A neuron in vivo receives a continuous bombardment of synaptic inputs that modify the integrative properties of dendritic arborizations by changing the specific membrane resistance (R(m)). To address the mechanisms by which the 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 motoneurons recorded and stained intracellularly, the motoneuron arborizations were reconstructed at high spatial resolution. The neuronal model was constrained by the geometric data describing the numerized arborization. The electrotonic structure and T(x) were computed under different R(m) values to mimic a highly excited neuron (1 kOhm x cm(2)) and a neuron at rest (100 kOhm x cm(2)). The authors found that the shape and the size of the effective dendritic fields varied in the function of R(m). In the highly excited neuron, the effective dendritic field was reduced spatially by switching off most of the distal dendritic branches, which were disconnected functionally from the somata. At rest, the entire dendritic field was highly efficient in transferring current to the somata, but there was a lack of spatial discrimination. Because the large motoneurons are more sensitive to variations in the upper range of R(m), they switch off their distal dendrites before the small motoneurons. Thus, the same anatomic structure that shrinks or expands according to the background synaptic activity can select the types of its synaptic inputs. The results of this study demonstrate that these reconfigurations of the effective dendritic field of the motoneurons are activity-dependent and geometry-dependent.

We studied the rapid changes in electrical properties of lumbar motoneurons between postnatal days 3 and 9 just before mice weight-bear and walk. The input conductance and rheobase significantly increased up to P8. A negative correlation exists between the input resistance (Rin) and rheobase. Both parameters are significantly correlated with the total dendritic surface area of motoneurons, the largest motoneurons having the lowest Rin and the highest rheobase. We classified the motoneurons into three groups according to their discharge firing patterns during current pulse injection (transient, delayed onset, sustained). The delayed onset firing type has the highest rheobase and the fastest action potential (AP) whereas the transient firing group has the lowest rheobase and the less mature AP. We found 32 and 10% of motoneurons with a transient firing at P3–P5 and P8, respectively. About 20% of motoneurons with delayed onset firing were detected at P8. At P9, all motoneurons exhibit a sustained firing. We defined five groups of motoneurons according to their discharge firing patterns in response to ascending and descending current ramps. In addition to the four classical types, we defined a fifth type called transient for the quasi-absence of discharge during the descending phase of the ramp. This transient type represents about 40% between P3–P5 and tends to disappear with age. Types 1 and 2 (linear and clockwise hysteresis) are the most preponderant at P6–P7. Types 3 and 4 (prolonged sustained and counter clockwise hysteresis) emerge at P8–P9. The emergence of types 3 and 4 probably depends on the maturation of L type calcium channels in the dendrites of motoneurons. No correlation was found between groups defined by step or triangular ramp of currents with the exception of transient firing patterns. Our data support the idea that a switch in the electrical properties of lumbar motoneurons might exist in the second postnatal week of life in mice.

The impact of dendritic geometry on somatopetal transfer of the current generated by steady uniform activation of excitatory synaptic conductance distributed over passive, or active (Hodgkin-Huxley type), dendrites was studied in simulated neurons. Such tonic activation was delivered to the uniform dendrite and to the dendrites with symmetric or asymmetric branching with various ratios of branch diameters. Transfer effectiveness of the dendrites with distributed sources was estimated by the core current increment directly related to the total membrane current per unit path length. The effectiveness decreased with increasing path distance from the soma along uniform branches. The primary reason for this was the asymmetry of somatopetal vs somatofugal input core conductance met by synaptic current due to a greater leak conductance at the proximal end of the dendrite. Under these conditions, an increasing somatopetal core current and a corresponding drop of the depolarization membrane potential occurred. The voltage-dependent extrasynaptic conductances, if present, followed this depolarization. Consequently, the driving potential and membrane current densities decreased with increasing path distance from the soma. All path profiles were perturbed at bifurcations, being identical in symmetrical branches and diverging in asymmetrical ones. These perturbations were caused by voltage gradient breaks (abrupt change in the profile slope) occurring at the branching node due to coincident inhomogeneity of the dendritic core cross-section area and its conductance. The gradient was greater on the side of the smaller effective cross-section. Correspondingly, the path profiles of the somatopetal current transfer effectiveness were broken and/or diverged. The dendrites, their paths, and sites which were more effective in the current transfer from distributed sources were also more effective in the transfer from single-site inputs. The effectiveness of the active dendrite depended on the activation-inactivation kinetics of its voltage-gated conductances. In particular, dendrites with the same geometry were less effective with the Hodgkin-Huxley membrane than with the passive membrane, because of the effect of the noninactivating K(+)-conductance associated with the hyperpolarization equilibrium potential. Such electrogeometrical coupling may form a basis for path-dependent input-output conversion in the dendritic neurons, as the output discharge rate is defined by the net current delivered to the soma.