Localizing sound sources requires discriminating differences of sound arrival time of a microsecond order between the two ears (interaural time difference, ITD). In nucleus laminaris (NL) of birds, neurons calculate ITDs by detecting the coincidence of binaural synaptic inputs. We utilized slice-patch recordings, immunohistochemistry and computer simulations to explore the acuity and cellular mechanisms of coincidence detection in NL neurons of the chick. At 40 °C, the avian body temperature, the acuity of coincidence detection was high enough to account for the animal behavior. This acuity was achieved by the acceleration of EPSP time course due to the activation of Kv1.2-mediated low-threshold K+ conductance. In NL, neurons are tuned to a specific frequency of sound (characteristic frequency, CF), and are arranged so that the CF decreases from rostro-medial (high-CF) to caudo-lateral (low-CF) direction. Along this tonotopic axis, NL neurons were specialized morphologically and functionally depending on their CF. In the high- and middle-CF neurons, dendrites were short and expression of Kv1.2 channels was strong, which made the EPSP time course rapid and improved the coincidence detection. In the mid-high CF neurons, the process of generating spikes was also specialized; the axon initial segment was myelinated and Nav channels were clustered at some distance from the soma (20-50 μm) in the axon. Theoretical model predicted that this unique distribution of Nav channels in the axon is essential for making the high-frequency generation of action potentials and enhancing the ITD detection. [J Physiol Sci. 2006;56 Suppl:S20]
