN Department of Physiology and Biophysics, University of Iowa Iowa City, Iowa 52242, USA Key Words: embryonic activity-intracellular recording-isolated spinal cord The mechanisms responsible for alternation of flexor and extensor motoneurons during motor activity were investigated in the developing spinal cord of the chick embryo. Embryonic preparations offer a number of advantages over adult animals for the analysis of motor activity. The chick spinal cord can be maintained in-vitro which allows ionic and pharmacological manipulations of function and offers increased stability for intra and extracellular recording. In addition, the isolated chick cord generates spontaneous episodes of motor activity in vitro. Becauseof the embryo's small size it is possible to use DC recordings from ventral roots and muscle nerves to monitor membrane potential electrotonically from populations of motoneurons, as an alternative to intracellular recording. DC potentials recorded from ventral roots and muscle nerves were similar in polarity and time course to intracellular recordings from motoneurons. Spontaneous motor activity generated by the isolated spinal cord consisted of recurring episodes in which motoneurons discharged cyclically. Single unit and whole nerve recordings showed that each cycle consisted of three phases: An i n i t i a l brief discharge that occurred synchronously in all motoneurons, a delay, and second more prolonged discharge. The second discharge occurred out of phase between antagonist motoneurons because the delay in flexor motoneurons (0.4-0.7 sec) was longer than in extensor motoneurons (0.0-0.1 sec) (Landmesser and O'Donovan 1984, O'Donovan 1986). The mechanisms responsible for this pattern of alternation were investigated using intracellular recordings from identified hinlimb motoneurons and DC recordings of electrotonic potentials from flexor and extensor muscle nerves. Intracellular recordings in 14-15 day old embryos showed that during each cycle motoneurons were depolarized by up to 30 mV for a varying fraction of the cycle. Motoneuronsdischarged on the rising and initial phase of the step and again after a delay. During the delay the trajectory of membrane potential remained flat without evident hyperpolarizationo The amplitude of the depolarization was reduced by injection of depolarizing current and increased by hyperpolarizing current suggesting that it was mediated by excitatory synapses. This conclusion was