Proportion and distribution of thorny and athorny pyramidal neurons in CA3.

A, Left, Image of 7 pyramidal neurons recorded simultaneously and filled with biocytin to reveal thorny and athorny morphologies. Right, magenta box contains a typical example of a thorny CA3 pyramid, grey boxes show close up of regions with thorns; yellow box shows a typical athorny pyramidal neuron. Scale bar in left image 100 µm, in magenta/yellow boxed insets 20 µm, in grey boxed insets 5 µm. B, Proportion of thorny and athorny cells in total recorded pyramidal neurons. C, Distance from soma to the first branch point for thorny (T) and athorny (A) CA3 pyramidal neurons. Di, Location of thorny and athorny cell somata across the deep-superficial axis of the pyramidal layer. Dii, Schematic depicting the distribution of thorny and athorny pyramids in the deep-superficial axis of the CA3 pyramidal layer.

Properties of excitatory connections between athorny and thorny CA3 pyramids.

A, Connection probabilities and example connections between: i, athorny cells, ii, thorny and athorny cells, iii, thorny cells and iv, athorny and thorny cells. Scalebars for presynaptic action potentials, 40 mV; for postsynaptic responses, 0.5 mV. B, Histograms of synaptic amplitudes of the different connection types: i, athorny-athorny, ii, thorny-athorny, iii, thorny-thorny, iv, athorny-thorny. Dashed line represents median value and shaded area interquartile range. C, Latency of synaptic connections onto postsynaptic athorny cells, individual points show single connection values. D, Failure rates of the different synaptic connection types. E, Short-term plasticity dynamics of different synaptic connection types. Synaptic amplitudes are normalised to the first EPSP in the train of 4.

Summary of overall impact of each connection type.

Ai, Matrix showing connection rates between the four combinations of connection types, ii, matrix showing mean connection strength for the four connection types, iii, proportion of each cell type found in the CA3 pyramidal population, iv, matrix showing the synaptic product, calculated as the product of the matrices in i and ii multiplied by the presynaptic population size shown in iii. B, Schematic depicting the connections between the two pyramidal cell types in the CA3, line colour is coded by connection impact.

Results of numerical simulations.

A, Network scheme. Bi, Firing rates before, during, and after a SPW. Inset: low-pass filtered estimate of the LFP over a longer window of 10 s. ii, Spike raster plot of a representative sample of each neuron type. iii, Relative increment of the average adaptive currents received by each population with respect to a 200 ms baseline before the event. C, Effects of varying each connectivity from its default value, marked by a black dashed line and dot. Continuous gold/magenta lines indicate the peak time of each population rate (with the peak of A always plotted at 0), while dashed ones represent the time at which the rate equals 25% of the respective peak. The peak size for each connectivity value is color-coded. Insets: firing trace of each population averaged over many events, for particular connectivity values highlighted by the gray arrows.

Single neuron parameters

Network parameters

Intrinsic properties of thorny (T) and athorny (A) cells. Significance calculated using Mann-Whitney-U and corrected for multiple comparisons. *** p < 0.001, * p < 0.05.

Onset f-I curves for each neuron type, calculated, for comparability, by delivering a constant current for 500 ms, like in Hunt et al. (2018). These curves (colored solid lines) are compared to experimental data (black dots) from Hunt et al. (2018) for A and T neurons, and from Fidzinski et al. (2015) for B neurons. Dashed lines represent transient firing. Insets: example of firing patterns displayed by the different neurons in response to the specific current values marked by vertical gray lines in the main figure.