Neurology · 2006 · 478 citations · 30 references
Life SciencesNeurobiologyNeuroscienceElectrophysiologyNeurophysiologySensory NeuroscienceAuditory Brainstem ResponsesAuditory NeurophysiologyAuditory PathwaysBrainstem CircuitsNeural Circuit DynamicsNeural ExcitabilityCoincidence DetectionInput ResistanceInteraural Time CuesMembrane Time ConstantSound Source LocalizationSource LocalizationTonotopic MappingTreadmill Training
The interaural time difference (ITD) is first determined in the nucleus laminaris (NL) of birds, where neurons are tonotopically organized to process ITDs separately at each characteristic frequency. The study examined excitability and coincidence detection of NL neurons along the tonotopic axis using a chick brainstem slice preparation. Morphological and electrophysiological properties of NL neurons varied systematically with characteristic frequency, with dendrite length, input capacitance, HCN conductance, and EPSC time course increasing, while low‑threshold K⁺ conductance and Kv1.2 expression peaked centrally; consequently, middle‑CF neurons had the lowest input resistance, shortest membrane time constant, fastest EPSP, and most accurate coincidence detection, explaining the high sound‑localization resolution in the middle‑frequency range.
The interaural time difference (ITD) is a cue for localizing a sound source along the horizontal plane and is first determined in the nucleus laminaris (NL) in birds. Neurons in NL are tonotopically organized, such that ITDs are processed separately at each characteristic frequency (CF). Here, we investigated the excitability and coincidence detection of neurons along the tonotopic axis in NL, using a chick brainstem slice preparation. Systematic changes with CF were observed in morphological and electrophysiological properties of NL neurons. These properties included the length of dendrites, the input capacitance, the conductance of hyperpolarization-activated current, and the EPSC time course. In contrast to these gradients, the conductance of low-threshold K<sup>+</sup> current and the expression of Kv1.2 channel protein were maximal in the central (middle-CF) region of NL. As a result, the middle-CF neuron had the smallest input resistance and membrane time constant, and consequently the fastest EPSP, and exhibited the most accurate coincidence detection. The specialization of middle-CF neurons as coincidence detectors may account for the high resolution of sound-source localization in the middle-frequency range observed in avians.
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