Need for Speed and Precision: Structural and Functional Specialization in the Cochlear Nucleus of the Avian Auditory System

Birds such as the barn owl and zebra finch are known for their remarkable hearing abilities that are critical for survival, communication, and vocal learning functions. A key to achieving these hearing abilities is the speed and precision required for the temporal coding of sound—a process heavily dependent on the structural, synaptic, and intrinsic specializations in the avian auditory brainstem. Here, we review recent work from us and others focusing on the specialization of neurons in the chicken cochlear nucleus magnocellularis (NM)—a first-order auditory brainstem structure analogous to bushy cells in the mammalian anteroventral cochlear nucleus. Similar to their mammalian counterpart, NM neurons are mostly adendritic and receive auditory nerve input through large axosomatic endbulb of Held synapses. Axonal projections from NM neurons to their downstream auditory targets are sophisticatedly programmed regarding their length, caliber, myelination, and conduction velocity. Specialized voltage-dependent potassium and sodium channel properties also play important and unique roles in shaping the functional phenotype of NM neurons. Working synergistically with potassium channels, an atypical current known as resurgent sodium current promotes rapid and precise action potential firing for NM neurons. Interestingly, these structural and functional specializations vary dramatically along the tonotopic axis and suggest a plethora of encoding strategies for sounds of different acoustic frequencies, mechanisms likely shared across species.