Figure 1.
Morphologies of electrically coupled PCs in the neocortex.
A. Images (upper panel) and reconstructions (lower panel) show the somata, primary dendrites and axonal trunks of 7 pairs. Putative contacts on dendrites and axons are marked with red asterisks on the reconstructions. The images of the coupled pairs were captured at 40× magnification and edited using the ‘merge images’ function of the Neurolucida program. Four of the pairs also had axonal trunks stained. The images of three putative axo-axonal contacts (marked with arrows in the insets) were captured at 60× magnification. The spatial arrangements of cells in the 3 unstained pairs are represented with schematic icons, which were derived through the visualization of recorded neurons under DIC optics during recording. In 8 of the 10 pairs, coupled PCs had touching or overlapping somata. The other 2 pairs (No. 5 & No. 6) had separations of only a few micrometers between somata of coupled PCs. The pair No. 5 had putative contacts on primary dendrites and axonal trunks. The pair No. 6 had one neuron's proximal apical dendrite overlapping the other cell's soma. Three (No. 2, No. 3 & No. 5), out of the 4 pairs with stained axonal trunks, displayed putative axo-axonal contacts (insets). These contacts were within distances of 16 to 150 µm from somata. B. The morphologies of coupled PCs were comparable. The 3D-computer reconstruction shows whole structures of the coupled pair No. 3. Note: For more information about each pair, please see Table S1.
Figure 2.
High junctional conductance and AP firing in pyramidal electrotonic couplings in the neocortex.
A. Bi-directional sub-threshold responses to stepped-current injections to either of the coupled PCs. B. The histogram shows notable disparities between pyramidal and interneuron electrotonic couplings. C. A prejunctional AP train induced a mixture of postjunctional APs and spikelets depending upon slight variations of membrane potential levels (inset: A broken line indicates a ‘threshold’ for the induction of either an AP or a spikelet. Asterisks mark truncated APs). D. AP firing was induced due to membrane potential depolarization from a resting level of −70 mV to −60 mV. E. Postjunctional AP firing was generated at resting membrane potential through the summation of spikelets when a 70 Hz prejunctional AP train was triggered. Spikelet summation, to a lesser extent, was also observed at 20 Hz as shown in D. Note: Traces were recorded respectively from the PFC slices of a 6 week old ferret for A, a P18 rat for C and a P32 rat for D & E.
Table 1.
Comparison between pyramidal and interneuron electrotonic couplings.
Figure 3.
Bi-directional tonic firing via pyramidal electrotonic couplings.
A. Tonic firing was ‘propagated’ from cell 1 to cell 2 (upper) and from cell 2 to cell 1 (lower). The firing patterns of the postjunctional cell mirrored those of the prejunctional cell. In elaboration, different initial firing patterns of cell 1 and cell 2 were exactly replicated postjunctionally (blank arrows). B. Corresponding to the direction of coupling conductance in A, the pre- (stimulating) and post-junctional (responding) APs had the identical slow rising and falling phases, but the different fast rising and falling phases (marked with arrows). C. The pre- and post-junctional APs of either cell (the same cells as in A) possessed the same fast rising and falling phases, but the different slow phases (marked with arrows). Note: AP traces in B & C were superimposed on each other by centering AP peaks without consideration for the onset delay of postjunctional responses. Recordings were obtained from the PFC slice of a P14 rat.
Figure 4.
Characteristics of the electrotonic coupling between PCs in the neocortex.
A. CCs are virtually symmetrical at the same current steps of depolarization and hyperpolarization. B. Linear relationship between pre- and post-junctional depolarizing responses. C. Linear relationship between pre- and post-junctional hyperpolarizing responses. D. Postjunctional spikelets are linearly correlated to the gradually enhancing prejunctional responses (CCs in the graph are color-coded with the superimposed traces of pre- and post-junctional responses). E. Amplitudes of spikelets induced by a pre-junctional response remained unchanged at different postjunctional membrane potentials. F. Comparison of asymmetrical CCs of pyramidal electrotonic couplings with symmetrical CCs of interneuron gap junctions. The CC label numbers correspond to the image numbers in Fig. 1. The CCs included step-CCs of pairs No. 2, No. 3, No. 6 & No. 10 and spikelet-CCs or AP-CCs of the other pairs. Note: All responses were recorded at −70 mV except those in E. Traces were recorded from PFC slices of a 6 week ferret for B and a P32 rat for A, C, D, and E, respectively.