Oertel D, Bal R, Gardner S M, Smith P H, Joris P X
Department of Physiology, University of Wisconsin Medical School, Madison, WI 53706, USA.
Proc Natl Acad Sci U S A. 2000 Oct 24;97(22):11773-9. doi: 10.1073/pnas.97.22.11773.
The anatomical and biophysical specializations of octopus cells allow them to detect the coincident firing of groups of auditory nerve fibers and to convey the precise timing of that coincidence to their targets. Octopus cells occupy a sharply defined region of the most caudal and dorsal part of the mammalian ventral cochlear nucleus. The dendrites of octopus cells cross the bundle of auditory nerve fibers just proximal to where the fibers leave the ventral and enter the dorsal cochlear nucleus, each octopus cell spanning about one-third of the tonotopic array. Octopus cells are excited by auditory nerve fibers through the activation of rapid, calcium-permeable, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors. Synaptic responses are shaped by the unusual biophysical characteristics of octopus cells. Octopus cells have very low input resistances (about 7 M Omega), and short time constants (about 200 microsec) as a consequence of the activation at rest of a hyperpolarization-activated mixed-cation conductance and a low-threshold, depolarization-activated potassium conductance. The low input resistance causes rapid synaptic currents to generate rapid and small synaptic potentials. Summation of small synaptic potentials from many fibers is required to bring an octopus cell to threshold. Not only does the low input resistance make individual excitatory postsynaptic potentials brief so that they must be generated within 1 msec to sum but also the voltage-sensitive conductances of octopus cells prevent firing if the activation of auditory nerve inputs is not sufficiently synchronous and depolarization is not sufficiently rapid. In vivo in cats, octopus cells can fire rapidly and respond with exceptionally well-timed action potentials to periodic, broadband sounds such as clicks. Thus both the anatomical specializations and the biophysical specializations make octopus cells detectors of the coincident firing of their auditory nerve fiber inputs.
章鱼细胞的解剖学和生物物理学特性使其能够检测听觉神经纤维群的同步放电，并将这种同步的精确时间传递给它们的目标。章鱼细胞占据哺乳动物腹侧耳蜗核最尾端和背侧部分的一个界限分明的区域。章鱼细胞的树突在听觉神经纤维束刚好靠近纤维离开腹侧并进入背侧耳蜗核的位置穿过该纤维束，每个章鱼细胞跨越约三分之一的音频定位阵列。听觉神经纤维通过激活快速的、钙通透性的α-氨基-3-羟基-5-甲基-4-异恶唑丙酸受体来激发章鱼细胞。突触反应由章鱼细胞不同寻常的生物物理学特性所塑造。章鱼细胞具有非常低的输入电阻（约7兆欧）和短的时间常数（约200微秒），这是由于静息时超极化激活的混合阳离子电导和低阈值、去极化激活的钾电导被激活所致。低输入电阻导致快速的突触电流产生快速且微小的突触电位。需要许多纤维产生的小突触电位进行总和才能使章鱼细胞达到阈值。低输入电阻不仅使单个兴奋性突触后电位短暂，以至于它们必须在1毫秒内产生才能总和，而且章鱼细胞的电压敏感电导会阻止放电，如果听觉神经输入的激活不够同步且去极化不够迅速。在猫的体内，章鱼细胞能够快速放电，并以特别及时的动作电位对周期性的宽带声音（如滴答声）做出反应。因此，无论是解剖学特性还是生物物理学特性都使章鱼细胞成为其听觉神经纤维输入同步放电的检测器。
