Figure 5. Convergence of different AN synapses and the response properties of postsynaptic bushy neurons in middle-aged mice.

(A-C) Example bushy neuron with I_a-D inputs (I_a: 92%). (A) Reconstructed VGluT1-labeled puncta from type I_a (yellow) and non-type I_a (green) synapses. Scale: 5 μm. (B) Responses of the bushy cell to current step injections. Scale: 10 mV and 20 ms. (C) The neuron fired sustained spikes to a train of auditory nerve stimulation at 100 Hz; scale: 10 mV and 1 s. Arrowhead: last spikes of the train expanded in the inset on the right; scale: 10 mV and 10 ms.

(D-F) Example bushy neuron with I_a-D but lower proportion of type I_a inputs (I_a: 69%).

(G-I, J-L) Two example bushy neurons that received Non-I_a-D inputs, and fired only transient spikes or failed to fire any spike to AN stimulus trains.

(M) Bushy neurons with different proportion of type I_a inputs show correlated distribution in the total volume of VGluT1-labeled puncta (black), volume of type I_a only puncta (yellow), and volume of non-type I_a only puncta (green). Linear regression lines: black, r² = 0.16, p = 0.025; yellow, r² = 0.62, p < 0.0001; green, r² = 0.40, p = 0.0001.

(N) The proportion of type I_a inputs in bushy neurons negatively correlates with their threshold spike amplitude. Linear regression line: r² = 0.15, p = 0.043.

(O) Spike adaptation index from 100 Hz trains in bushy neurons with different proportion of type I_a inputs. Linear regression line: r² = 0.08, p = 0.160.

(P-V) Comparisons between bushy neurons from middle-aged mice with I_a-D and Non-I_a-D inputs in resting potential (P), input resistance (Q), threshold spike amplitude (R), threshold current level (S), firing rate throughout the 100 Hz train (T), spike adaptation index (U), and vector strength of the spikes (V). Unpaired t-test or Mann-Whitney test: NS, p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001.