Roles of axonal sodium channels in precise auditory time coding at nucleus magnocellularis of the chick.

How the axonal distribution of Na(+) channels affects the precision of spike timing is not well understood. We addressed this question in auditory relay neurons of the avian nucleus magnocellularis. These neurons encode and convey information about the fine structure of sounds to which they are tuned by generating precisely timed action potentials in response to synaptic inputs. Patterns of synaptic inputs differ as a function of tuning. A small number of large inputs innervate high- and middle-frequency neurons, while a large number of small inputs innervate low-frequency neurons. We found that the distribution and density of Na(+) channels in the axon initial segments varied with the synaptic inputs, and were distinct in the low-frequency neurons. Low-frequency neurons had a higher density of Na(+) channels within a longer axonal stretch, and showed a larger spike amplitude and whole-cell Na(+) current than high/middle-frequency neurons. Computer simulations revealed that for low-frequency neurons, a large number of Na(+) channels were crucial for preserving spike timing because it overcame Na(+) current inactivation and K(+) current activation during compound EPSPs evoked by converging small inputs. In contrast, fewer channels were sufficient to generate a spike with high precision in response to an EPSP induced by a single massive input in the high/middle-frequency neurons. Thus the axonal Na(+) channel distribution is effectively coupled with synaptic inputs, allowing these neurons to convey auditory information in the timing of firing.