Activity-dependent reconfiguration of the effective dendritic field of motoneurons

A neuron in vivo receives a continuous bombardment of synaptic inputs that modify the integrative properties of dendritic arborizations by changing th e specific membrane resistance (R-m). To address the mechanisms by which th e synaptic background activity transforms the charge transfer effectiveness (T-x) of a dendritic arborization, the authors simulated a neuron at rest and a highly excited neuron. After in vivo identification of the motoneuron s recorded and stained intracellularly, the motoneuron arborizations were r econstructed at high spatial resolution. The neuronal model was constrained by the geometric data describing the numerized arborization. The electroto nic structure and T-x were computed under different R-m values to mimic a h ighly excited neuron (1 kOhm.cm(2)) and a neuron at rest (100 kOhm.cm(2)). The authors found that the shape and the size of the effective dendritic fi elds varied in the function of R-m. in the highly excited neuron, the effec tive dendritic field was reduced spatially by switching off most of the dis tal dendritic branches, which were disconnected functionally from the somat a. At rest, the entire dendritic field was highly efficient in transferring current to the somata, but there was a lack of spatial discrimination. Bec ause the large motoneurons are more sensitive to variations in the upper ra nge of R-m, they switch off their distal dendrites before the small motoneu rons. Thus, the same anatomic structure that shrinks or expands according t o the background synaptic activity can select the types of its synaptic inp uts. The results of this study demonstrate that these reconfigurations of t he effective dendritic field of the motoneurons are activity-dependent and geometry-dependent. (C) 2000 Wiley-Liss, Inc.