Investigations of the dendritic structure of motoneurons, the integration of synaptic inputs in motoneurons, and the function and identity of their ionic channels and associated intrinsic voltage- dependent properties have demonstrated a complexity which suggests that the motoneuron is potentially a site of complex integration in the formulation of motor commands. Yet, it remains to be demonstrated that the function of the motoneuron in motor control is any more complex than that of a simple summing device that produces a rate of discharge based on the sum of synaptic currents that reach the soma. The proposed study will apply the impedance analysis techniques developed and refined in the previous years of the NRSA to quantifying the distribution of both synaptic and intrinsic conductances. Either kind of conductance will produce a change in a motoneuron's response to white noise current injected at the soma, and this change can be used to estimate the location and magnitude of the conductance. Whether these properties vary with the electrical characteristics of the motoneuron will be investigated to determine the distribution of these conductances throughout the motoneuron pool. The parameters of a motoneuron model will be fit to the experimentally determined impedance function and used to determine how conductance parameters vary with electrotonic properties of motoneurons. Each of these sets of information would make an important contribution toward answering the question of how individual motoneurones, and motoneuron pools, integrate synaptic and intrinsic drive potentials to produce a final pattern of muscle activity.
