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1 Division of Neurobiology, Barrow Neurological Institute, Phoenix, AZ, USA
* To whom correspondence should be addressed. E-mail: thamm{at}chw.edu.
The mean location of Renshaw synapses on spinal motoneurons and their synaptic conductance were estimated from changes in impedance magnitude produced by sustained recurrent inhibition. Motoneuron impedance was determined by injecting quasiwhite noise current into lumbosacral motoneurons of pentobarbital-anesthetized cats. Synaptic location and conductance were estimated by comparing observed impedance changes to simulation results obtained using standard motoneuron models (Maltenfort et al. 2003) and compartmental models fit to each impedance function (Maltenfort and Hamm 2003). Estimated synaptic locations ranged from 0.10 to 0.41 
, with a mean of 0.19 or 0.24 , depending on the estimation method. Average dendritic path length was 262 µm (range: 128 to 412 µm). Average synaptic conductance was 23 to 27 nS (range: 6.7 to 57.9 nS), corresponding to conductance changes of 78 to 88% of resting membrane conductance. Estimate accuracy was supported by consistency using different estimation methods, agreement with morphological data of Fyffe (1991), and comparisons of observed and simulated recurrent IPSP amplitudes. Synaptic location, but not synaptic conductance, was correlated with rheobase, a measure of motoneuron excitability. Synaptic conductance did not depend on synaptic location. A regression analysis demonstrated that synaptic conductance and cell impedance were the principle factors determining recurrent IPSP amplitude. Simulations using the observed values and locations of Renshaw conductance demonstrate that recurrent inhibition can require as much as an additional 14 to 18% sustained excitatory synaptic conductance to depolarize motoneurons sufficiently to activate somatic or dendritic inward currents and recruit motoneurons or amplify excitatory synaptic inputs.
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