Figure 3. Functional implications of the low C_m value in human L2/3 cortical neurons for signal processing.

(a) The neuron model shown in Figure 1a, receiving an excitatory synaptic input on distal apical dendritic spine (top trace) and on a distal basal dendritic spine (lower traces). The model cell has either C_m = 0.45 μF/cm² (red traces) or C_m = 0.9 μF/cm² (blue traces), while the other cable parameters (R_a = 203 Ωcm, R_m = 38,907 Ωcm²) were kept fixed. Excitatory synapses were activated on the head of the modeled dendritic spine (inset, scale bar is 5 μm; see Materials and methods). Note the larger and faster somatic EPSP for the red case. (b) For the cell model shown in a, significantly smaller number of excitatory spinous synapses were required for initiation of a somatic spike when C_m = 0.45 μF/cm². Synapses were simultaneously activated and distributed randomly over the dendritic tree (see Materials and methods). (c) Top, schematics of soma and axon of the cell modeled in a., the axon had a diameter of 1 µm. Bottom, the velocity (θ) of the axonal spike, measured at 1 mm from the soma, is significantly increased (by about 65%) with C_m = 0.45 μF/cm² (red spike); the amplitude of the propagated axonal spike is also slightly increased in this case.

DOI: http://dx.doi.org/10.7554/eLife.16553.008

Figure 3—figure supplement 1. The functional implications of the low C_m value for the case where the membrane time constant, τ_m, is kept fixed by a corresponding change in R_m for both C_m = 0.45 μF/cm² and C_m = 0.9 μF/cm².

(a) As in Figure 3a, but here when C_m = 0.9 μF/cm² and R_m is halved (R_m = 19,453 Ωcm²). This resulted with smaller differences in the dendritic delay as compared to the case shown in Figure 3a, but with more strongly attenuated EPSPs. (b) As in Figure 3b, but here, due to the smaller EPSPs when C_m = 0.9 μF/cm², more excitatory inputs are required in order to initiate a somatic spike. (c) Conduction velocity is determined mostly by the axon’s diameter, the axial resistivity, R_a, and the membrane capacitance, C_m (Jack et al., 1975). Consequently, halving R_m had very small effect on spike propagation velocity (less than 1% change between the black traces shown here compared to the blue traces in Figure 3c).

DOI: http://dx.doi.org/10.7554/eLife.16553.009