FIGURE 1. Synaptic connections between individual Kenyon cells and -LNs are excitatory, powerful and varied in gain.

a, Schematic of the locust olfactory circuits: Projection neuron axons (n = 830) exit the antennal lobe (AL) and send collaterals into the mushroom body (MB) calyx. There, they excite Kenyon cells (n = 50,000) with 50% average connectivity¹⁶. Kenyon cells each send a bifurcating axon into the - and -lobes (-, -L), forming 'beams' of thousands of tightly packed axons (inset). The finer dendrites of -LNs run normal to Kenyon cell axons in -LN-specific sectors: two of these can be seen in the inset (a photomicrograph of a 10-m-thick section of the distal end of the -lobe). b, Spike-triggered averages of -LN intra-dendritic recordings from nine different KC–-LN pairs (9 Kenyon cells, 5 -LNs). All -LNs and Kenyon cells are recorded in vivo, with intracellular electrodes. Note the wide-range of spike-triggered average amplitudes. Same-symbol-marked spike-triggered averages are from the same -LNs, with different presynaptic Kenyon cells sampled successively. Inset, scaled spike-triggered averages in b, illustrating similarity of kinetics. c, Simultaneous dendritic impalements of one -LN with two separate electrodes (el. 1, 2). el. 2-el. 1 is the difference between the two voltage traces; note the high correlation of amplitudes (noise envelope is 2 s.d., red stipples), indicating similar electrotonic access to synaptic sites. Lower panel, overlay of selected EPSPs from above. d, Simultaneous dendritic impalements of two different -LNs (1 and 2). Note some common EPSPs (boxes) and EPSPs specific to either -LN (*).-LN2--LN1 is the difference between the two voltage traces; note significant variations on each side of the noise envelope (stippled lines as in panel c). Lower panel, overlay of selected EPSPs from above.