Figure 3.

Precise sodium channel distribution within the axon initial segment (AIS) fine-tunes neuronal responsiveness in the chick nucleus laminaris. The Na⁺ channel “hot spot” is arranged at greater distances from the cell body as the characteristic frequency (CF) of the neuron increases. A, B, Examples of Na⁺ channel hot spot localization in high-middle CF neurons (A) and low CF neurons (B). Na_v channels = red; retrogradely labeled nucleus laminaris neurons = green. C, Relationship between geometry of axonal Na⁺ hot spot (Na_v immunoreactivity) and CF found in nucleus laminaris (NL) neurons. High CF neurons had shorter hot spots (L) located at greater distance from the cell body (D), whereas low CF neurons hot spots were longer and closer to the cell body (yellow symbols are averages). D, Varying the geometry of the hot spot in multicompartment model simulations of NL neurons showed that the geometry producing the lowest threshold current (color coded; red corresponds to lowest threshold current) resembled the observed distribution of Na⁺ channels in NL neurons (shown in C). E–G, Dependence of interaural timing difference (ITD) sensitivity and threshold current on distance of Na⁺ hot spot from soma in model simulations of NL neurons. E, High CF (L = 10 μm). F, Middle CF (L = 15 μm). G, Low CF (L = 25 μm). Asterisks denote minimum threshold from data in panel D. Yellow symbols and red dotted lines indicate the actual observed average distances from data in panel C. Arrowheads show the maximum ITD sensitivity. Adapted by permission from Macmillan Publishers Ltd: Nature Kuba and others, copyright 2006.