Contributions of intrinsic motor neuron properties to the production of rhythmic motor output in the mammalian spinal cord

Motor neurons are endowed with intrinsic and conditional membrane propertie s that may shape the final motor output. In the first half of this paper we present data on the contribution of I-h, a hyperpolarization-activated inw ard cation current, to phase-transition in motor neurons during rhythmic fi ring, Motor neurons were recorded intracellularly during locomotion induced with a mixture of N-methyl-D-aspartate (NMDA) and serotonin, after pharmac ological blockade of I-h.I-h was then replaced by using dynamic clamp, a co mputer program that allows artificial conductances to be inserted into real neurons. I-h was simulated with biophysical parameters determined in volta ge clamp experiments. The data showed that electronic replacement of the na tive I-h caused a depolarization of the average membrane potential, a phase -advance of the locomotor drive potential, and increased motor neuron spiki ng. Introducing an artificial leak conductance could mimic all of these eff ects, The observed effects on phase-advance and firing, therefore, seem to be secondary to the tonic depolarization; i.e., I-h acts as a tonic leak co nductance during locomotion. In the second half of this paper we discuss re cent data showing that the neonatal rat spinal cord can produce a stable mo tor rhythm in the absence of spike activity in premotor interneuronal netwo rks. These coordinated motor neuron oscillations are dependent on NMDA-evok ed pacemaker properties, which are synchronized across gap junctions. We di scuss the functional relevance for such coordinated oscillations in immatur e and mature spinal motor systems, (C) 2001 Elsevier Science Inc.