While this approach might also be challenging in some cell types (GJs are dendrodendritic in most mammalian neurons), the M-cell and the

CEs offer several unusual anatomical and physiological characteristics Lumacaftor cost that make it possible to estimate these parameters in vivo: (1) CE afferents terminate with a single contact and are tightly segregated to the distal portion of the lateral dendrite of the M-cell; (2) the M-cell lateral dendrite as well as both the axons and terminals of CEs are accessible for intracellular recordings; and (3) the M-cell and the CEs have comparable and unusually fast membrane time constants, estimated to be 400 μs in the M-cell (Fukami et al., 1965) and 200 μs in CEs (Curti et al., 2008), which allow the use of physiological signals, such as action potentials, for measurements of CCs. Due to spatial considerations, measurements of CCs during simultaneous recordings of CE afferents in the VIIIth nerve root and the M-cell dendrite are useful to expose asymmetry of electrical transmission (Figures 4A and S3B) but not

accurate enough for estimating GJ conductance (see below). To overcome this problem, we calculated average values of CCs for the population of afferents, using values obtained under various experimental arrangements that maximize their accuracy (see below). The “population CC” in the orthodromic direction Thymidine kinase (CE to M-cell) for a number of CEs was estimated as the ratio between the average amplitude BMS-387032 in vitro of the electrical component (or coupling potential) of the unitary postsynaptic potential and the average amplitude of the presynaptic spike (CC,

postsynaptic coupling potential/presynaptic spike; Figure 4A). The orthodromic coupling potential (recorded during paired recordings with intradendritic recordings in the terminal field of CEs) averaged 0.73 ± 0.04 mV SEM (n = 76). (Because the strength of electrical synapses between individual CEs varies dramatically [Smith and Pereda, 2003], it was not possible to assign differences in the amplitude of individual coupling potentials to their relative position within the dendritic field and therefore correct for potential electrotonic attenuation. Thus, although potentially slightly underestimated, we believe the average amplitude of orthodromic coupling potentials represents the most appropriate value to use for calculating the CC in the orthodromic direction.) During simultaneous recordings, the amplitude of the presynaptic spike evoked at the recording site with long (200 ms) depolarizing pulses does not represent the spike that ultimately generates coupling, as the spike recorded at the site of depolarization regenerates in subsequent nodes and, finally, at the presynaptic terminal (see Figure S4).