抄録
Neurons in avian nucleus magnocellularis (NM) receive synaptic inputs from auditory nerve fibers, and extract the timing information of sounds. NM is tonotopically organized such that characteristic frequency (CF) of neurons decreases from rostro-medial (high CF) to caudo-lateral (low CF) direction within the nucleus. NM neurons show several CF-dependent specializations, including properties of postsynaptic membrane and synaptic transmission; high-CF cells express K channels abundantly and receive a few large end-bulb synapses, while K channels are scarce and multiple small bouton synapses converge in low-CF cells. These specializations are crucial for encoding the timing information precisely at each CF. On the other hand, NM neurons show a systematic difference in the amplitude of spikes depending on the CF and it becomes smaller toward the low CF. This may indicate that the process of generating spikes is also specialized along the tonotopic axis. In this study, we examined electrophysiologically and immunohistochemically the distribution and the properties of Na channels in NM neurons along the tonotopic axis using the chicken brainstem slices. We found that NM neurons accumulated Na channels at the proximal axon and varied their distribution in a CF-dependent manner; they were more abundant at the low CF. Computer simulation showed that the abundance of Na channels in the low-CF neurons was crucial to increase the reliability and precision of spike generation during the convergence of multiple small inputs. [J Physiol Sci. 2008;58 Suppl:S121]
