Compartmentalization of the dendritic trees of spinal neurons involved in motor control: The role of dendritic conductances

Spinal motoneurons are characterized by a large dendritic tree upon which operate numerous ligand- and voltage-gated conductances. This arrangement could lead to the segmentation of the dendritic tree into compartments with different signal processing capabilities. The goal of this thesis was to determine whether the dendritic tree of motoneurons could be compartmentalized.; We first estimated the distribution of L-type Ca2+ channels that mediate persistent inward currents that play a role in amplifying the effectiveness of synaptic inputs to motoneurons. By means of theoretical analysis of known characteristics of activation of these channels and computer simulations, we show that L-type Ca2+ channels are located in 50--200 mum long discrete regions (hot spot) centered 100 to 400 mum from the soma in the dendrites of motoneurons. This distribution leads to multiple compartments that are centered around each hot spot.; We then performed electrophysiological recordings of feline hindlimb motoneurons to examine the amplification by the presence of persistent inward currents (PICs) of inhibitory synaptic inputs from Renshaw cells. Two forms of amplification emerged where the time-course of the inhibitory postsynaptic currents (IPSCs) was changed in one form of amplification and unaltered in another. The basis for the two forms of amplification resided in the ability of the inhibitory synapses to deactivate PICs. Simulations of motoneuron models with different distributions of inhibitory synapses suggested that the ability to deactivate PICs is influenced by the spatial relationship between synapses and channels. In particular, the distribution of inputs from Renshaw cells seems to facilitate its ability to deactivate PICs. Therefore, a toric band overlapping the L-type Ca2+ channel hot spots represented a compartment in which the effectiveness of inhibitory synapses was augmented.; Finally, we show that the effectiveness of proximal synapses on Renshaw cells is augmented in suprathreshold conditions over subthreshold conditions due to the somatic quasi voltage-clamp produced by repetitive firing. Thus, in neurons such as motoneurons where the spatial extent of the voltage-clamp is also finite, a circular compartment extending from the soma represented a compartment where effectiveness of proximally distributed synapses could be modulated by the mode of operation of the neuron.